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Homework Help: Elastic collision speed and time problem

  1. Oct 1, 2007 #1
    A 0.45 kg ice puck, moving east with a speed of 3 m/s, has a head-on collision with a 0.9 kg ice puck. Assuming a perfectly elastic collision, what will be the speed and direction of each object?

    Alright, so the first time i go through this problem i solve it, no problem. the answer is 1 m/s west and 2 m/s east and thats also the answer in the book. but then i go back and i realize something weird. i set up the momentum equation like normal so that it's
    0.45*3 = 0.9v1 + 0.45 v2
    and then you can make it
    0.45*3 = 0.45*2v1 + 0.45 v2
    divide both sides by 0.45
    3 = 2v1 + v2
    and you get that but the values determined for the speeds dont match up with that. What's going on?
  2. jcsd
  3. Oct 2, 2007 #2


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    they match. If v1 is 2m/s east and v2 is 1m/s west... v1 = 2. v2 =-1.

    3 = 2(2) + (-1)
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