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Physics Lecture Demonstrations,
with some problems and puzzles, too.

Compiled and annotated by Donald Simanek

The internet has many sources of lecture demonstrations, particularly of demos suitable for large lecture courses. I list just a few.

• Physics demonstrations, Science Exhibits (Bill B's Homepages)
• Julien C. Sprott, University of Wisconsin. Descriptions, safety and presentation hints, data, literature references. A very rich and useful collection of physics demonstrations.
• University of California at Berkeley. Nice drawings of entire setups.
• University of Minnesota. Color photos.
• Boston University. Line drawings.

Often the university sites exist primarily for internal use, listing equipment and demos available to physics professors teaching large sections of introductory courses. They usually give lists of required equipment, they don't always give much information about setup, effective presentation and explanation of the physics.

In my little collection of demos I have included many which are appropriate for smaller classes and classrooms. I consider these demonstrations particularly well-suited for integration into a lecture/discussion in an informal setting.

I'm in the process of improving and refining this document, and intend to include more illustrations, discussion of the underlying physics, and presentation suggestions. Keep watching this space.

How and why to present demonstrations

Over the years since Newton, physics teachers have developed a vast catalog of physics lecture demonstrations, which fill many books.Too often these demo books merely describe the demonstration, but don't explain the physics behind it, nor suggest effective ways to present it. Too often teachers use these demonstrations primarily for entertainment in physics classes. My own philosophy on the use and misuse of demos is reflected in these questions you ought to ask before doing a demo.

1. Does the demonstration fit into the story line of your lectures?
2. Is the demonstration important enough to justify building a story line around it?
3. Does the demonstration clearly illustrate a phenomenon, principle or law of physics? If too many interacting principles affect the outcome you ought to select a "cleaner" demo.
4. Can the physics of the demonstration be understood at the level of the course? If not, don't do it.
5. Do you, the presenter, understand the demonstration fully? If not, don't do it.
6. Ask yourself whether you do the demonstration for entertainment, or because the physics behind itneeds this kind of reinforcement.

To do a demonstration without explanation and discussion is pointless. You should:

1. Practice the demonstration until you have it perfected. Present it to a critical observer you can trust to tell you what's wrong with it, what points are not clear, and what details of explanation you've omitted.
2. Be sure you know what the demonstration demonstrates—not what you intend it to demonstrate, but what the students see in it, and what they think about what they see. I.e., find out what it demonstrates to them and what they actually get out of it.
3. Prepare the students for the demonstration. Show the apparatus, describe what you intend to do before you do it. Tell your audience what the demonstration ought to teach us about physics, or what part of physics it will put to the test. Suggest what the audience should watch for.
4. Ask students to predict in advance what will happen. Do not proceed until at least some students have provided good physical reasoning for their predictions.
5. When you have made the experimental situation clear to everyone, and have their attention, do the demonstration.
6. Analyze the demo. If some predictions were wrong, find out why. Do the demonstration again during the discussion.
7. Be prepared to test hypotheses raised by students by doing a modification or variation of the demonstration, and making some measurements.

What does it demonstrate?

The staged bit of physics we call a demonstration is done to illustrate and reveal a physical process or principle. Are we sure what principles are being demonstrated? If not, we should rethink the matter.

The bottom line is what is demonstrated to the students, that is, "What do students see in it, and what do they understand about it?" Often that isn't what we intended. If students take concepts away from a demo which are incomplete, wrong, or misleading, our purpose has been thwarted.

Consider the classic tablecloth pull and its variants, often passed off as demonstrations of inertia. To say that objects on the table remain in place when the tablecloth is yanked from under them is because of their inertia isn't entirely wrong. But it isn't the whole story, and isn't helpful to understand the outcome of this experiment. The audience didn't see anything which showed that objects with different inertia (mass) behaved differently. I consider these to be primarily demonstrations of impulse. They show that in a situation where the size of a force is essentially independent of the time it is applied, then the impulse of that force can be made very small by reducing the time during which it is applied.

Another so-called "inertia demonstration" is that of the heavy ball suspended by a string with another string dangling. (See my discussion below, titled jerk.) Inertia of the heavy ball plays an important role here, but one doesn't understand the demo until one considers the mechanism of how string breaks and the fact that the string must be physically stretched before it breaks. One must analyze the tensions which cause the stretch. The fact that a steady pull causes the upper string to break is easy enough to grasp, for its tension is greater than the lower one by amount equal to the weight of the heavy ball. This part of the demo has little to do with inertia. But when the lower string is jerked, why does its tension increase to the breaking point before the upper string? Here's where the inertia of the ball is important. The tension on the lower string increases to the breaking point in so short a time that the ball hasn't moved far enough to stretch the upper string enough to break it. Unless these points are brought out, the demonstration is little more than a gee-whiz bit of stage magic.

One more example, from Martin Gardner's The Last Recreations, Springer-Verlag, 1997. (Used with Mr. Gardner's permission.)

Inertia is the secret behind the following bet. Get a kitchen knife with a sharp point, hold it vertically, and hang half an eggshell over the point [as shown in the figure]. Challenge someone to puncture the shell by rapping the knife handle on a table or kitchen counter. Each time one tries the shell bounces off the tip of the knife unharmed. But you, who understand physics, can crack it at will. The secret is to hold the blade loosely in your hand. Make this appear as if you rap the handle on the counter when in fact you allow the knife to fall by its own weight so that it hits the counter and bounces. The imperceptible bounce sends the knife point through the shell.

I can just hear Robert Ehrlich and Cliff Swartz groaning, complaining that this isn't a demo, it's a deception, and such deceptive tricks are best avoided in a physics class.

Does this "demonstrate" inertia? Inertia does play a role here (doesn't it play a role in almost everything?) But there's more physics to this demo than that. If a you want to bring out all of the physics in this demo, you need to discuss these details:

1. The person trying to forcefully rap the knife on the table will cause the knife to move downward faster than free-fall, therefore when the knife hits, it won't be in contact with the eggshell. The motion of the knife is damped by the hand holding it and is over by the time the eggshell drops onto the stationary knife-point. That impact isn't forceful enough to break the shell. The small inertia (mass) of the eggshell plays a role here.
2. But when the knife and eggshell are allowed to fall freely they remain nearly in contact because their accelerations are the same. There may be some aerodynamic drag on the inverted eggshell, but not enough to cause the shell and knife to separate much, and a little separation doesn't matter.
3. The knife bouncing on a hard (elastic) table-top rebounds upward with nearly the same speed it had when it hit the table. Therefore the relative speed of knife point and egg shell is roughly twice what it would be if you held onto the knife and forcefully hit it against the table. This is enough to puncture the eggshell.

Oh, yes, I almost forgot. What principle of inertia was demonstrated in this experiment? Seems rather trivial, doesn't it, now that we've analyzed the real physics secrets behind this experiment.

Showmanship tips

I have seen many shoddy presentations of physics demonstrations. Teachers should seek out members of the speech and drama department to critique their performances. Professional, or even amateur, magicians can give invaluable advice on how to communicate one's intentions to an audience (and hide one's deceptions, but that's not a skill you'll need in physics).

Use apparatus large enough for the audience to see clearly. Consider super-sizing it.

If something can be seen only in a narrow range of angles, stand to one side, toward a corner of the room, so the audience subtends a smaller angle from your position. Take the time to show it from different angles if necessary, so no one is cheated.

Don't ask the audience to trust a measurement you make but they can't see. Consider projection analog scales or projection digital meters for electrical experiments. Avoid electronic balances; use visible mechanical ones. Sometimes all that's necessary to make a point is a visual indication of which direction something moves.

Color helps in many ways, to distinguish one thing from another, and just to enhance visibility. Why use a white paper sheet when paper can be had in many colors? Ping-pong balls and golf balls come in many fluorescent colors now. White or black string can be replaced with colored kite string, mason's cord (hardware store item), or rug-maker's cord. Water can be made more visible with a bit of food coloring, but tell your audience that you've colored it so no one will think it is something other than water.

Black lecture tables are drab. Consider having a selection of unpatterned tablecloths in attractive colors. Equipment stands out a lot better against them. Stand back in the lecture room to judge the effect.

Any well-equipped lecture hall has portable steps so the experimenter may safely ascend to stand on the lecture table for greater visibility.

Your students may be in the dark much of the time, but you, the experimenter, should work in a well-lighted environment.

Use all sensory clues. If you want to remind the audience that something is rigid, tap it on the table so they can hear it. Yank the string to show it doesn't stretch.

Demonstration list

This document is an alphabetical listing of lecture demonstrations suitable for physics courses, with brief descriptions. I intend to continually add to this file. More demos, corrections, clarification and suggestions are welcome at the address shown to the right. Herbert H. Gottlieb contributed a large bunch to get the project started. As more come in I will flag each contribution with the author's initials and add them to the list of names and e-mail addresses.

I've included credit tags where I've used the exact details or text of a description supplied to me by a person or source, or when some particular idea of apparatus or presentation seems original to the person credited. Uncredited comments are my own.

Many of these references are old. This helps remind us that these demonstrations go back several generations. I have found many books of this kind in used book stores, flea markets and library book sales. Older ones are often reprinted and appear for a short time in mainstream bookstores.

In the spirit of Julius Sumner Miller's presentations, and his Millergrams books, I have added questions stimulated by the demonstrations, and sometimes I don't give answers! Readers are welcome to treat these as physics puzzles, and send me their own answers. The tag [Answer] links to an answer at the end of the document (so you won't peek at answers too quickly); the tag [<] links back to the question.

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ACCELEROMETER. Mount a liquid accelerometer on wheels. Predict the inclination of the water surface as it accelerates down an inclined plane. Three possibilities are shown. Explain what you observe, with free-body diagrams. [BERG]

Berg does this in his shows as a test of understanding. In a classroom situation I recommend that you first consider the acceleration on a level surface, to develop a clear understanding (with free body diagrams) of what makes the water surface tilt.

If you don't happen to have a liquid accelerometer, use a clear plastic glass or beaker mounted on a suitable cart. Cover the top in case of a crash.

Other ideas: Mount the accelerometer on strings and swing it as a pendulum. Suspend it from two parallel strings; wind it up by twisting the strings, then let it spin. The commercial version with colored liquid between two flat plates is best for the spinning demo.

ADIABATIC HEATING. A considerable temperature increase occurs when a rubber band is stretched. Stretch a thick rubber band briskly and forcefully several times, then touch it to your upper lip.Your lip can sense the increase in temperature. Allow the rubber band to contract rapidly and note that it suddenly becomes colder. [HG]

AIR GLUE. To demonstrate Bernoulli's principle, cut a circular piece of cardboard slightly larger than the end of a thread spool. Push a straight pin all the way into the center of the cardboard. hold the spool so its hole is vertical. Press the cardboard against the bottom of the spool with the free end of the pin inside the bottom of the spool hole. Blow into the top of the spool hole. The air acts like glue and the card board clings tightly to the spool as long as there is air motion through the spool. [HG]

AIR PRESSURE SUCKER. Is so-called suction a push or a pull? Arrange a flask fitted with a two hole stopper and a glass tube extending well down into the flask which is completely filled with water. Challenge a good-natured student to "suck" water up the glass tube while the instructor holds a finger over the other hole of the stopper. When the student fails to get any water out of the flask, the instructor might remark, "Jim is not as big a sucker as we thought." After appropriate comments and removing the finger from the stopper hole, the instructor asks the boy to try again. Suddenly the sucker succeeds. When the class becomes orderly again, explain how air pressure provides the necessary push. [HG]

AIR PUCK. Cut a six inch circle of plywood or pressed board. Cement a small cork at its center, Drill a very fine hole through the disc and cork. Inflate a balloon and fasten its mouth over the cork. Place the apparatus on a smooth surface to see almost frictionless motion on cushion of air. [HG]

ALTERNATING CURRENT MADE VISIBLE. Method 1: Obtain a long, lightweight and flexible AC power cord with a plug on one end and bare wires at the other. Wire a small neon glow tube (of the sort used in night lights) with a 10K resistor in series and solder to the bare wires. Insulate the bare wires fully. Plug it in. The neon bulb should glow. Now swing the bulb round and round in a circle in a darkened room, and you will see that the glow isn't continuous. The two electrodes cause the neon gas to glow only near the peaks of voltage.

Method 2: As you watch a stationary neon bulb, sweep your gaze across it. Persistence of vision will leave a trail of bright dots. I've seen this trick also used with the spot from a laser beam reflected from a vibrating mirror onto a blackboard. The beam moves up and down rapidly, but the waveform can be seen against the blackboard by sweeping the eyes horizontally. A poor person's oscilloscope!

ANTI-GRAVITY. Select two test tubes so that one barely fits inside the other. Partly fill the larger with water and float the smaller one on the water. Quickly invert the tubes. As water leaks out, the smaller tube rises, apparently defying gravity. [HG]

ARISTOTELIAN FALLING BODIES. [DES] Aristotle taught that the "natural motion" of a falling bodies is to fall equal distances in equal times. He also said that the speed of fall is proportional to the mass. Is this correct? Can we test it? Bodies falling in fluid media actually fall with constant speed once they reach terminal speed. So, at terminal speed a body does fall equal distances in equal times. This can be demonstrated with ordinary coffee filters from coffee machines, the paper cups sold for baking muffins, or the small cups used for candies or party favors. When dropped with closed end down, their motion is rather stable, and they reach terminal speed very quickly.

But what about the relation of speed to mass? This may be studied quantitatively. The cups may be nested, presenting nearly the same aerodynamical profile for one, two or more cups. Drop one cup and a stack of two simultaneously, from different heights. How much higher must you drop the stack so that it lands simultaneously with the single cup? How about a stack of three? Someone may have to stand on a ladder to drop the heavier stack. (Alternatively, both may be dropped simultaneously from the same level, one landing on the desk top, the other landing on the floor. Adjust starting level till they land simultaneously.) In our experiments a stack of four must be dropped from twice the height as a single one to land simultaneously. What does this tell you about the relation between speed and mass? In these times of shrinking equipment budgets a quantitative experiment such as this which uses only inexpensive materials is most welcome.[Answer]

AUDIBLE ACCELERATION. We cannot perceive acceleration of a falling body by eye. A classic way to convince students that falling bodies accelerate uses wooden balls affixed to a long cord that is hung from the ceiling. The balls are spaced apart so that, from bottom to top, they get farther apart, just the right amount so that when the upper end is released the sound of their impacts hitting the floor is a series of clicks equally spaced in time.

The bottom ball should always be initially touching the floor when the system is released.

This demo isn't often done because of the nuisance of the setup and hanging and releasing the cord at the ceiling. A more practical version uses Galileo's "diluted gravity" method, rolling a ball down a grooved track with a slight inclination. Obtan a 6 foot long extruded aluminum V-strip from the hardware store. Make a suitable block to incline one end a couple of inches above the lecture table. A metal ball will roll down the trough slowly. Students probably won't "see" the acceleration. But if you place small paper clips along one edge of the ramp, the metal ball will make a click as it encounters each clip. Now space the clips for equal clicks. It will now be apparent that the clips increase their separation down the track. Always start the metal ball from the same position and mark that position. When you have perfected this, you may want to calculate the clip positions, and replace each clip with a small groove filed in the aluminum. These will be permanent, and will produce audible clicks. Galileo would have loved it. The apparatus is easily stored in the stockroom. [DES]

BALL IN FUNNEL. Blow into the small end of a funnel containing a ping pong ball. The ball will not fall out even though the funnel is inverted, so long as air is moving between the ball and the funnel wall. [HG]

Stores that sold vacuum cleaners used to attract customer's attension by setting up this demo with a vacuum cleaner blowing air through a funnel, and a lightweight colorful ball bouncing above the funnel, suspended there. Would this also work if the vacuum cleaner were reversed, to "suck" air through the funnel? Are you sure that isn't the way they were doing it?

BALLOON ENIGMA. Two balloons, partly inflated, are connected at either end of a tube with a stopcock. (A pinched-off section of rubber tubing may be used.) One balloon is inflated more than the other. When air is allowed to pass through the connecting tube, what happens?

1. The large one deflates completely.
2. The small one deflates completely.
3. The large one deflates and the small one inflates until they attain the same size.

This demonstration doesn't work reliably with balloons. The outcome depends upon the degree of inflation of the two balloons. One could consistently use the same brand of balloons, and practice in advance to get the correct initial inflation, but isn't that something of a fraud? The demo is much more reliable with soap bubbles, but more trouble to do.

The only excuse for doing this demonstration is to illustrate a detailed calculation of the dependence of surface tension upon pressure. This is a bit too much math (vector free-body diagrams in three dimensions) for most high school courses.

If you do it, and do it successfully, there's a neat joke you can include. Do this demo with the largest balloon below the other, and say that gravity pulls air down into it to make it larger.

Question: Why do balloons rise in the atmosphere? Answer: Because someone let go of the string.

Question: Does an inflated balloon weigh more, less, or the same as it does when deflated? By "weigh" we mean what it would weigh when placed upon a sensitive balance scale. Answer: More. The air inside is compressed. This may be easily tested with an electronic balance.

Showmanship tip: Inflate the balloon, tie it off, then fasten the neck of it to the pan of the balance. Note the balance reading. Puncture the balloon and note the change in the balance reading. Better yet, use a sensitive simple mechanical balance, so that the audience can see the scale unbalance when the balloon is punctured.

Serious question: When is a balloon hardest to inflate? Answer: When it is smallest. Actually the pressure barely reaches 1.06 atm in the early stages of inflation, then drops to about 1.05 atm and changes very little from then to full inflation. That's why the two-balloon demo is so unpredictable. Sometimes the two different size balloons just sit there, without either one deflating.

BALLOON ELECTROSCOPE. Light rubber balloons suspended from long silk strings act as demonstration electroscope. Charge the balloons by rubbing them with woolen cloth or fur. When charged, the balloons can be made to stick to flat surfaces such as walls or ceilings. [HG] With young children, consider adhering the charged balloons to the clothing of a volunteer from the audience.

BERNOULLI ATTRACTION. [MG-1] Lay two books on the table with their spines near each other and parallel, forming a channel on which two ping pong balls can roll. Place the balls a short distance apart and blow horizontally in the space between them. They roll toward each other.

BERNOULLI BALANCING ACT. Bernoulli's principle can be shown by balancing an inflated balloon or beach ball on a jet of air from the output end of a vacuum cleaner. The balloon will hover near the ceiling and will not fall off even when the air jet is tipped at a considerable angle. A ping pong ball balanced on a fine jet of water illustrates the same principle. [HG] You can even lie on your back on the floor and blow upward to support a ping-pong ball. Or blow through a vertical tube. Or stand up and blow through a tube bent in a right angle, with the open end up. [DES]

BIG TORQUE. Hold the end of a broom handle in one hand and extend your arm and the broom handle horizontally in front of you. Tie a string around a book and hang the book under the stick a few centimeters from your hand. Try to keep the stick horizontal while someone slides the book toward the end of the stick. Although the weight of the stick and book do not change, the torque increases. Lever arm has real meaning here.[HG]

BOBBING ON THE WAVES? [BERG] A beaker of water is suspended from a spring. An object floats in the water. As the beaker-pendulum moves up and down, what happens to the floating object as the pendulum is at the bottom of its motion. Does it rise up out of the water, sink deeper, or stay at the same level relative to the water surface?

Result: It stays at the same level relative to the surface. Even when you whack the beaker from the bottom the float stays at the same level. Of course a detailed analysis with a free body diagram is in order here, or nothing will be learned from the demonstration.

BLACK BOX CONTENTS. Into a small box, place small objects and seal the box closed. Students can manipulate the box, and try to tell you: (a) How many pieces are in the box. (b) the shape of the pieces. (c) How heavy the pieces are (density). (d) How big the pieces are. (e) The color of the pieces. By doing this the student has reason to believe that scientists may know something about the atom even though it has never been seen; just as the student knows something about unseen objects in the box. [HG]

I recommend boxes with the following: (a) one wooden sphere, (b) one wooden or plastic cube (from a pair of dice), (c) one wooden rod or clininder, (d) a small amount of sand, (e) some water in a closed bottle, (d) some with two or more objects, (e) a wooden ball attached with a cord to the center of one cube face, (f) a wooden ball attached with a cord to the inside corner. In the last two, the cord should be shorter than the cube edge, so that it can only touch the nearest faces of the cube. A chemistry colleague also used to make some cubes with inner compartments, splitting the cube into two equal chambers with a wall down the middle. [DES]

BOILING WITH ICE. Fill a flask two-thirds full of water and bring it to a boil. Cork the flask and invert it, taking care that the hot water does not spill out. Place an ice cube on bottom of flask. As the ice melts, the water begins to boil again. If the flask is corked with a one-hole stopper with a glass tube extended almost to the bottom of the flask, boiling can be effected by reducing the pressure of the entrapped air. You can also run cold water over the flask. Have the student feel the flask temperature as the ice boils the water. of boiling water. [HG]

BUTTON SPINNER. Children used to make these for amusement, using a large coat-button from their mother's sewing basket. Loop a string or heavy thread through two opposing holes of the button, and tie the ends together. Put the loop over your thumbs with the button hanging. Spin the button around until the strings are well-twisted on both sides. Then pull the strings taut. The button will spin rapidly as the string unwinds. When nearly unwound, release tension, but keep the string relatively straight between the thumbs. The angular momentum of the button will cause the strings to twist again in the opposite direction. As the button slows, pull on the string again and its direction will reverse.

This is more interesting to do than to watch. One learns to modulate the tension during the spin for maximum performance. The tactile "feel" of this process is fun to experience. If the button has slits or a raised repetitive design it may make a musical "whirr" as it spins, the pitch being speed dependent. Lots of physics here!

Primarily this demonstrates conversion of energy from kinetic (spinning rapidly) to potential (string "wound up"). There's considerable friction loss, so one must supply energy by pulling on the string at the most advantageous times. Compare to the person pushing a child in a swing. The push must be timed to be just after the maximum amplitude is reached. If you push just before that you'll be less effective, wasting energy.

The string should be quite pliable, and not too heavy. In fact the optimum weight/length of the string is dependent on the weight of the spinner. The holes in the button should not be too close together.

Other spinners to consider: The plastic or metal gears or pierced disks from Erector (TM) or Meccano (TM) sets work well. Colorful spinners may be constructed from K-Nex (TM) construction sets. [DES]

Color mixing: Consider pasting colored sectors to one side of a larger size spinner, using the three primary colors. When spun, you may get white. The angular size of each sector can be tinkered until the white is quite pure (the three color sectors won't be equal in angle). Mixing just two sectors can give you green from yellow and blue, orange from red and yellow and purple or magenta from blue and red. This is additive color mixing. (Purple and magenta are not spectral colors, but are achieved only by mixing.) [DES]

Persistence of vision: A different version of button-string-spinner can be used. This uses a disk with four holes, in pairs of two on opposite sides.Two strings connect to these holes, so that the card may be spun, not in its own plane, but around its diameter. Experiment with hole spacing, card weight, and string weight for best performance. Now two pictures are put on opposite sides of the disk, which will be "mixed" when the disk is spun. A common illusion is a picture of a bird cage on one side and a bird on the other. When spun, the bird appears to be in the cage. Mixing of two colors may also be demonstrated. [DES]

BREAK IT. A wooden slat, or yardstick is suspended by two thread loops, then struck hard at its center. The wood will break without breaking the strings. The wood may be suspended from loops of paper held up on razor edges. Pins may be stuck in the ends of the wood, and the pins supported on the rims of two wine glasses. Once you convince yourself that very little impulse reaches the supports before the stick halves drop, you realize that the stick may be suspended from loops around the ears of two students, without any "Vincent van Gogh" outcomes. Carnival performers used to do this with the stick suspended from their eyelids.

CHAIN REACTION. Arrange wooden matches closely on a soft board by means of straight pins placed through them at their midpoint. Hold the board upright and ignite the bottom match. The others will follow in turn to simulate a chain reaction.The usual warnings about "playing with fire" apply here. [HG]

Push pins through a thin board in a regular triangular array about 1 cm apart. The board is in a vertical plane. Stick the back end of matches onto the pins. Light a match near the bottom of the board. Chances are it will ignite the two nearest matches above, and so on, demonstrating the multiplication factor of a chain reaction. I have not tried this, but it could conceivably work with the board in a horizontal plane if the matches are closely enough spaced. This would be more realistic, in that the reaction would spread in all directions from the initial match. But to make it visible by an audience, a large mirror would have to be placed above, at a 45° angle.

If this is feasible, one might consider dispensing with the pins and using a board of metal or other fireproof material with an array of holes drilled in it to anchor wooden matches. The pins in the above demos were only to protect the wooden board from being scorched. [DES]

Of course the best chain reaction simulation is still the large array of mousetraps, cocked, with two corks resting on each one. The entire array is in a chicken-wire enclosing box. Triggering one trap releases two corks (analogous to neutrons) which bounce within the box and may trigger two other traps, which release two more corks, and so on. When done properly, it seems that the whole thing "goes off" in an instant, ending up in a jumble of mousetraps and corks.

CHIMNEY TOPPLING. Old brick factory chimneys are demolished by explosive charges at their base. If tall enough the chimneys do not topple as a single unit, but break in two parts as they fall, at a point about halfway up. This may be easily demonstrated with two wooden cylinders on a tiltable board, as shown. Children demonstrate it with stacks of wooden blocks, doing physics at a very early age. But they don't try to explain why it happens. Can you? [DES]

Brick structures have very little resistance to being pulled apart at the mortar lines. Poured-concrete chimneys, especially if they have steel reinforcing bars, would behave differently under these conditions of demolition.

Show that this doesn't happen with a stack of two cubes. What does happen? Now try higher stacks of cubes or cylinders. How high must the stack be before you see the definite "breaking point" occur partway up the stack? What conclusions can you draw? Does the area of the base relative to height play a role? Does the cross-section of the stack at the point of the break play a role?

COIN DROP. Hold two quarters and a nickel as shown, between thumb and forefinger. Release pressure slowly, allowing the bottom two coins to drop. The quarter will land on top of the nickel if the distance of drop is correct. Practice to find that distance. Why do the two coins invariably turn over as they fall?[Answer]

COLOR ABSORPTION. Using colored pencils, draw a bird in a blue cage. Let the bird out of the cage by covering the drawing with a red filter. Try other color combinations to show the effects of color absorption by filters. [HG]

COLORED ROACHES. Many organic substances, dead or alive, show interesting characteristics under black light. Cockroaches are multicolored under ultra-violet light. [HG] The "whitening" agents in laundry products cause white shirts to glow in UV.

CONDENSATION OF WATER VAPOR. A simple cloud chamber can be made from a gallon jug fitted with a one-hole stopper with a short piece of glass tubing. Blow into the jug through the glass tubing to increase the pressure. Put a finger over end of tube and pull out the stopper, suddenly reducing the pressure. No cloud is formed. If some smoke is introduced into the jug it provides nuclei about which water vapor condenses. Repeat the performance and watch the clouds form in the jug. [HG]

COMING AND GOING. Weld two semicircular steel rods together and weld or fasten a heavy ball at each end. Paint the balls different colors, say red and green. At the center you need something to allow it to rotate with very small friction with respect to the flat plate which will be placed on your head. When wearing it you'll look like a space alien (see picture). Then when you turn and walk. The balls keep the same orientation in space. This comes close to being a pure demonstration of static inertia.

Presentation: "This is great for someone who never knows whether he's coming or going." Walk with the red ball in front of you. "If the red ball is in front of your face you are going." Turn around and walk the other way. "And if the blue ball is in front of you, you are coming." [Dick and Rae]

COTTRELL PRECIPITATOR. Attach one lead from a spark coil to a foil surrounding a glass tube of about one-inch diameter. Extend a wire from the other terminal of the spark coil through the tube, insulated from the foil. Place a small amount of hydrochloric acid in one flask and some ammonium hydroxide in a second flask. With glass tubing connect the flasks and large glass tubing in train. Blow air into the first flask causing ammonium chloride to be forced into the Cottrell precipitator. Activate the spark coil and see 'smoke' consumed. It works near instantly on cigarette smoke. [HG]

CRUSHED CAN. The force of normal external air pressure is sufficient to collapse a rectangular varnish can. In a clean can place a few tablespoons of water and bring it to a boil to expel the air with the water vapor. Close the cap tightly as soon as water boils vigorously. Cool the can by dashing cold water on it. Two comments: You don't need to run cold water over the can. Be sure to stopper the can very soon after you remove the source of heat. If you don't, the reverse effect happens. [HG]

DENSITY OF ICE. Some properties of water make interesting conversation pieces. Demonstrate that ice is lighter than water by placing large icicle in milk bottle (ice cubes may be used). Add cold water to fill jar while holding ice under the water. Let ice float and observe how much water overflows as the ice melts. [HG]

DIMPLES AND PIMPLES.[Use safety glasses and protect nearby students with a shatterproof glass shield between them and the experiment.] Heat a spot on a cold light bulb with a blow torch and a dimple will form in the glass. Light the bulb and again heat a spot until a pimple forms. [HG]

DISAPPEARING GLASS. Some liquids have an index of refraction very close to that of glass. When a glass object is lowered into the liquid, it almost disappears. For some materials the dispersion of the liquid won't match that of the glass, so a faint rainbow image of the object still can be seen.

For safety and good effect, the best combination I've found is Pyrex (TM) glass in Wesson canola oil. The oil is nearly clear; just slightly yellowish. Small Pyrex beakers immersed in it show only a faint outline, but, of course, the frosted label remains visible, like the grin of the Chesshire Cat. A diligent search of the lab storeroom may turn up some pyrex glassware without labels.

Clean the glass thoroughly to remove any surface film.

For the adventurous who'd like to use other materials, here's some information from various sources. Some readily available liquids already are near-matches for the refractive index of crown glass, which is 1.5170 for the sodium D line. Transparent solids

1. Borosilicate crown glass, n = 1.5170
2. Spectacle crown glass, n = 1.5230
3. Fused quartz, n = 1.4585
4. Pyrex glass, n = 1.48
5. Fluorite, n = 1.4338

Transparent liquids

1. Benzene (Benzol) and Carbon tetrachloride (1:4). This is the best for use with crown glass,but both ingredients are carcinogenic.
2. Anisole (expensive), n = 1.5179
3. Toluene (1.4961) and Methanol (1.3288). I don't know the ratio.
4. Glycerine, n = 1.47
5. Cedar oil.
6. Johnson's Baby Oil.
7. Castor oil, n = 1.48
8. Olive oil (unfortunately it's yellow)
9. Corn oil, or other vegetable cooking oils.
10. Xylene, n = 1.5055
11. m-Xylene, n = 1.4972
12. p-Xylene, n = 1.4958

Also consider these combinations:

1. Quartz (1.46) in glycerine (1.47).
2. Lucite (1.47) in glycerine (1.47).

Some mixtures may be concocted by trial and error. Be careful, because some of the ingredients are carcinogenic. If you use those, keep them in sealed bottles and avoid skin contact or breathing the vapors. The index of the mixture is not simply the average of the indices of the components.

Variations:

Lower a glass stirring rod into the liquid. The portion below the surface disappears. Lower the glass end of an eye-dropper (crown glass) into a liquid of the same refractive index. The glass disappears, but the image of the air inside punches into the liquid. Draw up some liquid. Remove the dropper. Squeeze out drops, and it seems if they are drops of glass from the end of the dropper.

Keep a small beaker in a stoppered bottle of the liquid. The beaker vanishes, but its frosted label won't. Tilt the bottle so the beaker is partly exposed to reveal that it's really there.

Let a coin rest on a small submerged transparent solid object. The coin seems to be suspended in the liquid.

DROPPING THINGS. Here's a simple scenario to convince students of the importance of words and of specifying things completely and precisely. Hold up a book and a sheet of paper. Drop them simultaneously from the same height. Note the behavior of the paper. Try dropping them with the paper dropped edge down. The book wins each time.

Hold the book in one hand, paper in the other, hands wide apart. "I claim that I am clever and skillful enough that I can drop both of these from the same height simultaneously, yet the paper will reach the floor first." Students are probably doubtful of this claim. Place the paper underneath the book and drop it. "Not fair!" say the students.

"OK, this time I promise not to put the paper under the book. Yet I claim that when the book hits the floor, the paper will be within 1 cm from reaching the table." This time you put the paper in the book, between the back cover and the pages.

"Still not fair, you say? This time I promise not to put the paper in the book or under it, yet I claim the paper and the book will fall with the same acceleration and have the same velocity at every instant." You smoothly press the paper on the upper cover of the book, then drop them.

Finally: "This time I promise to drop them simultaneously from the same height, one in each hand, so the paper never touches the book." After they agree that this sounds fair, crumple the paper and wad it into a tight ball before dropping. They land simultaneously.

DRY WATER. Let the student propose explanations why one can pick a coin from the bottom of a beaker of water which has been dusted with Lycopodium powder and not wet a finger. The beaker was filled with water and the powder dusted over the water surface. [HG] Lycopodium powder is the pollen of the Lycopodium tree. Frankly I don't have an explanation off the top of my head, and haven't encountered one in the literature. [DES]

EGG PHYSICS Here's some eggcellent demos with eggs, which we used to do when we were children, and so did our parents and grandparents. Some of these appeared in the Chicago Sunday Times, March 24, 1940, collected and presented by Martin Gardner.

Eggs, fresh or old, sink in cold water, at least in my experience. But they are so near the density of water that dissolving sufficient salt (several teaspoonsful to a glass) in the water will make them float. In fact, you can get a tall glass cylinder, put some salt water in it and float the egg on the salt water, then carefully add plain water so it doesn't mix with the salt water, and the egg seems to float halfway between the surface and the bottom. I'm not talking about cooking the egg here—that's another issue.

As-yet untested proposal. Since an egg is so close to the density of water, how about using it in an oversize cartesian diver, visible from the back row! You might have to salt the water a bit to make it barely float. Cap the container, and squeeze its sides.

An egg in boiling water often rises to the surface because of convection currents in the water. But why does it stay at the top? For the same reason that large pebbles in a container of small ones rise to the top and stay there when the container is shaken. The pebbles develop convection currents too.

Balance an egg on end. You can do this at the dinner table on a tablecloth, or on a smooth table surface. Surreptitiously make a small pile of salt. Balance the egg on it, then blow the loose grains away. Only the grains wedged under the egg (which are the only ones supporting it) will remain, and not be noticed.

Second method. Shake the egg forcefully to break the yolk. Then the egg can be balanced on its broad end because the heavier globs of yolk sink to the bottom, lowering the center of gravity. This can prompt a discussion of the center of gravity relative to the center of curvature and the conditions for equilibrium.

To balance a hard-boiled egg on end, gently crush the shell at the end and sit it there.

To tell a boiled egg from a fresh one, spin it on its side. The hardboiled eggs spin longer. The fresh ones lose energy from viscous drag. But a fresh one will spin longer when spun on one end around its long axis than it will when spun on its side. Why?

Spin a fresh egg on the table. Stop its rotation momentarily with your finger, then quickly release it. It resumes spinning for a bit. When you stop the shell abruptly, the yolk and white continue to rotate. If you release the shell before that rotation stops, viscous drag pulls the shell along with the yolk and white.

Most people cannot break an egg by clasping it between their palms, with the two ends touching the palms. Try this with one of the football players or wrestlers in your class. If they fail, they will challenge you to do it. And you can. How?

Coat an egg with soot from a burning candle. Immersed in a glass of water it looks silver.

A hardboiled, peeled, egg can be forced into a milk bottle by the usual physics trickery. Burn a wad of paper in the bottle. When the flame is established strongly, quickly place the egg in the bottle's mouth. It helps to moisten the egg first. As the air inside is heated it expands, forcing air out around the egg. Then as the air cools inside, the pressure drops inside the bottle, and the egg is pushed inside by the greater air pressure outside. The egg can be removed by turning the bottle upside down so the egg blocks the neck. Then close your lips around the bottle's mouth and blow into it, creating enough pressure to force the egg out. It pops out surprisingly quickly, so be ready to catch it. Note: Avoid any explanation you may have seen in books which involves the flame "using up oxygen" in the bottle, for such processes have negligible effect on the pressure in the bottle.

Another as-yet untested proposal. How about an unpeeled egg in a bottle? Soak an uncooked egg in strong vinegar until its shell softens. Then the egg may be sucked into the bottle by the method previously described. Now flood the egg with cold water till the shell hardens. Pour out the water and let everything dry. The egg in the bottle now seems impossible. I have not tried hardboiling the egg first, or boiling it after it is in the bottle.

Wrap a string around the egg. Try to burn the string with match or candle flame. The string won't burn because the thermal energy of the flame is absorbed by the shell and the string can't reach kindling temperature.

An egg can be tossed into the air and caught on a plate without breaking. It takes practice. Martin Gardner says that performer Bill Talent used this in his juggling act. He'd put an egg on the floor between his heels, jump with both feet, propelling egg into the air behind his back, then catch it on a plate held in one hand.

Of course, eggs are used in the so-called inertia demos. Fill four glasses with water. Put them under the four corners of a flat tray with raised edges. Roll up four playing-cards and secure them with rubber bands. Use them as supports for four eggs positioned directly over each glass. Knock out the tray with a sharp blow of the palm. Eggs will fall into the glasses. Water can be put in the glasses if you don't want the eggs scrambled, but the water will splash out on the table. The smashed eggs won't. (You hope!)

Write a message on the eggshell with a solution of one ounce alum to one pint vinegar. The writing is invisible, but penetrates the shell. Boil the egg. The writing still isn't visible, but when the egg is peeled, the message is seen on the solid egg white.

No matter how hard you throw a raw egg into a curtain or suspended bed sheet the egg will not crack open. Because the sheet "gives" as the egg makes contact, the deceleration time for the egg to change its velocity to zero is fairly long. Therefore the sheet applies a large stopping impulse (force x time) with a very small force and a long time. However, if the uncracked raw egg falls out of the sheet and hits the floor, it comes to a quick stop with a large force and... An omelet or scrambled eggs anyone? [HG]

This one is more of a trick than a demo. One must be careful about doing tricks in a class setting. This one can be used in this manner: "Here's a demo which seems to defy physics. But of course nothing violates correct physics. Sure it's a trick, but just like magician's tricks, there's a physical explanation. So what is a physical mechanism which would make this happen? What's your model of how it's done?"

First the preparation. Blow an egg. You all know how to do that, don't you? If not, here's the method. Shake it to scramble the contents. Put a needle hole in each end and blow into one of these to force all the contents out the other end so you have an empty shell. Leave it a few days to let it dry inside. Now that you know how to do it, blow another egg, for use later.

Actually, for these demos, I think it better not to make the holes in the ends of the egg, but elsewhere.

Close one hole in the egg with wax or white glue. Enlarge the other hole if necessary to allow fine sand to be put inside the shell. Close the hole with wax or white glue. This "egg" will now balance on either end or in any other cockeyed position. Use your ingenuity and balance it precariously on things. This is the "obedient" egg, for it balances any way you want it to.

The other egg you have prepared similarly. It is to be the disobedient egg, which will balance only one way. Decide, before you blow it, how you want it to balance. Perhaps on the small end? Then be sure to make the needle holes well-removed from the point of intended balance. This time mix finely shaved candle wax with the sand and put some inside the egg. Close both holes with glue. Let dry. Heat the egg, point down, till the wax melts and the wax-sand glob is adhering to the inside of the egg-shell. Let it cool, and it balances on its tip. Even worse, if you try to topple it, it comes right back to its original balance position, just like those little clown toys. A very contrary, disobedient egg.

Properly presented, it can eggsasperate students who try to figure out what's going on. Practice till you are eggspert, and you'll get eggcellent response from students.

If you don't want to bother with real eggs, consider using the colored plastic eggs seen in stores around Easter-time, or the old L'Eggs (TM) containers for panty-hose, or the small eggs silly-putty (TM) is sold in. Weights can be glued at appropriate places inside.

Don't omit a mathematical discussion of how the center of mass must relate to the radius of curvature at the table-top to ensure that the egg won't roll to a different position. This is a good demo for illustrating conditions of stability.

ELECTROSTATIC INFLUENCE I. Charge a glass rod by rubbing it with silk and bring it near the thinstream of water from a faucet. The stream will be deflected toward the charged object. Charge a rubber or plastic rod by rubbing it with wool or fur, and the water will also be drawn toward the charged object.

Presentation:

1. Always do both cases, with external charged objects of different sign of charge.
2. This demonstration shows that something charge-related is going on, and the water itself must contain charges.
3. It does not, however, show that the molecules are charged, or that they have a net charge. If they were, you'd expect attraction in one case, repulsion in the other.
4. The molecules are polar; they have a permanent dipole moment. In the external field they align themselves in that field, + ends in one direction along the field, - ends in the other direction.
5. They why is there an attraction? Because the field is nonuniform. The field exerts a stronger force on the molecule end near the external charged body than it does on the molecule end slightly farther away.
6. This should be followed by discussion, with diagrams, showing the forces and demonstrating that in either case the net force is toward the stronger part of the field.

As always, be sure to sweat the details to get the maximum instruction from these simple demonstrations. [DES]

ELECTROSTATIC INFLUENCE II. A charged comb attracts small scraps of paper, or a piece of string or hair. Can it also affect a paper match delicately balanced on a coin inside a glass? How does the electric influence pass through the insulating glass? Touch the charged comb to the glass to find out. [DES]

EFFECT OF GAS DENSITY ON SOUND. Fill several balloons with different gases such as air, carbon dioxide, natural gas, helium, and propane to about the same pressure.Fix a whistle to be blown to a short piece of glass tubing.Note the pitch as gas from the different balloons blows the whistle. [HG]

ELEMENTARY BATTERY. Show the emf produced between solutions of different concentrations by using two copper discs attached to insulated wire and suspended in a dilute copper sulfate solution, then drop a few crystals of copper sulfate in to make the bottom layer more concentrated. Connect the electrodes to a sensitive milliammeter or galvanometer. [HG]

ENERGY CONSERVATION. Suspend a bowling ball with a strong cord from the ceiling, Draw the ball back against your nose, with your head against the wall. Release it and stand nonchalantly awaiting its return. It cannot rise to greater than height from which it started. You are safe if you do not move or push the ball during its release. [HG] Experienced (and daring) demonstrators do this with the back of their head touching the classroom wall.

FALLING LEAKY BUCKET. A large Styrofoam (TM) cup is filled with water. Punch a small hole in the side near the bottom and a stream of water shoots out sidewise. Plug the hole with your finger before much has come out. Why does the water shoot out this way? Because the weight of the water above the hole causes the pressure just inside the hole to be above atmospheric pressure, and the water near the hole moves from high pressure to low pressure. What would happen if the cup were dropped from a considerable height? In the cup's frame of reference it and everything else are accelerating at the same rate and are (in this frame) weightless, so the pressure everywhere in the cup drops to atmospheric pressure. Then there's no pressure difference between the inside and outside of the hole, and no water shoots out. It's best to this outdoors, from and upper floor window, and particularly entertaining when a Dean or department chairman is walking beneath. If done indoors, from a balcony or high ladder, have it fall into a plastic wastebasket. [DES]

Richard DeLombard suggests a very nice extension of this demo. First, he prefers a water bottle with a hole punched in it, being easier to handle than a styrofoam cup. Do not cap the bottle. Now ask students what would happen if the bottle were carefully tossed upward without spin or tumbling? After it leaves your hand, will the water come out the hole? If not, what will happen, and why? This is best done outdoors. [Answer]

FINGER FRINGES. Observe a line-filament lamp through the space between two fingers, the fingers being parallel to the filament. Vary the width of the space until you see a pattern of fine lines between the fingers. Such line-filament lamps are about six inches long, have a single, essentially straight filament and are unfrosted. They are sold for use in store display cases. This demo can also be made to work by looking at a long fluorescent lamp from across the room. A backlighted hand may look as if you are seeing an X-ray of the bones of the hand. Observe objects through a bird's feather.

Here's the puzzle. What are these fringes? Diffraction patterns? Very likely they aren't. If they were diffraction patterns they would show spectral colors. Too many "gee whiz" science demo books for children tell you that they are diffraction patterns. They lie. [Answer]

FISSION BUBBLE. Activation of a nucleus to cause it to fission may be simulated by catching a soap bubble between two wire rings with handles. When caught, puncture the top and bottom areas leaving a cylinder between the rings. Carefully pull the rings apart, noticing the shape of the film, until it breaks in two films over each circle. [HG]

FLAME DISCHARGE. Ionization in a flame can be shown by holding a lighted match near a charged electroscope. Charged pith balls or balloons lose their charge rapidly when a flame is brought near. [HG]

FOOL'S TACKLE. [DES] The figure shows a pulley arrangement called the "Fool's Tackle", for only a fool would expect it to work. However, if the diagram is shown to students (without the forces included) you can ask them what force would be required to lift the load from the table: W, W/2, W/3, W/4, where W is the weight of the load.

The answer is "none of these". The upper movable pulley drops till it hits the bottom one. The forces don't add up. This illustrates the fact that you can draw something on a blackboard which can't work in nature. It also reinforces the advice we give to students in lab: Do the analysis using known physical principles and make a prediction before undertaking the experiment. I've seen students struggle for a half hour to make this system work before they ask for help.

In case you were wondering, the whole system can be inverted, with the free end pulling up. It doesn't work any better.

Some of us sneaky types like to treat this as a deception. We have the apparatus set up with the load resting on the table. Then we pull on the free end and the load rises. Best to do this after you've convinced the students that it can't possibly work. Of course, you have pinned or otherwise gimmicked the upper pulley so it can't turn. If the string slips on that pulley, coat it with rubber cement.

GAMES. [HG] Games can make both learning and instruction a pleasure. Build a puzzle of jumbled letters for other students to solve.An example:

CTVREO — has magnitude and direction
DSEPE — magnitude portion of velocity
OHNTPO — elementary light wave

GROWING SILVER CRYSTALS. Place a copper penny on the glass slide of a micro-projector. Put silver nitrate solution around the penny and watch silver crystals form on screen. Note the many peculiar characteristics they exhibit. [HG]

GYROSCOPE, LARGE. Weld bicycle axle nuts into one end of each of two iron pipes. Screw the pipes onto the wheel axle for handles. This makes an excellent gyroscope. It's even better when the rim is weighted by winding it with iron wire. Or fill the tire with sand for greater weight. [Be very careful not to get your fingers in the whirling spokes, for the weighted wheel has considerable inertia]. [HG]

GYROSCOPE BAT CIRCLES. While standing or sitting on a rotating platform, turn around by swinging a baseball bat in circles over your head. Reversal of the swing reverses the motion of your body. [HG]

[In this, and the following demos using a rotating platform, consider the policy of your school before letting students do them. It's not unreasonable to have wrestling-mat pads surrounding the platform, and have the platform surface padded as well. People become dizzy and have been known to fall off the seat or platform.]

GYROSCOPE HOME RUN SWING. Show action and reaction by standing on the rotating platform and swinging a baseball bat vigorously at a pitched ball. This should be amusing. Do it outside, of course. [HG]

GYROSCOPE MOMENTS. Again on the rotating platform, pirouette. Hold heavy weights at arm's length, have someone rotate you slowly. When you are "up to speed" bring the weights close to body. Explain the marked increase in speed. [HG] [The speed increase can be great enough to disorient an inexperienced person, causing them to lose balance and fall off the platform.]

GYROSCOPE PLATFORM. Construct a rotating platform from an automobile front-wheel and spindle. Rigidness, coupled with small friction and small play in the bearings is amazing. This is useful to demonstrate rotational inertia and maneuvering in space. [HG] There's great value in having each student do this to feel the direction of the reaction torques. If this isn't practical, the demonstrator should describe in detail what direction the wheel seems to "want to move" as one tries to make the wheel move in a particular direction.

GYROSCOPE PRECESSION. Stand on a rotating platform holding a spinning gyroscope wheel with its axis horizontal. Observe what happens when the axis is rotated to a perpendicular position to the right? To the left? [HG]

HAIR RADIO TRANSMITTER. Combing dry hair near the aerial of an AM radio produces static. [HG] You could call the static "long-hair" music, but that joke is lost on younger people.

HEAT TREATMENT. The effect of heat treatment and tempering of metals can be demonstrated by heating bobby pins to redness in a Bunsen flame. Dip one heated pin in cold water to chill it. Allow the other pin to cool slowly. Compare these pins with one that has not been heated by bending each one. [HG]

HEAVY FINGER. [DES] This is an old demonstration, but I think it is still worth doing and deserves special attention to its presentation. Use an old-fashioned double pan balance. Don't be tempted to use an electronic balance, for the students must be able to see the changes. A beaker of water is on one pan, balanced by weights on the other pan. Propose this question to the audience: "Suppose I were to stick my finger down into this water, being very careful not to touch the sides or bottom of the beaker, what will happen?" If there's any hesitation, or confusion say, "It's really an easy question! What will happen is that my finger will get wet."

"But that's obviously not what I meant. Let me phrase the question more carefully. Will the pan with the beaker move down, up, or would the scales remain balanced when I insert a finger into the water?"

During the discussion, demonstrate that if you press a finger on that pan, it goes down. If you press on the edge of the beaker, it goes down. "But if I'm careful to touch only the water, not the beaker, what happens?" Refuse to actually do it till serious discussion ensues.

Finally, do it. The pan and beaker move down. "Why?" you ask?" Don't peek at my answers until you've come up with at least two instructive answers of your own. [Answer]

HIGH ROAD AND LOW ROAD. [BERG] Two balls are propelled with equal velocity on two tracks. The tracks start out on the same level, but one has a smooth drop to lower level, then a rise to higher level. The total length is over a meter. Some care must be taken to ensure equal firing speeds.

An alternate version uses two incline ramps at the left to achieve equal velocities of the balls. I made one 5.5 feet long of steel construction set parts (Meccano, Erector, Steel-Tech, Temsi, etc.). It includes two parallel 45° 10 inch long inclines at the start of the track to give the balls equal initial speed. The lower level portion of the track is two feet long, and is 5 inches lower than the upper track. The smooth, curved rise is about a foot long horizontally.

Construction set model. The balls are posing at rest for the photo.

Which ball wins the race? The balls gain kinetic energy on the first incline, the one with the longer incline gains an additional kinetic energy of mgh compared to the other ball. h is the height difference between the horizontal portions of track. So the lower ball is ahead of the upper ball on the horizontal tracks, and having higher speed, continues to gain its distance lead over the upper ball. On the final rise, the lower ball loses as much kinetic energy (mgh) as it had gained early on, so on exit they have the same kinetic energy and the same speed. But the distance advantage the lower ball gained going at higher speed on the straight track ensures that it wins the race.

This is a version of the famous "Brachistochrone Problem" first solved by Johann Bernoulli in 1696. It is discussed in most intermediate mechanics books. The problem was to determine what curve of incline will get an object from point A to point B in the least time, when A and B are at different heights. The curve of least time is a cycloid. Such problems led to the development of the calculus of variations. [Answer]

"HOLEY" WATER. Do molecules of water have spaces between them? Pour water into a long test tube or graduate until it is three-fourths full. Then completely fill it to capacity with alcohol. Place your palm over the top of the container and invert it. Be careful that no liquid is lost as the water and alcohol mix. Observe that the container is no longer full. Evidently some alcohol has disappeared in water molecule holes. [HG]

HOT DOG WHISTLE. Tune two metal dog whistles to unison or absence of beats. Heat one whistle with a flame. Beats reappear as the pitch of the heated whistle rises. [Please don't burn your lips!] [HG]

HOT ROD BALANCE. Drill a brass rod for a screw in one end. Insert a screw about half way. Balance the rod at its center on a pivot. Throw off the balance by moving the small screw out. Heat the other end of the rod and it comes into balance again. [HG]

HYDROSTATIC SCALE. Weigh yourself by hydrostatic pressure. Use a hot water bottle with a stopper fitted with about two meters of rubber and glass tubing. Fill the bottle with water and connect the tubing so that it extends vertically. Lay the bottle on the floor and cover most of it with a small board of known area. Stand on the board and measure the increased height of the water in the tube. Your weight is equal to the area of the board times the water pressure increase. Calculate the water pressure by multiplying the density of water (1 gram per cubic centimeter) by the difference in the water level height when you stand on the scale. [HG]

HYDROSTATIC PUZZLE. [MG] A cork or wood ball floats on water. Add a layer of oil to the water surface. Does the floating ball rise, or sink, when the oil is added? Answer: It rises. Why?

INERTIA (BALL IN CUP). Attach a ball to the bottom of a cup with an elastic band. When the cup is dropped, what will happen to the ball? Will it stay outside the cup? Will it jump inside the cup?

Answer: It jumps inside the cup. In free fall, in the cup's frame of reference, the ball is weightless, and it was its weight (the force of gravity upon it) which was countering the upward tension of the rubber band when the cup was at rest (as shown in the diagram). As the cup falls, the tension force, unopposed, pulls the ball up and over the rim into the can. [BERG]

INTERRUPTED PENDULUM. Show conservation of energy in a swinging pendulum by noting that the bob returns to the same level each time. Place a peg or obstruction below the point of suspension so that the arc of swing will be changed to a shorter radius. Locate the peg at a point one half the distance between the lowest and highest levels of the bob and then do it again with the peg still lower. Explain why the bob loops over the peg. [HG]

INVISIBLE THWACK. [MG] Bend a playing card and stand it upright. Stretch a rubber band on another playing card perpendicular to it. The two cards are about a centimeter apart. The far end of the band is pulled back and released. The standing card is knocked over, even though you can't see that anything touched it. The front end of the rubber band actually moves forward, breaking contact with the card. But when does this happen, and why?

Presentation: A more durable apparatus consists of two large nails in a board, the rubber band being stretched over them, and a card of any sort set upright a short distance (about 1 cm) beyond one of the nails. First snap the band without the card in place. Ask students to describe the motion of the band in detail. They usually will not suggest that the front of the band ever breaks contact with the nail. Raise the issue. Then do the card demo to confirm your hypothesis. Some may still doubt that the band hit the card, thinking perhaps that you jiggled the table or the board, or even blew on the card to knock it over. If they don't suggest this, suggest it yourself. "How can we test the hypothesis that the band knocked the card over, when our eyes can't see it do that?" Substitute a sense which isn't as easily fooled as the eye. Have a student put a finger where the card was, to feel the band hit the finger.

One answer is commonly seen and seems superficially plausible: When the band is stretched, the front nail (B) exerts a force on it, but your hand balances that force. When you release the band, there's an unbalanced force of the front nail (B) forward on the front end of the band. This gives a forward impulse. The momentum of the band is forward, and its center of mass moves forward faster than the band can relax to its unstretched position.

I must admit that I was taken in by this bogus answer. I had forgotten that it takes some time for the front end of the band to "know" that the other end has been released. That is, it takes some time for any physical effect of the release to reach the front end.

But the real answer is more interesting. Loren Winters and Travis Williams at the North Carolina School of Science and Mathematics have taken high speed photos of the band in action. These may be seen at thisprojects page, along with other fascinating pictures of physical phenomena.

They intend to do more investigation. Perhaps they will answer this question: Does the forward motion from nail (B) begin only after the band is fully relaxed or the moving end hits the nail (A), or the compressional wave from end (A) has reached (B)? What if nail (A) weren't there?

This illustrates the principle that you should never give up thinking about a problem just because you've gotten what you think is a plausible answer. Even if you have a correct answer, there are always other ways to arrive at it, and some of these may give you more insight.

The explanation of the SLINKY DROP demo is related to this. Both are examples of a general principle. Changes in one part of a system cannot affect another part of the system until some physical influence passes from one part to the other. Such influences do not travel instantaneously. One common source of mistakes in doing mechanics problems is to assume (without thinking about it) that when the force is removed from one end of the rubber band or spring that the other end "knows" about it instantly.

IMPULSE AND INERTIA MAGIC. Done with graceful flourish, this brings down the house! The mechanics of friction, forces and inertia involved stimulate interesting discussion. Set a glass two-thirds full of water about three inches from the edge of a table. On the glass place a pie tin. On the pie tin and directly over the glass place a spool on end. Place an egg (fresh if you are confident) on the spool. With one foot on the bristles of a springy broom, pull back the handle and aim at the pie tin. The spool rolls on the table, the pie tin scoots to the floor, the glass and the water remain unmoved on the table with the egg unharmed in the water.Note: The pie tin overhangs the edge of the table. The table edge stops the broom's forward motion before it hits anything else. [HG]

The wood block must be at least as high as the edge of the pie tin.

Variations. Only your imagination limits the possible variations you might try. With practice, you can snatch the pie tin quickly enough with your hand, using a smooth arm motion. Don't hesitate.

JERK. Two heavy balls are suspended one above the other by strings. When you give a steady pull on the lower string, C, which string will break? [Caution: your hand must get out of the way quickly so it won't be hit by a heavy falling ball. Some demonstrators fasten the lower string to the center of a wooden dowel, then use both hands on the dowel ends to jerk it down.]

Which string will break when you jerk the lower string? [BERG]

This is a variant of the demonstration using only one ball and two strings, say the top ball and strings A and B in the diagram. When B is pulled slowly, A breaks, because the tension in A is larger than that in B by an amount equal to the weight of the ball. When B is jerked its tension rises almost immediately to the breaking point, while the tension in A rises more slowly, since it takes a short time for the heavy ball to move and stretch string A enough to reach its breaking point. The reason for the time delay is that in F=maapplied to that mass, m is large, so a is small.

To really understand this demonstration one must remember that a string doesn't break until it is stretched (elongated) to its breaking point.

In these "inertia" demos involving jerks (including the tablecloth yanking demo) one must consider the rapidity of the motion, and the fact that the duration of the force can be smaller than the time it takes to move something enough to cause a particular outcome. [DES]

Also see: Ehrlich's comments [Ehrlich, Toast] p. 16, and Arons' comments on so-called "inertia demonstrations". [Arons] sec. 3.22. Arons concludes that "Without visualization of the stretching of the strings, students acquire no understanding of the demonstration; they simply memorize, and repeat, that it had something to do with 'inertia.'" [<]

JERKS AND YANKS. [DES, MG] Quite a number of demonstrations depend upon impulses of brief duration. The classic demo of this type is the one in which a tablecloth is yanked from under a full table setting, plate, silverware, glasses of water, etc. without toppling or spilling anything and without moving the table setting to a significant velocity. Use a silk or smooth rayon cloth, and be sure that the trailing edge has no hem. It helps to roll up the edge you are pulling all the way to the edge of the table before yanking it, helping to ensure that the cloth doesn't "bunch up" anywhere during the yank.

Of course each object does move a bit. They move during the brief time while the tablecloth slides from under them. Then, this motion continues as they are brought to a stop by friction against the tabletop. It is important that both the cloth and the tabletop be smooth and low-friction. If the table surface slows them too quickly, tall objects like glasses and candlesticks can topple forward.

Silverware can cause problems in this demo, and is best avoided.

Often this demo is passed off with the explanation. "The objects are not disturbed because of their inertia." This explains nothing. When the tablecloth is pulled slowly, everything moves along with it. If it is pulled more rapidly, the wine glass topples. If it is pulled (yanked) very quickly, nothing is seriously disturbed. Why?

The force an object experiences is that of friction between it and the cloth, and that friction is proportional to the normal force (equal to the object's weight) and the coefficient of friction. The coefficients of static and sliding friction aren't different enough in size to account for the outcome of this experiment.

The impulse given by the cloth to an object on it is Ft, where F is the tension in the cloth and t is the duration of application of the force. The impulse changes the momentum of the objects on the cloth. F depends on the friction force, and this is nearly independent of the speed of movement of the cloth. Since the friction is proportional to the normal force, which is constant in all cases, the impulse depends only on the time, and if the time is short enough the impulse is small. Therefore it is the impulse which is the key to the understanding of these experiments.

One can also argue that the cloth is removed in a time shorter than that required for the objects to accelerate much. This is really the same as the explanation of the previous paragraph. But one must still emphasize the importance of the fact that the normal force is independent of the horizontal force and also independent of the time of application of the horizontal force. As always, in elementary mechanics courses, draw the free body diagrams as you talk.

See: Ehrlich's comments [Ehrlich, Toast] p. 16. [<]

These impulse demos can also be done using a strip of paper and one object, if you haven't yet acquired a table setting and a silk or rayon cloth. An 8.5 x 11 inch piece of paper will do. Place it under a beaker or glass of water. Pull slowly. The glass moves along with the paper. Pull it nearly to the edge of the table. With the glass in this precipitous position, say "I will now pull the paper from under the beaker without spilling any water." (Say "pull" not "yank".) Suspense! Then yank the paper. Follow up by using an empty plastic glass and show that if the paper isn't pulled quickly enough, the glass will topple. Practice first.

Other objects may be used: a lighted candle, a smooth-edged coin balanced on its edge, a smooth wooden ball. Try the coin trick with the coin in a plane perpendicular to the yank, and also parallel to the yank. For a brisker yank, hold the paper out horizontally from the table and do a downward karate chop on it with your finger (Fig. d).

A little more suspense. Balance that glass of water, on the paper, on your head. Then yank the paper. Practice, using a plastic tumbler. You have to hold your body still while doing the yank. Easier: do this on your palm, with a dollar bill under the glass. Yank out the bill. (Dinner-table entertainment!)

I have three brass cylinders of different weights and sizes, the largest being an inch in diameter. I set them on the paper. "Big inertia, medium inertia and small inertia" I say. They are so "touchy" that the slightest motion of the paper makes them roll. Then I do the karate chop move to yank the paper from under them, and none of them even jiggle. So what does this teach us about inertia? Nothing.

So, do any of these "demonstrate inertia"? All objects on the cloth or paper, whatever their mass, seem to behave the same. Even a lightweight folded napkin in the table setting behaves the same. So these aren't cleardemonstrations of the property of inertia. But they are nice demonstrations of impulse.

As one student observed, when asked to explain this, "If you apply a force quickly enough to something, it doesn't notice." At least that's a better description of what happened than "It was because of inertia."

LENZ'S LAW. Lenz's Law may be demonstrated with any toy wheel of nonmagnetic material and low friction attached to a convenient holder. The wheel should have spokes for clearest understanding. Spin the wheel in air then between the poles of a reasonably strong horseshoe magnet. Spokes cut lines of force, the induced current field opposes motion. [HG]

LOCATING THE CENTER OF GRAVITY. Start with your hands outstretched and palms facing each other about a meter apart. Rest a horizontal stick, cane or metal pipe on the index fingers of each hand. With your eyes blindfolded, slowly move your hands together until the palms meet. Regardless of the starting position of your hands, the center of gravity of the stick will be at the point where your hands come together. [HG]

A meter stick is good for this demo. One can even tape a weight at some point on the stick and it still works. A two-dimensional version: Use a large dinner plate, with objects on it, perhaps even a glass of water. Use three fingers spread wide to support it. Bring the fingers together! Your fingertips should be dry. [DES]

A mathematical analysis of this can be found in Arnold Sommerfeld's Lectures on Theoretical Physics, Volume 1, Mechanics Academic Press, 1964, pp. 83-85. It's a good exercise in forces, torques, and friction.

MAGNETIC WAVES. Suspend a bar magnet on a string. Rotate another magnet under it to show transfer of magnetic energy. What changes the direction of the poles? How can the change be effected without human movement. [HG]

MASS SPECTROGRAPH. Properties of alpha, beta, and gamma rays may be simulated by propping a smooth board of about eight inches by twelve inches on an incline and arranging a bin with a trap gate at the top so that three different sized balls can be released to roll down the board. Place a strong magnet below the board and just to one side of the gate. Note how each falling ball goes into a separate bin because of the amount of deflection. The gamma may be represented by a brass or aluminum ball, he beta would be the smaller of the steel balls. [HG]

MATCH DISCHARGE. Rubber bands or strips can be tied together in bundles and charged by stroking with fur or by other means.A lighted match near the repelling strands will cause them to collapse. [HG]

MATCH HEAD DIVER. A Cartesian diver can be made with a Coca-Cola bottle full of water and a match head. Continue cutting or breaking off the match stick until the head barely floats. Thumb pressure on the mouth of the bottle makes these little divers zip up and down in the bottle. [HG] Experiment with glass bottles with secure plastic caps. Large medicine bottles with flat sides can be pressed on the sides to make this work, since glass is slightly flexible. There's no surprise in doing this with plastic bottles. Presentation note: Since so many plastic bottles are made to look like glass ones, tap the bottle on the table to let the audience hear that it is glass.

MIRROR IMAGE REVERSED? Why does a plane mirror seem to reverse your image left-right but not up-down? This frequently-asked question has value for encouraging students to think more precisely and use words carefully. Why should this question even be interesting? Where's the "problem"? The discussion shouldn't be carried out entirely in the abstract. Have a large mirror to demonstrate. Rotate the mirror around its normal as a student looks at the image. The image doesn't rotate. The mirror seems to be operating with axial symmetry. Then why should the mirror treat up/down differently than right/left? (Some students will even suggest gravity has something to do with it, so have such students look in the mirror and lean their heads sidewise.) Have one student look into the mirror while another stands behind the mirror looking at the first student. Have each touch the top of their heads. Have each touch their right ear. Perhaps have each wear something distinctive on the right ear.

The figure shows that a pair of plane mirrors at right angles behaves differently than does a single plane mirror. Have two mirrors hinged (for ease of storage) with a precise arrangement to hold them at perfect right angle alignment for the demo. Curved cylindrical mirrors can be made by forcing plastic mirrors into a slight curve. This may not seem like physics, but it is certainly a good exercise in three-dimensional thinking.

NUT DROP. Tie six or seven metal nuts on a string at distances in proportion to (1/2) gt² where the time is 1, 2, 3, 4, etc. seconds. Hold the string vertical and still and then let it drop. Note there is no difference in the time intervals as nuts strike the floor. [HG] To get full value from this demonstration, have another string with nuts tied on it with equal spacings. Drop it first, to demonstrate that the sound of the impacts with the floor do not occur at equal time intervals. Here's a conclusive test of the Aristotelian vs. Galilean laws of motion; a test accessible directly to the senses without the need for timing devices. In a large room it's worth putting two pulleys near the ceiling to hold the strings of nuts until they are released. Or use light threads over supports near the ceiling to suspend the two strings of nuts, then cut the supporting threads. [DES]

OSCILLATING BEAM. [K&K] A heavy uniform bar or beam rests on top of two identical rollers which are continuously turned in opposite directions, as shown. There's friction between the rollers and the bar, and it's constant, independent of the relative speed of the surfaces. Find the motion of the bar. [Answer]

Follow-up question: What happens if the rotation direction of both wheels are reversed?

This homework problem from Kleppner and Kolenkow may be demonstrated. I made a model from Erector set parts. The rollers are 1 inch pulleys with thick rubber o-ring tires. The rollers are about 6 inches apart, driven by a pulley arrangement with a long rubber o-ring belt, driven by a standard Erector set motor geared down to slow speed. The beam is a 10 or 12 inch angle girder. Place the girder on the rollers so that its angle is like an upside down V: ^. When the rollers turn, the girder oscillates back and forth with simple harmonic motion, without falling off the rollers. This is just as predicted by the mathematical analysis. Sometimes the friction is a bit erratic, but the girder stubbornly refuses to fall off. I've had a model in a display case with a button to activate the motor, and many people have tried to topple the girder without success. Of course, if the motor were reversed, both rollers turn the other way, and the girder smoothly moves in one direction and falls off.

Steel construction set model of the oscillating beam.

Some students argue that sometimes, once in a blue moon, the stopping position of the girder would be just at a balance position, and when the motor is started again the girder would just sit there without moving in either direction as both wheels slid underneath it. It has never happened yet. This must be telling us something profound about how nature works.

This is related to the "locating the center of gravity" demo in which you support a meter stick with two fingers, then move the fingers together.

PAPER KETTLE. Boil water in a paper cup. The paper will not burn until the water has boiled away. [HG]

PAPER WEIGHT. The usual demonstration: Cover a thin wooden slat or shingle with a sheet of newspaper except for a few inches which project beyond the edge of the table. Hit the protruding part of the slat with a sharp downward blow of a sturdy broomstick. The slat breaks without tearing the paper. [HG]

The usual explanation: The paper has been smoothed down over table and slat. Some books simply say that the pressure of the air downward on the slat is what secures it in place on the table. If that were all there were to it, the paper would not be needed. If the paper is carelessly placed and not smoothed down, this shouldn't work, according to the usual explanation. Any correct explanation must include the fact that smoothing the paper helps prevent air from getting under the paper easily.

The paper is somewhat flexible, so when the shingle is struck, the portion under the paper rises a bit, creating a space with very low air pressure. The force on the slat is now unbalanced, with much greater force of air pressure above. If the slat breaks before air has a chance to flow into the space under the paper, the demonstration is a success.

Consider the old experiment in which two of the rubber flat sink-stoppers (about six inches diameter) are pressed together, then one attempts to pull them apart. (The poor man's Magdeberg hemisphere experiment. If you are even poorer, use one stopper on a smooth tabletop.) If the stopper surfaces are not very smooth, they pull apart easily. But if you sand them with very fine emery paper, or moisten them, they are difficult to separate. Can we say it is simply the air pressure which prevents them from separating? No. The fact that they are flexible, allowing creation of a low pressure space between them, is essential. The fact that air can't easily get between them is also essential. So what did Magdeberg demonstrate in 1654? He used iron hemispheres, and deliberately used the newly invented air pump to remove most of the air between them. Could we separate the two hemispheres (or sink-stoppers) if they were surrounded by a vacuum?

But this demo of breaking of the slat is misleading. Demonstrate it using a wooden yardstick lying on the table with a bit of overhang. Strike the overhang with your hand, and the stick goes flying up in the air. Now replace the stick and strike the overhanging portion forcefully with a broomstick. The end breaks off. No newspaper is needed! I often do this with old strips of lath of the kind used in old houses under plaster walls. This may also be playing a role in the usual demonstration.

Explanation of the paperless breaking demo: Air pressure plays no role in this version. This works best if the portion of the lath lying on the table is long compared to the overhang. The blow to the overhanging end gives an impulse of short duration. This stresses the overhanging end, as it starts to bend. The internal stresses cause a wave of stressed wood to move toward the table. When this reaches the edge of the table the wood is forced down against the table edge, and an upward reaction force from the table acts on the wood. This force and the downward force of the broomstick act as a couple, tending to rotate the portion of the yardstick extending beyond the table. The more massive portion of the yardstick has not yet received any of the stress wave, and has not yet been acted on by any forces tending to cause it to move up away from the table. Crudely, "It doesn't yet know anything has happened." The yardstick breaks before the longer portion has a chance to move.

PENCIL POINT BALANCE. To demonstrate center of gravity outside of a body, and the criterion for stability, borrow two pocket knives from students. Push the blades firmly (but carefully) into a pencil near the sharpened end with the handles beyond the point of the pencil. Balance the pencil point on your finger. Since the system's center of gravity falls below the point of balance, the system is stable. [HG] Today, students are not allowed to carry knives in schools. How times have changed! When I was in school in the 1940s and 50s, nearly every boy carried a pocket knife, for whittling, playing mumbledy peg, trimming wooden slingshots made from tree branches, and sharpening pencils. And I never heard of a case where a knife was used as a weapon. [DES]

PINHOLE EFFECTS. An interesting conversation piece can be made from an empty 35 mm film can. In the center of one end punch one hole with a sharp needle. About the center of the other end punch three pinholes at the corners of an equilateral triangle about two millimeters apart.Look through the one hole and see the three holes. Look through the other end at the one hole and explain what is seen. Label the box "Drunk-O-Meter" and list the following directions: 1 hole—sober, 2 holes—nipping, 3 holes—dog drunk, 4 holes or no holes at all—dead drunk. [HG]

POURING AIR. Submerge a beaker full of water in a large water filled container or fish tank. Invert the beaker so its open end is down. Invert a second beaker and submerge it so that air is trapped inside. Pour air from one beaker into another, pouring up. Note the fluid behavior of the gas. [HG]

POURING CARBON DIOXIDE. Construct a series of three 5-cm steps that will fit into a wide mouth jar. Set a lighted candle on each step. Slowly pour carbon dioxide gas from an open container into the jar. Carbon dioxide is heavier than air. As it settles it extinguishes the candles one by one starting with the candle at the lowest level. There are many ways to generate carbon dioxide. Try mixing some vinegar with bicarbonate of soda. [HG]

PRECISION IN ADVERTISING. Encourage students to think, speak, and write more precisely. Illustrate by using a meaningless advertising slogan: "The Rolls-mobile is bigger and better than (a) a kiddie car (b) a freight car (c) last year's model. [HG]

RADIOMETER. The little toys with four metal vanes inside an evacuated glass globe are readily available. They are sometimes called "light-mills", but the proper name is "Crookes Radiometer." Many books give incorrect explanations of how these work, and the correct explanation may be more than you wish to deal with. Before you demonstrate one, read Bill Beaty's How does a light-mill work?.

RATE OF HEAT CONDUCTION. Three students, each holding a rod of a different substance in a flame, will demonstrate the difference in conductivity of heat by their object from the flame. Use about the same sized rods of iron, aluminum, glass, copper. [HG]

REACTION OF THE ROAD. Place a plank on rollers (doweling). With a string, tie a small cart to one end of the plank and stretch a long rubber band between the cart and the other end of the plank. Add weights to the cart to increase its mass. Burn the string to release the system. The road goes one way, the cart goes another. [HG]

RETINAL AFTER EFFECTS. Draw a circle in the center of a piece of white paper with colored crayon. Stare at the circle at arms length for a time, then look at a blank wall. A circle of the complimentary color appears on the wall. [HG]

PULLING SPOOL. Select a large spool and wrap several turns of ribbon or cord around it. Place the spool on a table so it can roll when the free end of the ribbon is pulled out from the spool bottom. Observe the direction that the spool rolls when the ribbon is pulled straight up and when it is pulled at other angles closer to the horizontal. With a little practice, the spool can be made to roll in either direction as the ribbon angle is changed. Encourage students to explain the phenomena using terms such as torque, friction, and vector direction of force. [HG]

At a particular angle of pull the spool slides without rolling, if the angle is maintained as it moves. Ask students to determine, from physics principles (forces and torques), precisely how that angle can be predicted from the nature of the spool. Answer: The angle is such that a line extended along the ribbon passes exactly through the point of contact of the spool with the table, therefore the ribbon exerts no torque about that point. The force due to friction and the gravitational force mg also pass through that point, so the net torque about that point is zero, and no rotation can occur around that point. [DES]

A variation, to challenge the better students: Find a suitable sized cylindrical solid object. Tape one end of a length of cash register tape to it and wrap the tape around it. Now pull on the tape. The cylinder rolls away from you, as expected. Now bring the tape to the table top, and if you are very, very, careful, pulling slowly, the cylinder moves toward you without rolling. Doesn't this seem to defy what we learned above about the angle of pull? What's going on here? The tension of the tape exerts a horizontal force on the cylinder, but that's matched by the friction force, so there's no net horizontal force, and no acceleration. The only torque on the cylinder is the tension of the tape, with a lever arm equal to the radius of the cylinder. That torque should produce a constant angular acceleration. But we observe no angular acceleration, and no angular velocity. Is some physics being violated here? [DES] [Answer]

Challenge trick: When you've gotten the feel of this, tell students that you can actually make the spool rolltoward your hand. Use a long lecture table, and slowly accelerate the system, so the cylinder slides without rolling. Then suddenly stop. The cylinder's inertia causes it to roll toward your hand. [DES]

SEEING THE SUN BEFORE SUNRISE. The fact that one may see the sun while it is still below the horizon can be simulated by looking at a penny at the bottom of a bowl filled with water.Note that the penny cannot be seen over the rim of the bowl unless there is water in the bowl. When the sun first appears in the morning, it is still our of sight below the horizon. Refraction of the sunlight by the atmosphere makes the sun appear higher than it really is. [HG]

SELECTIVE IMAGE INVERSION. Print with capital letters the word TITANIUM DIOXIDE. Use a red pencil for the first word and a blue pencil for the second. View both words through the side of a test tube filled with water. Only the red word looks inverted. [HG]

SELECTIVE LIGHT SCATTERING. Demonstrate the effect of the sun setting through the dust-laden atmosphere. Add five grams of sodium thiosulfate and 5 mL of concentrated hydrochloric acid to a liter of water in a clear container. Shine a light through the solution and on to a wall or screen. Observe the changes as the colloidal sulfur forms. Scattered blue light can be seen in the solution at a ninety degree angle from the beam.On the screen or wall the spot slowly changes from white to yellow, to red, and then is finally is blacked out completely. [HG]

SHADOW REFRACTION. Place an object on the bottom of a metal pan so that its shadow may be measured. Fill the pan with water and remeasure the shadow. Refraction is evident if the pan, object, and light source are kept stationary. [HG]

SINGING FLAME VARIATION. Hold a four foot 1-1/2 inch glass tube vertical. Insert in the bottom end at a predetermined resonance point a heavy disc of wire gauze. Heat the wire gauze with burner, then remove the flame and hear a phenomenon. [HG]

SINGING TUBES. A straight metal blow pipe connected to a gas supply is fixed in an upright position on the demonstration desk and lighted.A thirty to sixty centimeter glass tube of large diameter is lowered over the flame until at a certain position a sound is heard. [HG]

SKY HOOK. Cut a four inch piece of wire from a coat hanger.Bend one half inch back on one end so that a leather belt will slip in the hook. Rest the free end of the wire on a finger tip. The belt and wire will hang out in space without apparent support underneath. [HG]

SLINKY DROP. [BERG] Stand on the lecture desk for extra height. Hold a SLINKY (TM) spring at one end. The other end shouldn't reach the floor. Ask what will happen if you release the upper end. Of course all will agree that the center of mass of the spring falls with acceleration g, and that the spring begins to contract when you let go of it. But what are the relative motions? Among the possible outcomes:

1. The lower end immediately falls with acceleration less than g.
2. The lower end rises to meet the rest of the spring, till the spring contracts fully, then the whole spring falls with acceleration g.
3. The lower end remains at rest, waiting for the rest of the spring to contract on its way down.

The outcome surprises students. The lower end remains at rest at constant height until the rest of the Slinky closes completely, then the whole thing falls to the floor. The center of mass of the spring falls with accelerationg. In the center of mass frame of reference, the spring collapses toward its center of mass.

Note that the acceleration of the upper end of the spring is initially approximately 2g. When the spring is hanging, the center of mass is not at its midpoint, but is lower because each part of the spring must support the weight of everything below, so the separation of the coils is greater in the upper part than in the lower part. Does this affect the outcome? Why, or why not?

But without getting mired in these details, we know that the lower end cannot react until information reaches it from the top end. At the instant the upper end is released, the lower end "doesn't yet 'know' anything has changed." No physical influence from the upper end reaches it immediately.

Some students think this has something to do with gravity, or with the non-linear stretch of the Slinky. Not so. The same Slinky spring can be strung over a taut horizontal wire or strong nylon cord (about 3 meters long for a plastic Slinky. A weight (W) attached to one end hangs over a pulley. The other end is pulled to stretch the Slinky, holding the weight in equilibrium. That end is released. The weight does not begin to fall immediately, but only when the Slinky is nearly collapsed. (The end may be attached to a string, which is then cut.)

For more instructional value, show the students the speed of a compression pulse along the Slinky. With the Slinky stretched, compress a small portion near one end and then let go. The pulse travels to the other end slowly enough to watch. The same thing happens when one end is released. The weight begins to fall only when the compressed portion travels from one end of the Slinky to the other end. So long as the coils at the left end of the Slinky have the same spacing they had initially, the tension there is the same as it was initially, thereby holding the weight (W) in static equilibrium.

Related experiment: INVISIBLE THWACK.

SOAP BUBBLES AND SOAP FILMS. [Dick and Rae] This recipe is from Richard B. Minnix and D. Rae Carpenter. Use Joy diswashing liquid (no other brands work as well). 140cc Joy, 300cc glycerine, and 450cc water. Approximately 1:2:3 ratios. Let the solution age a week before use for best results!

SODA-STRAW WHISTLE. [MG, DES] If you can still find paper soda straws, try this. Flatten one end. Snip off the edges of that end to form two free flaps of paper. Blow in that end. The paper acts like the reed of an oboe. Flatten the flaps more (or less) as necessary to produce the sound. Different lengths give different pitch. Start with a long one, and snip off lengths with scissors, making it successively shorter. When it gets shorter than an inch or so, be careful not to snip your nose with the scissors. If you can find a slightly larger soda straw, you can telescope it over the other one and make a slide-whistle. We farm boys used to make goose-feather whistles. Snip off and discard the portion with the feathers, leaving only about a two inch piece. Make a diagonal cut at the closed end, raising up a portion of the shank so it looks like a miniature clarinet mouthpiece. Blow into it for a high-pitched sound. Can be concealed in the mouth.

Find another slightly larger diameter straw to slip over the first to make a slide-whistle. Cut two to such a length that when played together you hear beats. Mark one in advance with the proper lengths so you can cut successively at the marks and produce a perfect ascending scale. Cut several, properly tuned, to play a simple tune. The opening of "Jingle Bells" requires only three.

SPECTRUM FROM A BOWL OF WATER. Submerge a mirror in a bowl of water as shown. When sunlight falls on the water surface at a low angle, a spectrum will be cast on the ceiling. Place this near a sunny window of the classroom. [MG]

SPECTRUM FROM A BALL POINT PEN. Some brands of ball-point pen have a clear plastic barrel with hexagonal cross section. Pilot © pens do. When these catch the sunlight they cast a nice spectrum, since the angle between alternate facets is 60°. You don't need to remove the ink cylinder from the pen. Save these pens for lab use. [DES]

Such a pen may also be used in the classroom with an overhead projector. Mask off all but a narrow 1mm slit on the projector light table. Notice that only a narrow strip (1 or 2 cm wide) of the projection lens has light passing through it. Place the pen in this strip of light (and parallel to it)near the projector lens, and adjust it so that the spectrum it produces is at minimum deviation. Narrow the slit if necessary to get purer colors. [DES]

Of course a long prism, or diffraction grating, or holographic diffraction grating can be used with the overhead projector to produce a spectrum, but the ball-point pen is far less expensive and the result is very good. [DES]

Question 1: Why does this spectrum form a curved rainbow-like arc on the wall? The answer to this has no connection whatever to the fact that a rainbow is curved in the sky.

More questions: As you rotate the pen, the spectrum moves through different angles of deviation from the unaltered beam from the projector. But there's a distinct position where the deviation is least, called theminimum deviation angle. Why is this? When the spectrum is at minimum deviation, the light rays pass through the prism symmetrically, making equal incident and emergent angles. Why? Why is the dispersion (spread of colors) greater when the deviation is larger than minimum? What has this to do with question 1?

SPECTRUM FROM A PROJECTOR. For classroom demonstration an overhead projector may be used to cast a large circular spectrum on the ceiling.

Obtain a disposable plastic glass, such as those used for cocktails at parties. These glasses have sloped sides. Nearly fill it with water. Cut a hole in an opaque paper just a bit larger than the bottom of the glass, and place this mask on the easel of the overhead projector, with the glass sitting in the center of the hole.This arrangement casts a circular rainbow on the ceiling. The size of the hole in the mask may need adjustment. In some cases the mask is not needed. Block the light on one side of the plastic glass to demonstrate that the path of the light is upward at an angle into the sloping side of the plastic glass, through the water, then out the surface of the water on the other side of the glass. This is especially effective in a fully darkened room. If the motor of the projector vibrates the water surface, the spectrum jiggles beautifully. [DES]

Spectrum from light reflected by a needle. From Laboratory Experiments in Practical Physics, Macmillan, 1927.

SPECTRUM OF THE SUN. Obviously, looking at the sun with unprotected eyes can be very dangerous. But here's a way the solar spectrum may be observed safely. Place a metal knitting neele, sewing needle, or polished straight metal wire, in the sunlight. With your back to the sun, look at the reflected sunlight from the needle through a prism whose axis is parallel to the wire. The sunlight reflected from the wire (acting as a cylindrical mirror) looks like a very narrow line and performs as a very narrow slit. But it spreads the reflected light so its intensity is low enough to be safe to look at.

A diffraction grating may be used in place of the prism. If you place the needle on the top of an open cardboard box, you can arrange things so that the spectrum is seen against the shaded inside of the box, for better visibility of the colors. Some of the Fraunhoffer dark absorption lines may be observed in the spectrum if you use a prism or grating of high optical quality. The prisms sold in science stores are usually not good enough.

I've seen this demo in older books. For example, Henry Black, Laboratory Experiments in Practical Physics. Macmillan, 1927, p. 227. The needle method was obviously well-known then, but I've not seen it in modern books.

More about this, with spectrum photos.

SPINNING HAMMER. Mark the center of gravity of a hammer with a spot of paint. Toss the hammer into the air with a spin and note that the spot is the most stationary point of the hammer as it spins. Note also that the paint spot moves in a smooth (not wobbly) path, in fact, its path is a parabola.[HG]

Footnote: Is the path of a projectile a parabola, or an ellipse. Isn't a projectile just a body in orbit around the center of the earth? Aren't such orbits always ellipses? [Answer]

SPOOKY BALANCE. Find a large cylindrical object, about 10 or more inches in diameter, and secure it onto a base so it doesn't roll or move. You can easily balance a textbook on it. If one end of the book is pushed down a bit, the book restores to its original position. It can even be placed offside a bit, and it still balances, though tilted. Try wooden blocks of various size and height. Try a stack of blocks, or books. If the height is too great, balance can't be achieved. What determines the maximum height? Note, there must be sufficient friction to allow the balanced object to roll a bit without slipping. But even with large friction (felt on felt) when the height of the balanced object is too great, it won't stay in place. See Kleppner and Kolenkow, problem 6.35. [Answer]

STACKED CHIPS. Stack coins or smooth plastic disks, then aim another coin at the stack. It will knock out the bottom coin, leaving the stack undisturbed. A thin ruler, less than the thickness of a coin, may be used to knock out the bottom coin. Stack alternating colors of chips; swing the ruler back and forth, separating the two colors of chips.

STANDING WAVES ON A STRING. Wave motion and standing waves can be demonstrated by attaching a vertical string to almost any small electric motor or vibrator. An electric shaver is ideal. Hang weights of varying amounts on the string. As the tension increases, there will be changes in the length of the standing waves. [HG]

UNLIMITED WATER SUPPLY. Suspend a water faucet above a sink by thin wires. Water can be made to flow continuously in a strong stream from the faucet bottom despite the fact that the faucet is not connected to any water pipes. The secret is to use a concealed electric water pump that pushes water up a glass tube into the bottom of the faucet. As the water emerges, it flows over the outside of the glass tube concealing the tube from view. This makes an interesting corridor display. [HG]

STEREOSCOPIC VISION. Look through a paper tube at some distant object with the right eye while holding a book over the other eye and close to the tube. It will appear that one is looking through a hole in the book. [HG] Or, as shown in the figure, hold up your hand and it will appear that you have a hole in your hand.

STRETCH CHARGE. Stretch a rubber band tightly and rub it against an electroscope. Determine the nature of the charge produced. [HG]

SUSPENDING SOAP BUBBLES. Show that a soap bubble filled with air will float on carbon dioxide. Using a bubble pipe or straw, blow a soap bubble and carefully place it in the center of a jar partially filled with carbon dioxide. Because the carbon dioxide is invisible the bubble appears to be suspended without any support. [HG]

THUNDER BUBBLES. [Demonstrator and observers should use ear protection.] Prepare a soap bubble solution in a shallow dish or pan. Fill a balloon or beach ball with a 2 to 1 mixture of hydrogen and oxygen. Using a small nozzle delivery tube, blow the gas mixture from the balloon through the soap solution to produce copious bubbles.Pick up handful of bubbles and hold them far out in front of you. Ignite the bubbles with a match. The explosion will not be felt by you.However, someone standing too close to the bubbles may have an ear injured. The sound is indeed intense. Try this in a long hallway for good reverberation. [HG]

THUNDERING GRADUATE. Suspend a coiled platinum wire just below the lip and inside an English graduate containing a few milliliters of ammonium hydroxide (concentrated). Bubble oxygen through the ammonium hydroxide. Violent explosions occur. (I don't even know what an English graduate is!) [HG] But an ungraduated cylinder is one that flunked out of school. [DES]

TUBELESS TELEVISION. Prepare a slide with a few simple words such as "THAT'S ALL, FOLKS" cut in a piece of metal foil. Aim the slide projector so the beam goes out an open door and does not attract attention. In a darkened room, wave a white wand in the plane where the image is focused. Persistence of vision creates a complete image, apparently materializing the words in space. [HG]

TUG OF WAR. Use two equal weights, suitable for hanging from a cord. Weigh each with a spring balance to convince students that they have the same weight, W. Arrange the weights and the spring balance as shown, by hanging the two weights on strings passing over pulleys to either end of the balance. Ask students to predict the reading of the balance. Suggest options: 0, W, W/2, 2W, 3W. You may think this isn't worth the trouble to set up. But if you do, you may be surprised/dismayed at how many students predict 0 or 2W, or even W/2.

VARIABLE DENSITY. Moth balls rolled in sodium bicarbonate and put into a cylinder of very dilute hydrochloric acid will rise and fall with regularity. [HG]

VARIABLE VOLUME SPONGE. A dry plastic sponge can be measured for volume. Ask how much water you can pick up with it. When wet, it may pick up more than its initial volume. It expands slightly and is mostly a "lot of air holes fastened together." [HG]

VISCOSITY REACTIONS. Let a few students with egg beaters beat oils of different viscosity. Those having to beat the heaviest oils will tire first. [HG]

VISUALIZED WORK UNITS. Mount a 12 inch length of 2x2 inch wood upright on a base. Mount another one slightly over 9 inches long on the same base. Lifting a one-pound weight from the base to the top of the one foot tower represents one foot-pound of work.Lifting the weight to the top of the 0.76 foot tower represents one joule of work.A feather lifted from the table top on to a piece of paper laying on the table represents an erg of work. A millionth of a millionth of an erg is called an electron-volt. [HG]

WATER GLASS SUCKER. Carry a glass brim-full of water up a ladder and press it to the ceiling. Ask the "sucker" to push against the bottom of the glass with a long pole while you climb down and put away the ladder and go on about other business. If the ceiling is smooth, the student need not fear that the glass will fall. [HG]

WATER GLASS PRANK. This is an old restaurant prank. Cover a brimful glass of water with a thin card (such as the restaurant menu).Carefully invert it on a smooth table top, then pull out the card. Leave it for that surly or snobbish waiter or waitress at the greasy spoon diner. Better yet, include the tip (in coins) in the glass first. The clever waitress will know what to do to retrieve the tip without spilling water all over the table and floor. Do you? [DES] [Answer]

WHISTLE BEATS. Prepare two small whistles with a screw in the end so that the length of the air column can be adjusted for different frequencies. Attach the whistles to a Y tube so that they can be blown simultaneously. When the pitch of one is slightly different than the other, a low pitch "beat note" is heard. [HG]

WHISTLE DIFFERENCE TONES. Small and inexpensive double (and sometimes triple) plastic whistles are sold in museum shops, toy stores, science stores, and places where novelties are sold. They have two or three whistles side-by-side so they can be blown simultaneously. The large (foot-long) wooden whistles simulating locomotive whistles are also made this way, with two or more inner chambers of different length. They produce difference tones, equal to the difference of frequency between the pipes, but in the aubible range. (Beats are in the subaudible range.) This is a result of non-linear mixing of the turbulent air near the fipples of the pipes, which are close enough together to allow this. If one cut such a whistle, physically separating the pipes, the difference tone would not be heard when they are blown simultaneously. [DES]

WOOD'S METAL. Carve a spoon mold in wood and fill the mold with molten Wood's metal. The spoon will melt in hot water, coffee, or tea. Save the mold to recast the spoon as part of the demonstration. [HG]

Wood's metal melts at 65.5° Celsius. It may be made by melting together 4 parts by weight of bismuth, 2 parts lead, 1 part tin, and 1 part cadmium. It can be purchased from various web stores.[Sutton, p. 208]

Presentation idea: Casually use the spoon to stir your coffee. Remove it and the bowl is gone. Remark that the coffee is really strong today.

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Some Answers

This section is continually under construction. Watch for potholes.

I've received many inquiries (a couple or three, at least) asking "What's that guy doing in the lead illustration of your demo document?" The illustration is from p. 161 of Hopkins' book Experimental Science (1890). It's caption is "Reflection of Light and Sound." Oddly, it isn't specifically explained in the text, but may have been in the issue of Scientific American from which it was taken. As near as I can figure, the sound of the ticking watch at E is reflected from mirror B to mirror A and then focused at C and conducted to the ear through the tube. The flame at the focus of the left mirror illustrates that the mirrors reflect the light in the same manner, back to illuminate the face of the watch. It could also be demonstrating the effect of the focused sound wave on the flame. Anyway, I think it appropriate to illustrate the spirit of physics demonstrations, and the fact that the audience doesn't always know exactly what the demonstrator is doing.

ARISTOTELIAN FALLING BODIES. At terminal speed the drag force on the body is equal in size to the force due to gravity, F_drag = mg. If the drag is proportional to some power of the speed, say the second power, F_drag = kv², and the body reaches terminal speed quickly, then the distance of fall will be approximately d = vt. Let's find the ratio of distances two objects of different mass will fall.

So,d₂/d₁ = (F₂/F₁)^1/2 = (m₂/m₁)^1/2.

Then if the mass ratio is 4, the distance ratio is 2. If the mass ratio is 2, the distance ratio is 1.414. These may be tested by holding the two objects at heights in these ratios and releasing them simultaneously. They will plop on the floor simultaneously, much to the surprise of students. [<]

COIN DROP. The requirement of retaining the top quarter prevents you from releasing the coins quickly. One edge of the quarter drags on one finger, initiating the flip-over. Try it with a single quarter, nickel, or dime, and tally the statistics. [<]

FALLING LEAKY BUCKET. After the container leaves your hand, moving downward, it is in "free fall" with an acceleration g. The pressure on the water inside is the same everywhere: atmospheric pressure. So there's no pressure difference across the hole to push water out. The leak stops.

The case of upward motion is especially instructive. After the container leaves your hand, moving upward, it is again in "free fall" and has accleration g, the same as if the cup were falling downward. So, again, there is no pressure difference across the hole and the water flow through the hole stops. During the time your hand was in contact with it, while throwing, accelerating it upward, the water pressures inside are larger than when the container was at rest, and the leak velocity is larger. You could hold your finger over the hole while tossing the bottle upward. During the entire motion up, then down, the acceleration of the bottle and its contents has the same value and the same direction (downward), and that includes the very top of its trajectory. Now if it were spinning, it would have an angular acceleration, and then you would observe leaking water. [<]

FINGER FRINGES. With a point or very fine line source you could see colored diffraction patterns in this situation. But not with a broad source of incoherent light. The fringes seen here are an artifact of our vision. The receptors in our eyes, along with our brain, processes signals to detect contrast and therefore emphasizes edges. This causes fringes to be seen around solid objects.See Robert P. Bauman "Diffraction and Non-Diffraction Fringes" The Physics Teacher 34 p. 339 (Sept 1996). [DES] [<]

HEAVY FINGER. This demo is rich in educational value if the details are discussed. Have a student immerse a finger into the beaker. Grill the student to describe what is felt. Eventually bring out that the student feels an upward force on the finger. "Aha!" you say. "What does Newton say about this?" Answer: If the water exerts a force upward on the finger, the finger exerts a downward force on the water.

"But doesn't the water, being fluid, just move aside to accommodate your finger? How does the balance pan know you've put a finger in?"

Answer 1: The water, after insertion of the finger, is in equilibrium, so the net force on the water is zero. The forces on the water from the sides of the beaker add to zero by symmetry. The force of the atmosphere on the water surface is essentially unchanged, except that the surface area is reduced by the inserted finger. But that must be a smaller effect, for it is in the wrong direction to account for the observed result. So if the finger exerts a downward force on the water, the bottom of the beaker must exert an equal size upward force on the water. And if so, the water exerts a downward force on the bottom of the beaker, etc., accounting for the extra force on the pan which pushes it down. But still, there's the nagging question of how, in colloquial terms, "The force of the finger is communicated to the bottom of the beaker." Point out that this language is rather loose, and we seek a better and more detailed answer.

Answer 2: Have a graduated cylinder of water nearby on the table. Push a finger into it. Note that the water level rises. "What does that rise of the water level do? It increases the pressure on the bottom, for that pressure is due to the weight of a column of water supported by the bottom, and we have increased the height of that column of water. This is what happened in the original experiment, but we may not have noticed the water level rise." Do the original experiment again, so everyone does notice. [<]

HIGH ROAD AND LOW ROAD. The balls are neck and neck as the first inclined drop is reached. The ball which takes the low track attains a greater kinetic energy than that of the ball on the high path by amount mgh, where h is the height difference between the level paths. As the low ball enters the level portion of track, it is already ahead of the high ball because of the speed it gained on its downward incline, which increases its horizontal component of velocity. The low ball continues to get farther ahead of the high ball because of its greater speed. The low ball then loses an amount of kinetic energy on the upward incline equal to the energy it gained on the downward incline, but when the two balls are at the same level exiting the track, they have the same kinetic energy and the same speed. So, is the low ball still ahead of the other one on exit? Yes, because the low ball was ahead of the high ball as it entered the upward incline, and the low ball's speed up the incline was at all times greater than the speed of the high ball. Therefore its horizontal component of velocity was greater at all times also. [<]

OSCILLATING BEAM. The forces on the beam are shown. The centers of the wheels are each a distance L from the centerline (dotted) of the apparatus. The friction forces are in opposite directions:

f₁ = µN₁ and f₂ = µN₂

Take torques about the center of mass of the beam, shown by the + symbol in the diagram.

-Wx - N₁L + N₂L = 0

Then: Wx/L = N₁ - N₂

The net force on the beam is F = f₁ - f₂ = µ(N₁ - N₂)

So,-Wx/L = (f₁ - f₂)/µ

F = f₁ - f₂ = -µxW/L = -(µW/L)x = ma

So the acceleration of the beam is proportional to the negative of its displacement, and therefore its motion is simple harmonic. So what happens if both wheels turn the other direction? The friction forces change sign, and we get the result +(µW/L)x = ma, which tells us the acceleration of the beam increases with displacement, so the slightest displacement increases till the beam moves off the wheels. [<]

ROLLING SPOOL. The demonstration of the cylinder sliding without rolling on a horizontal surface is a cheat, but a subtle one. I use a solid wooden cylinder about two inches thick and ten inches diameter. Cash register paper is attached to the cylinder, given a wrap, and pulled slowly. The weight of the cylinder crushes the paper beneath it, making it slightly thinner. So the spool rides in a shallow depression in the paper. The paper exerts a force on the spool which isn't exactly normal, but is tilted slightly in the backward direction (opposite to the motion). This off-normal force provides a torque of equal size and opposite direction to the torque due to friction. The friction force is mainly responsible for the forward motion, reduced slightly by the small backward horizontal component of the off-normal force.

Better yet, use a full roll of cash register paper. I use rolls about three inches wide by three inches diameter for larger audiences. To ensure that it works every time you can "cheat" by piling weights on the roll for a while as it lays on its side, producing two flattened regions. Of course we physicists would never cheat, would we?

This geometry of forces and torques on the spool is similar to that responsible for rolling resistance. Here's a physics puzzle related to this:

A perfectly round ball (or cylinder) rolls without slipping on a smooth horizontal surface. The rolling object eventually slows and comes to rest. Suppose we argue that friction was responsible, since the rolling object obviously loses energy. Whether or not the friction force is strictly horizontal, the horizontal component of friction must oppose the forward motion, thereby decreasing the speed of the rolling body. But the torque due to friction acts in a direction which would speed up the object, not slow it down. Explain this apparent paradox. What's wrong with this explanation?

Rolling resistance is the key to understanding why the body slows. Friction isn't the reason. Let's ignore friction for the moment. The cylinder and surface both distort at the area of contact with the table (see diagram). The forces normal to this contact surface aren't uniform, and are larger on the forward side. So the net force of the table on the cylinder isn't quite normal, this vector leans a bit backward, opposite the direction of the motion. This force exerts a torque to reduce the angular momentum and slow down the angular velocity. The horizontal component of this force acts to reduce the linear momentum and slow the linear velocity. The vertical component of this force balances the cylinder's weight. The nonlinear forces responsible for the distortion of the surfaces account for the energy loss, and if there's also friction in the normal sense (actually the tangential sense) it accounts for some of the energy loss, and slightly affects the sizes of the other forces we discussed. The interesting thing is that all of these considerations are interrelated and their relative sizes are fixed by the geometry of the distortion of the surfaces. Rolling resistance is seldom dealt with in physics books, but may be found in some intermediate mechanics books for engineers. Cliff Swartz's bookTeaching Introductory Physics, a Sourcebook (AIP, 1997) has a discussion of it (pp. 93-95) but calls itrolling friction.

Similar geometry applies to the case of steel locomotive wheels on steel track. However here we know that some friction must be present, or each wheel would spin in an indentation in the track. Both friction and rolling resistance must be taken into account. Also, in this case, the wheels are driven. In the example discussed above, the rolling object was not driven. [<]

SPOOKY BALANCE. Let H be the distance from the bottom of the balanced object and its center of mass, measured perpendicular from the bottom surface. M is its mass. When the balanced object is disturbed, it can roll without slipping to a position tilted at angle ß. The point of contact has moved through the same angle with respect to the center of the cylinder, and through an arc of length Rß, where R is the radius of the cylinder. The block is stable if the line of action of Mg falls within Rß of center, so Mg produces a restoring torque. The block is unstable if (H/2) tanß > Rß. For small angles ß, tanß is approximately equal to ß, therefore the block is unstable if H > 2R. [K&K, problem 6.35] [<]

TRAJECTORIES. The paths of projectiles near the earth's surfaces (ignoring air drag) are portions of ellipses, as are any periodic orbits, such as that of the moon around the earth, or artificial earth satellites. The projectile's elliptical path, if extended, would pass through the volume of the earth, and the center of the earth would be at one of its foci. Then why do measurements of height and range of projectiles obey equations which describe parabolas?

Transofrming an ellipse to a parabloa.

y = v_oy + (1/2)at²

x = v_oxt

Here y is height above the earth's surface and x is distance along the earth's surface. We may graph these on cartesian paper, but that's not quite fair. Heights are actually radial lines from the center of the earth, and aren't parallel, but divergent. Distances along the earth's surface aren't straight, but are circular arcs. They should be graphed on polar coordinate paper. When you do this correctly on polar coordinate paper, the path is a segment of an ellipse. When graphed on cartesian paper it is a segment of a parabola. When a parabola on cartesian paper is mapped onto polar coordinate paper it becomes an ellipse. And vice versa. Isn't that remarkable! Can the mathematically inclined among you prove this?

A textbook asks, "Is the path of a trajectory an ellipse, or a parabola?" What's the answer? Actually textbooks shouldn't ask such dumb questions. Here's two pictures. One is of a parabola, one is of an ellipse. Can you tell which is which?

Trajectory with and without air drag.

Answer: The ellipse is on the left, the parabola on the right. When only part of either curve is shown, the differences are negligible. In fact, if we take air drag into account, the resulting curve is also hardly distinguishable from these. This figure shows a trajectory without drag (solid) and a trajectory with air drag (dotted). The range is shorter, but the distance to the highest point is also shorter. Either one would "fit" the mathematical form of an ellipse, or of a parabola, to within the usual experimental error. See how textbooks deceive?

The culprit here is the use of "is" in the question. This is one of those dangerous words in science. Another dangerous word is "because".

[<]

WATER GLASS PRANK. Slide the glass to the edge of the table, and catch the water in a container below the edge of the table. It's also possible to insert a thin card (or menu) under the inverted water glass without losing much water. Then invert the glass. [<]

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Sources, contributors and additional references

1. Anon. Social Tricks for Entertaining. No author listed. Illinois Herb Co. (1930s?)
2. Arons, Arnold B. Teaching Introductory Physics. Wiley, 1997. This is an essential resource for the physics teacher who wants to "get it right." [Arons]
3. Berg, Richard. University of Maryland, from his Physics IQ Test demo show. [BERG]
4. Black, Henry. Laboratory Experiments in Practical Physics. Macmillan, 1927.
5. Brown, Bob. Science Treasures, How to Repeat the Great Experiments. Cornerstone Library, 1969.
6. Carpenter, D. Rae Jr., and Richard B. Minnix. The Dick and Rae Physics Demo Handbook. Dick and Rae, Inc., 1993. Dick and Rae are from Virginia Military Institute. I haven't seen their book, but have seen their shows. [Dick and Rae]
7. Ehrlich, Robert. Why Toast Lands Jelly-Side Down, Zen and the Art of Physics Demonstrations. Princeton University Press, 1997. I discovered this book after I'd begun this web page, and reading it made me wonder why I bothered. It is one of the few demo books which explains the underlying physics, with mathematical analysis. Better yet, Ehrlich shares my opinion that simple apparatus is best, and that demos aren't worth doing unless you are willing to examine all of the details with a class. [Ehrlich, Toast]
8. Ehrlich, Robert. Turning the World Inside Out and 174 Other Simple Physics Demonstrations.Princeton University Press, 1990. [Ehrlich, World]
9. Everett, J. D. Deschanel's Natural Philosophy. Appleton and Company, 1873. Out of print. Another good source of old engravings.
10. Frank, J. O. and Guy J. Barlow. Mystery Experiments and Problems for Science Classes and Science Clubs. J. O. Frank & Sons, 1934.
11. Frier, George D. University Physics. Meredith, 1965. A physics textbook with marginal notes including pictures and brief descriptions of suitable demonstrations for the concepts being discussed.
12. Frier, George D. and Frances J. Anderson. A Demonstration Handbook for Physics. 1972, 1978, reprinted by American Institute of Physics, 1981.
13. Gardner, Martin. Various sources and his personal files. See also his "Physics Trick of the Month" contributions to The Physics Teacher. [MG-PT]
14. Gardner, Martin. Science Puzzlers. Scholastic Book Services, 1960. Republished as Entertaining Science Experiments With Everyday Objects Dover, 1981. [MG-SP]
15. Gardner, Martin. Encyclopedia of Impromptu Magic. Magic, Inc. 1978, 1985, 1991. [MG-1] Illustrations from this book have been used below, with Mr. Gardner's permission.
16. Gardner, Martin. Martin Gardner Presents. Richard Kaufman and Alan Greenberg, 1993. [MG-2]
17. Good, Arthur. 100 Amazing Magic Tricks. Translated and Adapted by David Roberts. Pinnacle Books, 1977. Yet another reworking of the 1890 Columbus' Egg materials (see Lanners),
18. Hopkins, George M. Experimental Science, Elementary Practical and Experimental Physics.Munn & Co. 1890. Out of print. I hope to use some of the 680 fine engravings from this book here if I'm granted enough disk space.
19. Kleppner Daniel and Robert Kolenkow. An Introduction to Mechanics, McGraw-Hill, 1973. Some problems in this book are derived from standard classroom demonstration apparatus or are easily constructed. Several of these are in the collection below. The book has a solutions manual, available to teachers. [K&K]
20. Lanners, Edi. Secrets of 123 Old-Time Science Tricks & Experiments. TAB Books, 1981. Original by Verlag C. J. Bucher, Lucerne and Frankfurt, 1976. First English Language Edition by Paddington Press U.K. Ltd., 1978. Translation of Kolumbus-Eier, based on a larger collection entitled Columbus' Egg, first issued by the editors of Der Gute Kamerad in, two volumes, 1890. Most of the illustrations are by Poyet.
21. Lynde, Carleton John. Science Experiments with Home Equipment. International Textbook company, 1937, Van Nostrand 1949. [Lynde, 1949]
22. Lynde, Carleton John. Science Experiences with Ten-Cent Store Equipment. International Textbook Company, 1941.[Lynde, 1941]
23. Meiners, Harry F., Ed. Physics Demonstration Experiments. Two volumes. The Ronald Press Company, 1970.
24. van Rensselaer, Alexander. Fun With Stunts. Blaikston, 1945. Reprint of Betcha Can't Do It (1940) and Try This One! (1941).
25. Sutton, Richard Manliffe. Demonstration Experiments in Physics. McGraw-Hill, 1938. [Sutton]
26. Herb Gottlieb. [HG].
27. Donald Simanek. [DES].