Exploratories

Super Bounce With Super Balls

Problem:

The instructional purpose of this demonstration is to show the conservation of energy and momentum.

Materials

A large super ball (the one used had a mass 3 times the smaller), a small super ball and a ping pong ball.

Procedure:

Take the balls one at a time and drop them. Observe the height to which they rebound. Now take the two together, placing the small one on top of the large one and let them fall to the floor. (This takes a little practice because the small ball must ride down on top of the large one.)

Have the students explain why the balls behave as they do. As an option, repeat the above activity but place a ping pong ball on top of the second super ball--have the students predict what will happen.

Because of the unpredictable speed and direction of the rebounding ball, it is
necessary to supervise this activity closely. It is recommended that one set of super
balls be used at one time in a classroom. Warn nearby students to be ready to
protect themselves.

Options:

A basketball and a softball, or a basketball and an egg may be easier to control and
allow better visibility for a larger class. A commercial toy is the Astro blaster. A series of 6 stacked super balls of varying size that have a plastic rod through their centers. The smaller balls on the top are free to leave the rod.

Calculations:

Assume the large ball is three times the mass of the smaller ball and total system PE before dropping is equal to the total system PE after rebound. (This assumes a perfectly elastic collision.)
If all of the original potential energy is transferred after the rebound to the small ball, then only 1/4 of the original total mass must absorb all of the original potential energy. This would demand that the small ball rebound four times higher than the original height of the two balls. This ratio would be true only if the heavier ball is three times the smaller ball.