# Q & A: Bouncing Balls - It’s All About Forces

Q:
Does force effect the bounce of a tennis ball?
- Evan (age 14)
Central middle school, PA
A:
Evan -

It sure does. Newton's first law of motion says that anything keeps going at the same speed in the same direction unless something exerts a force on it. So a tennis ball can't bounce unless something - a racquet, the ground, a wall- exerts a force on it.

Now you might wonder why some balls bounce more than others. Tennis balls usually bounce well, but lumps of clay usually don't bounce much at all. You could describe the reasons in terms of forces, but it is actually easier to describe in terms of energy.

When a nice springy tennis ball hits a wall, it squashes like a spring. The kinetic energy that it had when it was moving mostly goes into a type of potential energy of the squashed tennis ball. For a tennis ball, that gets a little complicated to describe, but
for a bouncy solid superball, the energy goes into compressing, stretching, and bending spring-like molecules. As the ball springs back to its round shape most of that energy comes back into the ball's motion. Some goes away as sound, or into slightly heating up
the molecules in the tennis ball.

When a lump of clay hits a wall, it also squashes. However, a squashed lump of clay doesn't spring back. As the molecules in the clay slid past each other, while it squashed, the friction between them heated them up. Most of the energy got dumped into heating up the clay, leaving little for its return bounce.

Mike

(published on 10/22/2007)