PHYSICS DEMOS FROM
THE WOODROW WILSON PHYSICS INSTITUTE
compiled by Pat Cannan
submitted by Preston "The Boom" Boomer
Woodrow Wilson National Fellowship Foundation
Princeton, NJ 08542
When introducing acceleration of gravity, discuss it in terms of a falling banana (or rutabaga, or whatever). Demonstrate the fall and then compare to a heavy banana (filled with lead shot and rubber latex or aquarium sealant). Drop both bananas at once by quickly pulling a book out from under them.
Conclusion: All bananas accelerate at the same rate. This can then be quoted for the rest of the year to remind students of the demonstration.
And/or another variation:
Galileo's home country-- Italy. National fruit of Italy-- Grapes.
So all grapes fall at the same rate whether dropped individually or in a bunch. Show it. Bunching them makes no difference! Each atom accelerates at g regardless of its companions.
Kepler's dad was a plumber. So take two plumber's helpers (preferably with short handles) and stick them onto the chalkboard for the foci of the ellipse. Then using a looped string and chalk, draw the ellipse around the two helpers.
How can an astronaut distinguish between a lead banana and one that is just a hollowed out (and reinforced) peel? By sensing their resistance to motion by shaking them. Pass bananas around so students can FEEL the mass.
Gravity discovered-- the real story.
One day, young Isaac Newton, then in his mid-twenties, was sitting under the banana tree in his back yard...
Bend a coat hanger and its hook so that a penny will balance on the upturned hook. Hold the hanger by your index finger and swing it in a circle. The penny will (with practice) remain in place.
Swingin' big scare:
Suspend a small (25cm diam) board from three strings so it can be vertically swung around. Place objects on the board and scare everyone! Practice this before trying beakers of water etc.
Cars come equipped with a positive accelerator, a negative accelerator, and a change sign lever. When the lever is in the + position, the positive and negative accelerators work as designed.
When the lever is in the - position, the positive accelerator produces a negative acceleration, the negative accelerator a positive acceleration. (for those doubting students, what would have happened in the second case if the - accelerator had indeed accelerated the car negatively?)
Single-ply toilet paper takes a force of about 10 newtons to separate. A rapid linear acceleration of the paper takes advantage of the rotational inertia of the roll to help stretch and tear the paper. The build-up to the breaking point must occur quickly so that angular velocity of the roll is kept small and paper is not dumped onto the floor. As the roll is used up, the moment of inertia decreases making it increasingly difficult to get paper off with one hand.
Place a new roll of TP and an almost empty roll on a bar held by two students. Give the new roll a yank, and the paper should tear nicely. Give the small roll a yank, and it should unravel onto the floor.
Discuss the moment of inertia. The new roll approximates a disk, the old roll a hoop.
With a small transistor radio blaring away, enclose it in a cage of wire mesh. The Faraday cage will shield the radio from any electric fields and hence will shield it from radio waves. (The electric waves of light enter and leave the cage because their wavelengths are much smaller than the mesh size.)
When Chocolate Chip Speaks, Students Listen:
Take about 50 turns of fine, insulated wire and tape to the back of an ice cream carton (or whatever), leaving the two leads of the wire to attached to the output of an amplifier. Bring a large magnet up to the back of the voice coil when the amplifier is signaling appropriate music.
You may construct the speaker in class, discussing it in abstract terms so students are taken by surprise. If you do not have a carton, the daily bulletin or and administrative pronouncement will do.
Tie large steel hex nuts (of varying mass) on a string at spaces of S=1/2gt2 where t=.1, .2, .3, etc. Release the string onto a noisy flat plate and listen for a constant rat-a-tat-tat as th nut hit. First drop a string with evenly spaced nuts.
Put tape on a meter stick at intervals similar to the Mixed Nuts. Mark off a parabola on the board by moving equal horizontal distances as you mark off vertical positions from the tape.
Challenge students to toss an object that matches the parabola, or drag a student in a wagon while she tosses an object straight up.
The Beat Goes On:
Drafting supply stores have stick-on tape with parallel bars. By Xeroxing this and shrinking it various amounts, waves of different length are obtained. Make transparencies and place them over each other to project beats or group velocity onto the screen.
By choosing l = fl (l= lambda) where f is a simple fraction, beat frequencies are harmonic with the generating frequencies and give a pleasing sound. (Thank you, Pythagoras).
Concentric ring patterns may be purchases from drafting suppliers of about $3 per sheet. Make different wave lengths by enlarging or reducing the pattern. With these made into overheads, you may demonstrate (1) 2-slit interference, (2) the effect of changing slit spacing or wavelength (3) n-slit interference (4) diffraction grating (5) effect of telescope aperture and incident wavelength on resolving power.
Measuring the wavelength of light with a diffraction grating demands and act of faith-- are there really all those lines on the grating? You can diminish those concerns by using a metal ruler with scored divisions of less than a millimeter or so ($2.50 at a hardware store). Allow laser light to reflect off it at a grazing angle and project the pattern onto a wall.
Kitchen Scale Equilibrium:
Take a two-meter or so 2X4, mark it at 30 cm intervals and show your class its weight on the scale. Now support one end on the scale, one on a block, and ask the class to predict weight on the balance before you actually release the board. Then reverse block and scale and ask again. Try various locations of the block and scale and even add extra weights to the beam. This demonstrates moments under a variety of conditions.
Back to Normalcy:
Clamp a weight to the scale and tilt it to show normal force variance with angle. In general, you will find a kitchen scale to be a frequently used piece of apparatus for all sorts of phenomena. They are available with metric readouts.
Reflections on the Wave Nature of Light:
Reflect a laser beam off a flat mirror, make incident and reflected beams visible with clouds of chalk dust. Then reflect light off a good quality diffraction grating. Ask students what is going on.
Kids do not outgrow the desire to take something home with them. Diffraction gratings and polarizing materials are so cheap that they should be given to every student. Tape 1cm X 1cm pieces of each behind punched holes on a card. Challenge students to write down observations when they look at clouds, reflected light, neon or sodium lamps, stars, etc.
Trapped in the corner:
A corner reflector may be made by cementing three mirrors together at right angles to each other. Use aquarium sealant for adhesive and drafting triangles to insure accurate right angles.
Reflect laser beam off the interior of the cube. Students will be able to see how three reflections are required for it to work, and they will actually be able to follow the light path. Rock the cube around so it is clear that the return path is not dependent upon orientation.
Make a soap solution as follows: 70ml of Joy, 200ml glycerin, 230 ml water. Roll a cone from a piece of paper and blow a large bubble onto a glass plate on an overhead projector. Ignore projection on screen and look at beautiful, iridescent interference on the bubble itself. With the right mixture of bubble soap the bubble should get thin enough to become totally transparent to reflected light, just before it breaks.
Reflect a strong light off a soap film across the end of a can. With a lens of appropriate focal length, focus an image of the bubble on a screen. A series of spectra characteristic of a thin film is visible. Now bring a speaker to the back of the can, and interesting distortions of the image will occur. If you hook up a signal generator, very the frequencies to get resonance on the soap film.
The Swing Era:
Hang several pendula of different lengths from a semi-rigid support. Challenge students to get a particular pendulum swinging to the exclusion of the others by pulling on a rubber band attached to the support.
The classic demonstration of a mass suspended between two strings, protecting the upper string from breaking by its inertia does not communicate the importance of a stretchable string. If the string were absolutely unyielding, the upper string would break every time.
By replacing the upper string with a spring, a slow motion of the mass downward stretches the spring and visibly puts tension on it. A rapid jerk on the string breaks it without significant stretch of the spring.
The Big Attraction:
With a charged lucite rod, rubber rod, or golf tube, attract an empty pop can. Balance objects on the dome of a watch glass and observe the effects-- everything up to a 2-meter 2X4 will work.Small charged objects may be discharged with an anti-static gun available at a record store. The gun has a piezoelectric crystal connected to a sharp pin. The potential developed on the point creates ions that stream off the point and discharge whatever.
Suspend a large funnel from a support so that it can spin freely. Fill it with sand and release it, giving it a small initial angular velocity.
Soda Straw Symphony:
Clip the flattened end of a drinking straw to a point, forming a double reed. Pinch the reed end slightly with your lips as you blow HARD to get something like a high-pitched duck call. (No self-respecting beaver would respond to such a noise). While blowing, clip the other end off to change resonant frequencies. Remind the class that shorter tube lengths produce higher resonant frequencies.in the straw just as it did on the soap bubble. Connect multiple straws and flex straws for good bass notes, insert smaller straw to make a slide trombone, cut holes for advanced work. Cut appropriate lengths so class can play school fight song, Christmas carols, etc.
No Strings Attached:
Poke a hole 2/3 of the way into a Nerf ball and imbed a 30g sinker attached to a string in the middle of the foam. Swing the ball in a circle over your head and ask students at what moment in its path you should release it to hit a target. The demonstration is more forceful if some of the student predictions result in the ball flying into the class.
Shifting to Doppler:
Get a code oscillator circuit (e.g. Radio Shack #20-115), a 5cm speaker, a small switch and a 9-volt battery clip. With a sharp knife slice into a Nerf ball and imbed all parts inside. Turn on the switch and throw the ball to students in the class. Pitch will change noticeably depending on whether the ball is approaching or receding.
(Total cost is about $9).
Mount a length of piano wire under tension (from a spring or weights). Place a large magnet over it and hook the ends of the wire to the input of an amplifier. Plucking the wire will induce currents that will amplify as musical sounds.
A human hair held in the laser beam will produce a single-slit interference pattern. (The hair forms a single thin barrier.) The width of the hair can be determined by measuring the spacing of the secondary maxima and using the single-slit equation.
Learning the Ropes:
A convincing session in vectors: Have two burley guys pull a rope between them as tight as they can. Then have your smallest kid pull sideways in the center of the rope. He will have no trouble pulling the burleys toward each other.
If two soap bubbles (or balloons) are connected at opposite ends of a pipe, the smaller bubble (or balloon) will force air into the larger one. The pressure inside a bubble varies inversely as the radius of the bubble. It's neat to have a valve in the pipe and set up the bubbles or balloons and ask the kids to hypothesize on what will happen when the valve is opened and why.
Soap Film Trampoline:
Use a coat hanger or a large wire frame. Dip it into a bubble solution. With practice you can cause the large soap film to undergo many interesting modes of vibration.
Try spinning the following objects on a bare floor or on a smooth table top: Small toy football, hollow egg-shaped plastic container, hard-boiled egge, full-size football. Although the football begins its rotations about its short axis, it reorients itself to a lower energy state by standing up and rotating about its longer axis.
The Levitating Screwdriver:
When various objects are individually placed in a narrow stream of fast moving air, they seem to float. Objects which have been used include: golf balls, small footballs, styrofoam balls, rubber balls, steel balls, hollow egg-shaped plastic containers, and smooth handled screwdrivers.
Pat the Pipe:
Pat the end of the pipe with the flat palm of your hand. Use two distinctly different motions: 1. Leave the hand against the end of the pipe after striking it. 2. Quickly remove the hand away from the end of the pipe immediately after striking it. If you listen carefully, you should hear two different octaves because an open pipe has antinodes at both ends while a closed pipe has a node at one end.
Hold on to the midpoint of a solid aluminum rod (18mm diameter is good) with the thumb and forefinger of one hand. Stroke the rod with the thumb and forefinger of the other hand. A LOUD tone emerges from the longitudinal oscillations set up in the rod. By holding the rod at other locations, higher harmonics are heard. It is helpful (essential to get resin, stick-um, or some kind of frictional material on the rod. Consult your local sports store.
Baffle the Speaker: Purchase an ear phone attachment for a cassette player. Cut off the ear piece and in its place solder a small (5cm) speaker (Radio Shack). Plug this speaker into the cassette player and listen to the musical sounds before and after the speaker is placed near the opening of each of the following objects: plastic pipe, bottomless styrofoam cup, a sheet of 60cm square cardboard with a 5cm hole cut in the center.
Light My Balloon:
Fill one balloon with water. Fill another balloon with air. Place a lighted match beneath each of the balloons. Explain.
Follow the Vector:
Use a bicycle wheel or a circular disk (such as a disk stroboscope). Attach a styrofoam ball and arrows radially inward and tangential to the circle. Use shadow projection and watch the length of the arrows shadows simulate the acceleration and velocity vectors of a body in simple harmonic motion. A 1cm hole in an aluminum slide in the projector helps narrow the beam of light.
An Uplifting Experience:
Use duct tape to attach a plastic garbage bag to the outlet hose of an air blower (vacuum cleaner). Have someone sit on the bag as you turn on the air. Watch the person being lifted as the bag fills with air. An alternative is to attach several hoses and have classmates blow up the bag with lung pressure. This method is sometimes used to lift automobiles in accident situations. Big total force from a small pressure over a large area.
How to construct a smoke cannon: Remove one end of a cardboard box. Attach long rubber bands to each of the remaining 4 corners. Use duct tape to attach a sheet of flexible plastic across the open end of the box. Attach each of the four free ends of the rubber bands to a knob near the middle of the plastic sheet. Cut a circular hole in the side of the box opposite the plastic sheet. Saturate the interior of the box with smoke and blow smoke rings across the room.
A simpler version of the Smoke Cannon. Use a plastic milk bottle. Whap it on the side. Nice vortex rings shoot out. Fill the bottle with methane from your gas jets. Shoot the vortex rings at a burning burner. Watch the rings ignite with nice sound action!
Better still super great Vortex Blammer:
Take a plastic bucket (10 liter) and cut a 25cm hole in the bottom. Cover the top with rubber (wet suit) material (surfers can usually supply you with old wet suits). Tie or clamp the rubber on (with hose clamp stock). It should be tight as a drum. Bam the rubber and WOW! Great vortex rings. Shoot the kids across the room. Fill it with methane, shoot the Bunsen Burner and oooohhhhhh.... NEAT! Big roaring rings of fire!
Drop two full cans of pop into a tank of water. One a can of diet pop, the other regular. The diet drink will float. The sugar content of the regular drink will increase its density sufficiently to sink it.
Take an empty pop can, put in a few ml of water. Heat over a burner until steam emerges vigorously from the opening. Quickly invert the can into a pan of cold water. Condensing of the steam is so rapid that the low pressure created allows the atmosphere to crush the can instantly. (Unless you are a real man, you'll want to hold the can in tongs). The pressure of the atmosphere is 1kg/cm2 (9.8n)/cm2.
The sugars in Karo syrup rotate the plane of polarized light and rotate different colors different degrees. Place a bottle of it on a polarizing filter on the overhead. Students looking at the bottle through their own polarizers see spectacular colors.
The trick may also be done by stretching scotch tape and layering it randomly on a glass plate. The stretching is essential to straighten out long coiled organic molecules.
The Silver Lining (or a cloud in a 4-liter jug):
Take a 4-liter jug (you may first have to consume the contents the previous weekend), swirl a few ml of water around in it and pour out the excess to raise the humidity. Introduce some smoke from a freshly blown out match for hydroscopic particles. With your mouth over the mouth of the jug (mouth-to mouth) blow hard into the jug and then release the pressure. Ah, fog in a 4-liter jug!
Or connect a rubber tube to the jug with a one-hole stopper and glass tube and blow or suck on the tube. Nice adiabatic warming and cooling.
Discuss capacity of air, absolute humidity, relative humidity, and the variance of the capacity with temperature. And do not forget the dew point.
Mix corn starch (from the grocer) with water to make a fluid paste. Pour it into a beaker and challenge students to QUICKLY punch their finger into it and withdraw. Under rapidly applied force the mixture becomes an elastic solid. Scoop out some of the fluid and squeeze it into a ball. As soon as you quit squeezing it will become a liquid in your hands. This is completely washable, so you can really get into it.
Take a commercial disectible Layden jar and charge it by holding the base in your hand and reaching toward a Van de Graaf generator with the central plate. Be sure your body is grounded or it won t charge. With a wire between the inside and outside plate, show it is charged. Great Spark! Set it on a table and with faked care lift the center plate out with an insulated rod and hand it to a reluctant student. Remove the glass and with the insulated rod hand the outer cup to another student. Ask them to touch hands. Nothing happens.
Now touch the cups to show there is no charge. Nothing happens. Reconstruct the jar and VOILA there is a spark. You can make your own jar with the bottom half of a soda can in a peanut butter jar with a steel can on the outside. The charge resides on the glass, not the metal.
Change in the Wind:
Place a dime about 3 cm in from the edge of a table. Set a can at an angle near the coin. Challenge students to get the coin into the can without touching either. Solution is to blow SHARPLY over the top of the coin. The Bernoulli effect will lift it and pop it into the can. A real expert can lift a quarter.
Milk, Motion, and Molecules:
Dilute homogenized milk with about 10 parts water to 1 part milk. Place a drop on a microscope slide, cover and view under high power. The tiny fat particles will be seen to dance continually from the collisions of the water molecules. The effect is called Brownian Motion and was crucial evidence for the existence of molecules. Einstein did the math on it.
Dissolve about 10g of erbium chloride or any rare earth chloride in a large test tube. Hold the tube in front of a showcase bulb while students look through diffraction gratings. They will see a beautiful absorption spectrum.
The Incredible Inverting Pin:
Take a plastic film container and poke a small pinhole in the bottom center. Then push the pin through the side near the front so the head is centered in the opening. Hold it up to you eye and look at the pin silhouetted in the little circle of light. It will appear to be inverted.
Take a short piece of flexible plastic hose (60 cm long) and swing it briskly in a circle. Three or four resonant frequencies can be heard. With a little practice you can play Taps on it.
Tie about 1.5m of thread or very fine wire to a coat hanger. Loop it over your head as you lean forward, and with your index fingers stick the tread into you ears. Have a friend strike the hanging hanger with a pencil, and revel in the sound produced. Ah, GreatTom of Westminster!
Take two PSSC air core solenoids. Attach lamp cord directly to the terminals of one. to the other attach the terminals from a low-watt household light bulb. Through the first solenoid place a bundle of straightened coat hanger wires. Plug the assembly into the 120 volt line. The coil will jump a little onto the iron core.
To find the pressure on a balloon, simply press the inflated balloon down onto a balance until it flattens to a circle of known radius (cm). Read the balance for force and calculate pressure. P = fA.
Two students hold a sheet by its four corners. Someone throws a raw egg into the sheet as hard as possible. It does not break. Be careful not to miss the sheet.
The is a good follow-up to the closed-tube resonance one gets from reflecting tuning-fork sound into a graduated cylinder with water in it. Just take two cardboard tubes that fit one inside the other. Adjust the total length by sliding them in and out until resonance with the tuning fork is found. Your open tube will resonate at twice the length of the closed tube.
Print DICK JANE on a card. DICK in blue, JANE in red. When viewed through a water-filled test tube, JANE appears inverted, while DICK appears normal. Let the kids hypothesise this one. The fact is that they are both inverted, but DICK looks the same either way.
Famous Archimedian problem. Ask class... A row boat has an anchor in it as it floats on a lake. When the anchor is thrown overboard, will the lake level rise, fall, or stay the same?
Collect student answers and justifications. Then try this experiment. Float a 400 or 500 ml beaker with mass in it (say 200g) in a larger beaker of water (say 2000 ml beaker). Mark the water level. Then remove the mass and place it in the bottom of the larger beaker (the lake). The lake lowers because in the boat, the anchor displaces its weight of water but in the lake it displaces its volume of water.
Get a Pom-Pon from a cheerleader. Place it on the Van de Graaf machine and charge it up. Approach the charged pom pon with a sharp pointed object. It will discharge fast. Ben Franklin found that charge accumulates on sharp points... The Lightning Rod.
Cook a hot dog by inserting nail electrodes at each end and connecting to the 120 v line. Watch out for shock hazard! it takes only a few moments to cook.
Electric Jump Rope:
Connect a long wire to the terminals on a galvanometer. Swing it like a jump rope. In the earth s magnetic field it will generate current. Swing only one loop of the wire so that the opposite loop won't cancel the current.
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