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Homework Help: Momentum Issues

  1. Mar 16, 2009 #1
    1. The problem statement:
    A 98 kg man lying on a surface of negligible friction shoves a 86 g stone away from himself, giving it a speed of 4.5 m/s. What speed does the man acquire as a result?

    2. Relevant equations:
    Pi = Pf
    M1V1 + M2V2 = M1V1 + M2V2

    3. The attempt at a solution:

    I've tried the formula above, and I got this far:

    98 x 0 + 0.086 x 0 = 98 x X + 0.086 x 4.5
    0 = 98X + 0.387
    -0.387 = 98X
    -0.387/98 = X
    X = -0.00394897959 m/s

    That's what I got. I have a feeling that I've done something wrong. The online Assignment system is telling me my answer is wrong, so what am I doing wrong? Have I missed something? All help is greatly appreciated :D
  2. jcsd
  3. Mar 16, 2009 #2

    When you are substituting the velocity into the equation, the left side of the equation. The equation normally should look more like this:


    By the way, I state "u" as the initial velocity of the object(s), and "v" as the final.
  4. Mar 16, 2009 #3
    If you're taking the direction of the stone's velocity to be positive, the other has to be negative since its in the opposite direction. ;)
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