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Elastic collision HW problem

  1. Dec 5, 2011 #1
    An "infinitely massive" Ping-Pong paddle moving with a velocity of 20 m/s hits
    a "massless" Ping-Pong ball at rest. The collision is elastic. What is the velocity of
    the ball after the collision?

    i get v2f = (2(infinite mass)/(infinite mass - 0))*20m/s = 40m/s

    is this correct?? i dont know why im doubting myself.. its just a weird question lol
  2. jcsd
  3. Dec 5, 2011 #2


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    Homework Helper

    Correct answer. I don't follow your calc, though. Usually it just isn't possible to calculate anything sensible when there is an infinite quantity involved.

    This question is really about frames of reference. How is it different from a ball moving at 20 m/s relative to a stationary wall?
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