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)
(published on 10/22/2007)
