Michelle Goyette, age 11, of Fairport, N.Y., for the questions

How does a rubber ball bounce?

If you drop a rubber ball onto a hard floor, it will bounce up and down several times. Each bounce is a little less high until the up and down activity finally stops. If you give the ball a little tap each time it bounces up, the up and down activity will go on indefinitely. But if you drop or even smack a rubber ball into a bed of sand, it will not bounce back at all.

The bounce in the ball has something to do with the nature of the rubber and a hard surface. Most important, energy is involved. It may be the pulling energy of the earths s gravity and the pjLt or smack you give to the ball as you send it down.

You can understand something of the nature of rubber by stretching a rubber band. You pull until it is, say, twice its normal size and then let go. The rubber springs back to its original size and in the process it gives you a little sting as a bonus. This ability to stretch and spring back into shape is called elasticity. It operates when a ball bounces, though too fast for your eyes to see.

You may pat the ball down to the ground, or let it fall. If you let it fall, the force of gravity will pull it, down. If you pat it, you add some energy to this pull. In any case, the falling ball gathers up a certain amount of energy. It tries to use this energy to keep on going but it hits the solid ground.

For the fraction of a second, the ball tries to go down through the ground. It tries to move that solid surface out of the way. But this cannot be done. Instead, the elasticity of the ball comes into play. The part of the ball near the ground is pushed flat, out of shape.

In a moment, the elasticity of the rubber springs the ball back into its original shapes The squashed part of the ball uncoils.

This sudden uncoiling turns the energy in the ball back into the opposite direction. Up it flies into the air again. If left alone, it will soar up until the force of gravity pulls it down again. Each time it rises and falls a little of the original energy is lost until the ball finally comes to rest. If you pat it each time it rises, it gets anew spurt of energy to make up for the loss.

When the ball falls into a bed of sand it succeeds in moving the surface out of the way. It spends its energy in digging into the sand, for the sand is soft and movable. If the bail merely falls into the sand, it will dig in only a little way for it has only the energy from the earths gravity. If you hit it down hard, it digs in deeper because it has the energy from gravity, plus the energy from the mighty muscles in your arm.