by Lois Nicholson

former teacher of high-school advanced-placement chemistry and physics
from a presentation at the Science is Fun! Symposium
at the 1996 annual meeting of the American Association for the Advancement of Science in Baltimore, Maryland

Balancing Nails on a Nail

Pound a large nail far enough into a board so that the nail stands securely upright. Lay a second large nail on a flat surface and place other nails across this nail, head to head as shown above. Finally, place another nail on top of this assembly, head to tail with the second nail. Carefully pick up the assembly and balance it on the upright nail.

In a gravitational field, any object is most stable when its center of mass is as low as possible. The center of mass of the nail assembly is below the point of support and at its lowest when the assembly is balanced. If the assembly swings to the side, its center of mass rises. Gravity that exerts a restoring force to bring the assembly back into balance.

Rolling Coin in Balloon

Place a coin in a large balloon, and then inflate and tie off the balloon. Swirl the balloon rapidly to cause the coin to roll inside the balloon. The coin will roll for a very long time on the smooth balloon surface. At high coin speeds, the frequency with which the coin circles the balloon may resonate with one of the balloon's "natural frequencies," and the balloon may hum loudly.

Racquet Ball Conserves Energy!

With a sharp knife or razor blade (caution!!!), slice a racquet ball into two halves. Trim each half so that it is slightly smaller than a hemisphere. Turn the he hemisphere inside-out and drop it, bulge-side-up, on a hard surface. The ball will snap and rebound to a height much greater than that from which it was dropped.

Work is required to turn the hemisphere inside-out and this work is stored as potential energy. As the dropped ball hits the hard surface, this potential energy is released and converted to kinetic energy, allowing the ball to rebound to a greater height.

Dropping a Light Ball on Top of a Heavy Ball

Simultaneously drop a light ball (such as a ping pong ball) resting on top of a much heavier ball (such as a superball or golf ball). If the balls are properly aligned, ideally the light ball will rebound to nearly nine times its original height. If three stacked balls each considerably heavier than the one above it--are dropped, ideally the lightest ball rebounds to nearly forty-nine times its original height. If two stacked balls with mass ratio of 3:1 are dropped (approximated by a baseball on top of a basketball), the bottom ball will remain dead on the floor ant the top ball will hit the ceiling.

The explanation for these phenomena involve conservation of momentum and kinetic energy. Further details can be found in physics texts or Turning the World Inside Out, by Robert Erlich, Princeton University Press, 1990.

Balancing a Ball with a Hair Dryer

A light ball, such as a ping pong ball or Styrofoam ball, can be balanced in the air stream of a hair dryer. According to Bernoulli's Principle, the pressure in the fast-moving air stream is less than the pressure of the surrounding quiet air. If the ball strays from the air stream, the surrounding higher pressure air tends to push it back.

Dinner Table Optics

Use a filled round-bottom wine goblet as a lens to focus the light from a candle or from the filament of a chandelier bulb onto a wall. How does the image on the wall compare to the original object?

Look carefully at the world through the wine goblet. Then look carefully at the world through a beer mug. How are the optics of a wine goblet and beer mug similar? Different?

Use a large shiny spoon as a mirror and compare your image in the bowl and back of the spoon. Observe carefully the image of your pointed finger as you move it toward the bowl of the spoon until it touches the spoon.

Deli Optics

Fill a large test tube or tall narrow pickle or olive jar with water to serve as a cylindrical lens. Hold the lens above a piece of paper on which the words "DICK" and "JANE" have been written in bold capital letters. Hold the jar horizontally in front of your eyes and look at the world through the jar. While holding the jar in this position, have someone else look at your eyes through the jar!

Film-Canister Optics

Make a small pinhole in the center of the bottom of an opaque film canister. About halfway up the can, push a pin through the side of the can from the inside, so that the head of the pin is on the inside of the can directly above the pinhole in the base. Look into the open end of canister toward any bright source of light and adjust the head of the pin until it is in line with the light coming through the pinhole in the bottom. You should see the pin head inside the canister, but it seems to be pointing in the opposite direction!

The view of the pin head is different from ordinary image production by a pinhole, for in that case the object and the image are on opposite sides of the pinhole. What you are seeing here is the shadow of the pin head, which is right side up your retina, superimposed on the scene from beyond the pinhole, which is inverted on your retina. Your brain interprets the messages from your retina by turning them upside down, making the image from beyond the pinhole look right side up and the shadow of the pin seem upside down. A thorough explanation of this phenomenon is given by Curt Gabrielson of The Exploratorium, in The Physics Teacher, September 1993, p 380.

Flashlight Fiber Optics

Make two small holes in the cap of a small mustard jar, diagonally opposite each other. Attach the jar to a flashlight, bottom end of jar against the flashlight lens, using several layers of duct tape. Make sure that the glass walls of the jar are completely covered with tape so that no light comes through the side of the jar. Fill the jar with water and cap it, turn on the flashlight, and turn out the lights. Allow a thin continuous stream of water to pour out of one of the holes in the cap (the other hole allows air into the jar). The light will be captured within the stream due to total internal reflection. The stream will not be visible from the side and the light will only be seen when the stream breaks up or hits something.

Lightning in Your Mouth

Turn out the lights and allow your eyes to become accustomed to the dark. Then, while looking into a mirror, pop a wintergreen Life Saver into your mouth and chew. Charge separation is produced as the candy fragments, and the discharge produces light. This phenomenon is called triboluminescence and can be observed with other flavors of hard candy--or even sugar cubes, but wintergreen flavor seems to work best.

Coat-Hanger Chimes

Tie the middle of a 3-foot long piece of thread to the hook of a cost hanger. Wrap the ends of the thread around the second finger of each hand, stick these fingers in your ears, and allow the hanger to bump against a rigid object. Big Ben never sounded this good!

Non-Newtonian Fluid

Make a thick slurry of cornstarch and water. Unlike ordinary liquids, the slurry will flow easily under a small pressure but resists moving when a sudden or strong force is applied. (Maybe there's a lesson in this about human nature!)

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