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Velocity from an elastic collision

  1. May 13, 2017 #1
    1. The problem statement, all variables and given/known data
    A body X moving with a velocity v makes an elastic collision with a stationary body Y of equal mass on a smooth horizontal surface. Which statement gives the velocities of the two bodies after the collision? (multiple choice question)





    3. The attempt at a solution
    The answer according to the mark scheme is
    velocity of X = 0
    velocity of Y = v



    So I can't understand how this is worked out. But this as far as I got:

    MUx+MUy = MVx + MVy
    Ux +Uy = Vx +Vy
    Ux = Vx + Vy

    How do you know that Vx is 0 and Vy is V (or Ux)?
     
  2. jcsd
  3. May 13, 2017 #2
    You've used the conservation of momentum which is true for all collisions. Since the question specifies that it's an elastic collision, you know that kinetic energy will be conserved. Try including that in your equations.
     
  4. May 13, 2017 #3
    Thank you, I understand now!
     
  5. May 13, 2017 #4

    haruspex

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    When you have conservation of both energy and momentum the two equations combined produce this neat result: the relative velocity reverses. I.e. v1f-v2f=v2i-v1i. See "Newton's Experimental Law" for a generalised version.
     
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