Postby bert_the_turtle » Tue Jul 16, 2013 10:55 am
NeatNit, since one of your troubles seems to be that the 1/2, 1/4... subdivision of the process is arbitrary, try this different problem:
Consider an idealized bouncing ball in a vacuum (no air friction). Assume the bounce is instantaneous, it skips the compression/expansion phase. But it is not lossless: each jump reaches only sqrt(0.5) times the height of the previous jump (which means the air time will be half of that of the previous jump). The ball is also allowed to rest still on the ground.
Drop the ball from 1.25 m height. After 1/2 a second, it will hit the floor (let's arbitrarily define that as our zero point in time, t=0s) and bounce back up, reaching only, umm, about 0.8 m height, and hit the floor 1/2 s later. The next jump will reach 0.625 m and last only 1/4 s.
So, summing up, the first jump finishes at t=1/2 s. The second jump at t=3/4 s. The third jump at t=7/8 s. And so on, the n-th jump concludes at 1-0.5^n. What happens at and after t=1 s?
Well, obviously, the ball is not jumping any more at t > 1s. Each jump demonstrably concludes before t=1 s. So it must be resting on the ground. (*) When did the transition between jumping and resting happen? Again, obviously, at t=1.
And now, the really important question is: How did the transition happen at t=1?
Sub-Question: Was there a last jump that ended at t=1?
Clearly, there must have been, because at t<1, the ball is jumping and at t>1 it is resting. So it stopped jumping at t=1. So there was a jump that ended then.
But equally clearly, there can't have been. All jumps end before t=1, none ends precisely at t=1.
So.... now what?
And before you claim that the model is incomplete and insufficient: Yes, it is. However, a full model, when you afterwards take the limit to infinitely fast bouncing without compression, yields precisely the same motion. The full model gives one result you would need to wrestle from the simplified model: the speed of the ball at t=1 s is zero. You are allowed to use that.
*: You may be tempted to evade by saying the ball was blown up by the increasing frequency of impact shocks. You would be wrong.Yes, their frequency approaches infinity, but their magnitude approaches zero in such a way that any average containing more than one shock has a finite average force bound by about twice the ball's weight. If you want to blow up ideal bouncing balls in thought experiments, put one between two mercilessly converging plates. Or trains.