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Homework Help: Elastic collision between two moving objects

  1. May 11, 2013 #1
    1. The problem statement, all variables and given/known data
    A 0.2 kg block, moving at 6 m/s, is catching up and colliding elastically with a 0.6 kg block that is moving along the same line and in the same direction. Find the velocities of the ball after this one-dimensional collision.

    2. Relevant equations
    Conservation of momentum, conservation of energy

    3. The attempt at a solution
    First, I figured the momentum and kinetic energy would be conserved so I setup a couple of equations:

    m1v1i + m2v2i = m1v1f + m2v2f
    m1v1i^2 + m2v2i^2 = m1v1f^2 + m2v2f^2

    At this point, I realize I have 3 unknowns (v2i, v1f and v2f) and I simply don't know what I can even do with this information. Any ideas at all would be helpful.
  2. jcsd
  3. May 12, 2013 #2


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    Homework Helper
    Gold Member
    2017 Award

    Hi, captainmustard. Welcome to PF!

    You are right. There is not enough information to answer the question. You have set up the correct equations.
  4. May 12, 2013 #3
    Thank you for the response. I thought I was going crazy for a moment there. Would this be possible if it were a completely inelastic collision? I would imagine it would involve some tedious algebra if so.
  5. May 12, 2013 #4


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    No, you need the initial velocity of the 0.6 kg ball, too.

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