Why Does a Ball Bounce?
There are two kinds of balls that bounce, those that are solid, like a solid rubber ball or a golf ball, and those that are hollow, like a tennis ball.
No matter whether the ball is solid or hollow, its bounce is due to the fact that it is elastic. A thing is said to be elastic when it tends to return to its former size or shape after being pressed down or stretched. Rubber is a very elastic substance, as you know. Press a rubber ball on the ground. It loses its perfect roundness, but the instant you take away the pressure of your hand, it springs back into shape. Throw it sharply on the ground; it loses shape against the ground but springs back so sharply that it bounces.
Now a great law of science is that nothing is ever lost and that everything has to be paid for.
When the ball starts bouncing it has a certain amount of motion in it, which is force, or power, or energy. When it stops, that has gone. Either we must show that the energy has gone somewhere and has not been destroyed, or, according to the great law of the persistence of power, the ball should bounce forever. If it did not bounce forever, the law would be false. It is, however, quite easy to show that the ball does lose the power with which it started. What, then? Why does it stop bouncing? And what happens to the energy when it stops?
To begin with, the ball is moving, both up and down, through the air, and forcing millions of particles of air aside. All the motion it gives to the air it loses.
If a ball were bounced in a space as far as possible emptied of air, it would bounce far longer than it does in the atmosphere, just as a top will spin longer in the same circumstances. Suppose that, instead of bouncing the ball on something hard, we bounce it on a pillow or on loose sand. It will not bounce long in such a case. Its power has gone to move the pillow or the sand as well as the air. The ball itself is not completely elastic, nor is the ground. If the ball and the ground were completely elastic, and there were no air to move, and the ball never turned and rubbed the ground in falling, it would bounce forever.
In the case of a hollow rubber ball, it is not by any means the rubber only that explains why the ball is so elastic. The ball is filled with a mixture of several gases, which we call air. The air is elastic. It is pressed in upon itself when the ball hits the ground, but quickly tends to go back to its old space in the ball. We can see how much the air bounces if we compare an ordinary soft rubber ball with another one which has a small hole in it.
The air is expelled from the hole when the ball is bounced, and we find that the ball bounces very little, because its elasticity is so poor. But the other ball bounces exceedingly well, because, when it is bounced, the air in it is not squeezed out, but only compressed for an instant.