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Q & A: Why do basketballs bounce?

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Q:
why and how do basketballs bounce?
- maree (age 12)
carlingford, syndney
A:
Maree -

Awesome question! Basketballs (and other types of hollow balls) bounce because of the pressurized air inside of them.
When you drop a basketball, the first thing that affects it is gravity, which pulls it straight towards the ground. Gravity makes the ball accelerate (get faster and faster) as it falls, so when it gets close to the ground, it's going pretty fast. As it hits the ground, there's also a force between the wall of the ball and the ground. The ball pushes down on the ground and the ground pushes up on the ball. The ball compresses a little as it squashes, and the energy it picked up as it fell mostly goes into compressing the air inside. The extra air pressure pushes against the bottom of the ball, making it push harder against the ground. So the ground pushes back equally hard, and the ball bounces back up. The energy that had gone into compressing the air mostly comes back into the ball's motion as the air expands again.

If there's not enough air pressure inside, most of the energy just goes into bending the molecules in the rubber of the ball. Unless a ball is made of special rubber (like a superball), most of that energy doesn't get back out, but just heats the ball up. To get more pressure, you just add more air (by using a bike-pump, for example). Then, there's so much air squished into the space inside the ball that it pushes out really hard, and keeps the walls from flexing too much. (You can think of this like a balloon. The more air you blow into it, the harder the air is pushing on the sides of the balloon, so the stiffer it feels.)

-Tamara (w Mike)

(republished on 07/11/06)

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