# Why do Tennis Balls Lose Their Bounce?

Q:
I need to know why tennis balls loose their bounce when removed from their vacuum can.I can prove that this happens, but need help with the WHY part. Any Help? Also any source that might help? thanks,
- Michael (age 13)
Lost Mountain Middle School, Acworth, GA, USA
A:
Michael -

The reason that tennis balls bounce in the first place is because the air on the inside of the ball pushes outwards. When the ball hits the ground, the side of the ball squishes inwards and the air on the inside of the ball pushes out again, and that's what pushes the ball back up off the ground.

Tennis balls (which are different from some other types of balls, like racquetballs) are made so that there's actually more air pressure on the inside of the ball than on the outside. The air pushes on the inside of the ball with a force of about 27 pounds per square inch, and on the outside with a force of 13.7 pounds per square inch.

The container that the balls are stored in is also pressurized to the same pressure as the balls. (The can is actually a "pressure can ," not a "vacuum can." A vacuum can would have less air in it, while a pressure tube has more air in it than it would if filled with ordinary atmosphere.) Because there's the same pressure of air in the can as in the balls, there's the same amount of air pushing on the inside of the balls as on the outside.

Once you take the balls out of the can, there's more air pushing on the inside than on the outside, and a little bit of that air actually manages to push its way through the outside of the ball. So as the ball gets older, more air escapes from the inside and there's not as much air pushing on the inside of the ball when it bounces, so it doesn't bounce as well. [see below]

Thanks to the for their help on this question!

-Tamara

(published on 10/22/2007)

## Follow-Up #1: Tennis Ball Physics

Q:
why do tennis balls loose their bounce?
- lavoisie tyson (age 15)
jacksonville,florida,united states
A:
The previous answer tells part of the story As a ball ages, it leaks air. With less air inside it squashes more before it has pushed enough to start the ball upward again.   Why should that make it not bounce as high? For that we have to first explain why the ball loses a little height on each bounce even when it's new.

To get back to the starting height, the ball needs to have all the energy it started with. But on each bounce it loses a little. You can hear the bounce, which tells you that some energy left as sound.  Even more important, as the ball squashes, the rubber molecules in it sort of slide past each other, and that heats the ball up the same way sliding friction always heats things up. So some energy gets lost as heat.

As the ball ages, it squashes more. That means that there's more chance to lose heat to the molecules inside. So it doesn't bounce as high as it used to.

-Chris and mbw

(published on 03/23/2010)

## Follow-Up #2: tennis ball pressures revisited

Q:
A little confusion here. Per wikipedia: Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used. Two atmospheres would be (2 x 14.7) or 29.4 psi The article does not state whether this is gauge or absolute - but convention would mean gauge - so the pressure difference would be approx. 29.4 delta psi - not the little bit as stated in the first answer. This makes sense because a delta pressures stated above would not give any bounce at all!
- Richard (age 55)
Denver, CO
A:
Richard- Thanks! We've fixed it, I think,

It turns out that the correct pressures were there in another answer from our same volunteer, and we'd missed the discrepancy. It turns out that the two atmospheres is the absolute pressure, not the increment.

Mike W.

(published on 09/10/2011)

## Follow-Up #3: Re-pressurizing tennis balls

Q:
If I was to place a bunch of tennis balls that had lost their pressure into a tube or container that I could pump up to a higher pressure than the balls would the pressure in theory go back into the balls and make them "like new" again?
- james brown (age 37)
UK
A:

Hello James,

Yes, one can re-pressure tennis balls.  You need to place them in a container with about 14 psi relative pressure and leave them there for a few days.    There are several commercial products available that will do the trick.  It's probably easier to buy one than make one yourself.

If you type tennis into the PVan search box you can find a raft of tennis related questions and answers.

LeeH

(published on 12/11/2014)

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