I know tennis balls go flat (even if all they do is sit there) because the interior of the ball is actually pressurized above normal atmospheric pressure, and over time some of the air leaks out.

The question is, why aren't the balls manufactured with some material lining the interior that's properly airtight (creating balls that should never go flat)? Do the rules of the game prohibit that sort of thing? Or is it more that manufacturers would rather keep selling you new balls? Or something else?

  • Aren't most balls( tennis balls, footballs, volleyballs, basketballs,... )not airtight? So I'm guessing there's a simple explanation for it, and not being a rule. BTW: most tennis balls are apparently filled with nitrogen rather than air. Only balls that are airtight (and not solid) that I can think of are pingpongballs. And they break rather quickly.
    – Don_Biglia
    Jan 30 '15 at 13:51

Tennis balls are mostly airtight. They're made of rubber covered with felt; the rubber is quite good at keeping air in, but not perfect. Some of the answer as to why they are not more airtight is due to cost (it would cost more), some is tradition (tennis balls are largely built the same way they always have been, at least in a long time), and some is physics (many of the things you might do to make them completely airtight would alter the rigidity of the ball). Realistically a rubber ball is so good at keeping air in and is so cheap that it's not really worth the extra cost to make it last longer.

This is a difference in quality of ball, though: the much nicer, and more expensive, balls are less leaky than the cheaper balls you find at Wal-Mart. I've also seen that some are filled with Sulphur Hexaflouride, which is a very heavy, inert gas; being heavy means it doesn't leak out nearly as fast as Nitrogen would (the other major option).

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