How bouncy a ball is depends on the fraction of the energy which is
lost in the collision between the ball and the floor. For very hard
balls, this depends at least as much on the floor as it does on the
ball.
Balls with more air pressure in them bounce better because air,
when compressed, will uncompress (spring back) with little or no energy
loss, while the rubber the ball's made out of isn't quite as nice. When
the rubber flexes, it heats up and makes a noise, dissipating energy. A
ball that has higher air pressure in it will not squish as much during
the collision, and so less energy will be lost.
A hot ball will usually have more air pressure in it than the same
ball, colder (unless it's leaky), because air likes to expand when hot.
That's one reason it's bouncier. A second reason is that the rubber may
be less stiff when hotter, and dissipate less energy when it squishes.
If your ball gets very very cold (like a racquetball dipped in
liquid nitrogen), it may even shatter into lots of pieces when it hits
a hard floor. Not too bouncy. But if you don't throw it so hard at the
floor, it may bounce instead.
Now if your ball is made out of something solid, the effects
depend on the materials. If the ball is made of rubber, then the
stiffness argument above would incline me to predict that the ball will
still be less bouncy when cold.
Some balls are not very bouncy to begin with, like squash balls.
Some kinds of stiff materials do not dissipate energy very much, like
steel. A steel ball on a steel floor is amazingly bouncy. But drop it
on an unvarnished wood floor and it will just go thud and make a dent
in the floor. You might be able to make a squash ball "bouncier" by
cooling it to liquid nitrogen temperatures and dropping it on a hard
but springy floor. Don't drop it from so high that it shatters or even
cracks, but my guess is that while a squash ball may become less bouncy
as it becomes colder, it will then become more bouncy as it freezes and
turns into a rigid solid. Then it probably depends on how springy the
floor is.
Tom
(published on 10/22/2007)
