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 Wilson Sporting Goods Company
for their help on this question!
(published on 09/10/11)