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Q & A: Superball

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Most recent answer: 10/22/2007
Q:
The superball is a solid hard rubber. If it does not deform much, why does it bounce so high? Where does the energy come from?
- Julia (age 14)
London
A:
Julia-

That's a really nice question. We always love it when somebody tries to figure out how different things can make sense together, instead of just collecting statements.
You're absolutely right that the amount the superball deforms is the key question. When you say that it doesn’t deform much, though, the thing to wonder about is ’compared to what?". When the superball bounces on a hard floor, the superball still does almost all the deforming, not the floor. That means that the energy that had been kinetic energy of the ball goes mostly into internal energy of the ball, not the floor. Superballs are made of very springy (elastic) polymers, so that most of that stored spring potential energy can come back out as kinetic energy- the same as for an ordinary spring. For less elastic material, most of the energy dribbles away into internal vibrations, heating up the ball.

Your basic idea becomes very important when the ball is hard compared to the floor. If you drop a steel ball on a wood floor, I bet that most of the deforming is done by the the floor, not the ball. Then how high the ball bounces depends more on the floor’s elasticity than on the ball’s.

Mike W.

(published on 10/22/2007)

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