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Q & A: Materials for bouncy balls

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Most recent answer: 10/22/2007
Q:
what materials make balls bounce the best and from what height?
- Jonny
Minnesota
A:
A ball bounces the best when it springs back after being compressed, trading the potential energy it gets when squeezed back again for kinetic energy of motion. Balls that bounce the least well absorb some of the energy and heat up or make a noise.

Good materials for bouncy balls are rubber and substances like rubber (like the stuff they make superballs out of, for instance). You can make a nice bouncy ball by inflating a hollow rubber ball with air at high pressure. Air is springy -- squeeze it and it pushes back, absorbing very little of the energy. Tennis balls, racquetballs and basketballs are examples of bouncy inflated rubber balls. If the ball is not fully inflated, it can be less bouncy. The reason for this is that the rubber walls deform and change shape more, heating up and making a noise, absorbing more energy, when the ball doesn't have enough air pressure inside.

Stiff balls that do not deform much also can be bouncy on stiff surfaces. Ping-pong balls are an example, and are made of stiff plastic. A solid steel ball bouncing on a solid steel floor is remarkably bouncy. But if the floor is soft (say, made of unvarnished wood or something even softer), then the steel ball may just fall with a thud.

The bounciness of a ball depends on what the ball and the floor are made out of (and how well inflated the ball is), and does not depend on the height. A ball should bounce to some fraction of its initial height, and this fraction shouldn't depend too much on the initial height. If you drop a fragile ball from too high on a hard surface, it might tear or break, though.

Tom

(published on 10/22/2007)

Follow-Up #1: bounciness and air resistance

Q:
"The bounciness of a ball depends on what the ball and the floor are made out of (and how well inflated the ball is),and does not depend on the height." I suppose itís a matter of how you define "bounciness", but this statement doesnít seem to hold true for air-filled balloons, which donít bounce very high because they donít develop enough momentium when dropped to overcome their air resistance, or heilum ballons which donít bounce at all when dropped, but just rise.
- Mike (age 42)
New York, NY
A:
Hi Mike,

Good job thinking critically about a complex situation! That's the most important part of being a good scientist.

We normally think of the bounce and subsequent motion separately. An object that can only travel with difficulty through the air won't seem to bounce very well because it won't move very well after the bounce. But in a vacuum both of these balloons should bounce just fine (assuming they don't explode). Helium-filled balloons also fall with the same accelerations as rocks, as long as there is no air around them to supply a buoyant force. And they might bounce better (but then again, that depends on the material the floor's made out of).

Good job thinking!

Tom

(published on 10/22/2007)

Follow-up on this answer.