Hey Bug! Natalie K. from Illinois wants to know: What makes a bouncy ball bounce?

When you drop a ball, the ball pushes down on the floor and the floor pushes back up on the ball. (Weird!) In slow motion you would see that the side of the ball squishes or dents when it hits the floor. According to physics professor Lou Bloomfield, the energy of the falling ball is now stored in the dented ball. The ball then uses that energy to push back on the floor. It undents—and bounces.

The ball doesn't bounce back as high as it fell because it cannot store all the energy it had when it hit the ground. Some of that energy is lost to heat. That's why a basketball warms up when you dribble it. Bounciness depends upon what a ball is made of and how it's put together. In a “Bounciness Olympics,” superballs would take the gold. Made from a special hard rubber, superballs return to their original shape when dented without losing too much energy. Most hard surfaces store and release energy better than soft ones. That's why beanbags don't bounce very well and why rubber balls bounce higher on cement than on carpet. Now, when you're told the living room is a bad place to play ball, you'll know why.

Activity

1. Does a ball gain or lose speed as it bounces? Why? Explain your answer in a sentence or two.
[anno: A ball loses speed as it bounces because some of the energy is lost each time it bounces. Some of the energy is lost because the ball dents. Some of the energy is lost as the floor absorbs energy.]
2. If you wanted to speed up the motion of a ball after it has bounced, what might you do?
[anno: Answers may vary but could include hitting the ball again.]
3. Think of a tennis game or a ping pong match. Describe how the motion of the ball changes after someone serves the ball. How does the motion change after someone hits the ball back to the server? Is the ball being pushed or pulled? What role does friction play? Describe the motion of the ball in a few sentences.
[anno: When a person serves the ball, the person applies a force to the ball that pushes it towards the other player or other side of the court. The ball hits the other side of the court and loses some energy to the court surface and to friction. Then the other player hits the ball and applies a force to the ball that pushes it back to the server.]