Balancing Nails on a 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.
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.