Factors Affected a Bouncing Tennis Ball

i am doing a science fair project on how external forces affect the way that a tennis ball bounces. Some of my ideas include: freezing it, heating it up, soaking it in water, and boring holes in it. i was wondering if you had any other ideas for external forces and if there should be a large difference in the heights that the tennis balls bounce based on the forces I have mentioned.
- laura (age 13)
Florida

In order to understand why a tennis ball bounces, a few things must be mentioned. When you throw or drop a tennis ball to the ground, it has a certain amount of kinetic energy when it hits the ground (this is energy of moving objects). When it hits the ground, some energy is transferred into the ground or the air as heat or sound. The larger the energy left in the ball, the higher it will bounce on the way back up. One major reason why a tennis ball rebounds is because when it hits the ground, the ball is contorted into an elliptical (oval) shape.

The ball is most stable in a round shape, so the gas inside expands to push the ball back to form the round shape. This forces the outside of the ball to push out and therefore bounce back up. For this reason, boring holes in the ball will decrease the bouncing ability because the gas will escape when hitting the ground and won't be able to reform the shape of the ball as easily. Also, the holes will decrease the stability of the shape of the ball, so it won't bounce as high. When you freeze a ball, the outer shell turns very hard, which in most cases will result in a lower bounce since the ball is not as elastic as it was before freezing. The ball therefore does not change formation and will not bounce high. Heating will not greatly change the bouncing because it will not greatly change the composition of the shell until it gets to melting temperature. Heating it can affect the gas inside of the ball though. Finally, soaking the ball in water will decrease bouncing height because the water which is absorbed by the "fur" will move around and absorb kinetic energy when the ball hits the ground, leaving less energy to help the ball bounce back up.

I hope these lengthy answers will provide you with a sufficient background to do your project. The only other factor I could suggest would be to shave the yellow hairs off the outside of the ball. This might make the ball bounce a bit higher because the hairs won't be able to absorb energy. For further research, look up completely inelastic collisions (which would be the case if a ball didn't bounce back up at all) and elastic collisions (if the ball bounced back to its original height).

Finally, the best thing to do when you have questions like this is to have a grown-up help you try it for yourself and see what happens.

(published on 10/22/2007)