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Linear momentum conservation vs mecanical energy conservation

  1. Sep 7, 2011 #1
    A ball A (mass ma) with initial velocity v colides with a ball B (mb) initially stopped. A and B gets the same direction/velocity v', Calculate v'

    By linear momentum conservation
    ma.v = (ma + mb).v'
    v' = mav(ma + mb)

    But by mecanical energy conservation

    ma.v²/2 = (ma + mb).v'²/2
    v' = v (ma/(ma + mb))^(1/2), which is wrong

    Why we can't use mecanical energy conservation, is there a energy dissipation?
  2. jcsd
  3. Sep 7, 2011 #2


    User Avatar
    Science Advisor

    Well, for A and B to get the same direction/velocity v' after the collision, don't they have to stick?

    Does that sound like an elastic collision?
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