# Bouncing Objects

Q:
We did an experiment where you drop a tennis ball and a foam ball onto different sufaces from a height of 2 meters, I would love to know the answers to 3 questions: 1.Why do they fall? 2.Why do they stop bouncing? 3.Why do you get different results(heights) for different surfaces?
- James (age 13)
Kyneton, VIC, Australia
A:
1) All objects fall due to gravity. There is gravity between every particle of matter. The equation for gravity is G*M*m/R^2 where G is the universal gravitation constant, M & m are the masses of the objects , and R is the distance between the centers of the objects. Even though G is very small (G=.000000000067 in MKS units), and the radius of the earth is pretty big (about 6.4 million meters), the mass if the earth is so big (M=5980000000000000000000000 kg) that when you multiply it all out the force of gravity on a ball is enough to make it accelerate toward the earth quite rapidly.

2) The reason they stop bouncing is because they are loosing energy. When you lift them up, you are giving them potential energy. As they fall, the potential energy changes into kinetic energy. While the object falls and as it moves back up after a bounce, the air is trying to slow it down. That’s like trying to walk through the water in a pool or beach. It slows us down. Also, when the object bounces, it’s temperature goes up. You hear a sound as it bounces. And the object vibrates a bit. All these things take energy away from the object. Eventually, it will not have enough energy to go back up into the air.

3) The main reason for the differences you see are that the objects are made of different materials. For example, a rubber ball bounces better than a plastic ball. If you bounce a ball on hard surface, like the floor, it will bounce better than on a bed. This is because the springs in a bed help absorb the energy from the ball by moving while they are touching. Also, some things don’t move as well through the air. A feather falls very slowly because the air slows it down quickly.

I hope this helps you out.