benzado

Tennis serves

benjaminapple:

At no point does energy transfer from the wall to the ball, because the wall is inelastic and not moving. Right now we’re not buying into Einstein’s relativity thing yet, so we share a reference point with the wall.

Actually, since we’re talking classical mechanics, it’s Galileo’s principle of relativity. (Einstein just generalized it to other things, like electromagnetism.) Quantities like energy and momentum are frame dependent. If you use the perspective of the ball, the wall is the one with all the kinetic energy.

That’s what I mean when I say it doesn’t make sense to say no energy transfers to the wall. “Energy” is just part of the model we’re using to explain what is happening. Pick a frame to make the math easier.

Anyhow, remember that the ball is highly elastic. We’re not hitting a rock. If we were hitting a rock, cool, I get it. But at the moment at which the ball’s velocity hits zero (during the player’s return swing), it has a LOT of potential energy stored in the form of compression, just like when it’s bouncing against a wall.

I think you are overestimating how much potential energy can be stored in a tennis ball by deforming it. Most of it is used to push the racket backwards.

Sorry, did you think I was trying to trump something? I’m trying to understand it.

Fair enough. But you were arguing that it seems like it should be easier to hit a ball faster on return than on serve. We have evidence that it isn’t.

I’ll admit, my handle on this stuff isn’t good enough to explain it very well.