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Homework Help: Calculating Final Speed in Elastic collision (momentum)

  1. May 22, 2012 #1
    1. The problem statement, all variables and given/known data

    If it is eg. two balls, both going to the right before and after an elastic shock, how do you determine their velocities after the collision? (speeds before and weights are given)


    M = 50 kg
    u1 = 20 m / s

    m = 100 kg
    v1 = 10 m / s

    (M, m = mass)
    (u1, v1 = velocity before the collision)
    (u2, v2 = velocity after collision)

    2. Relevant equations

    The law of conservation of momentum:
    m*v1+M*u1= m+M*v2*u2

    Momentum before = Momentum after

    3. The attempt at a solution

    m*v1+M*u1= m+M*v2*u2 →

    100 * 10 +50 * 20 = 100 +50 * v2 * u2

    How do I continue from here?
    Last edited: May 22, 2012
  2. jcsd
  3. May 22, 2012 #2
    In elastic collisions energy is also conserved , so you can set up one more equation .
  4. May 22, 2012 #3
    Oh, right. Think "momentum before equals momentum after" is what you meant.
    Forgot to put it there.
  5. May 22, 2012 #4
    In elastic collisions :

    Momentum is conserved

    Energy Is conserved

    In inelastic collisions:

    Momentum Is conserved

    Energy is not conserved

    Just to be more clear :P
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