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Homework Help: Composite Body Momentum

  1. Dec 15, 2008 #1
    1. The problem statement, all variables and given/known data
    A disc of mass m is moving horizontally to the right with speed v on a table with neglibible friction when it collides with a second disc of mass 2m. The second disc is moving horizontally to the right with speed v/2 at the moment of impact. The two disc stick together upon impact. The speed of the body immediately after the collision is:

    A)v/3
    B)v/2
    C)2v/3
    D)3v/2
    E)2V

    2. Relevant equations

    p = mv (that wasn't given, but that's the equation I used)

    3. The attempt at a solution

    mv = mv + 2mv/2

    m(v) = m(v+2v/2)

    v= v+2v/2

    v= 2v/2 + 2v/2

    for the final answer, I got: v= 2V, according to my answer sheet, the correct answer is 2v/3. I can't seem to figure out what I'm doing wrong. Please help.
     
    Last edited: Dec 15, 2008
  2. jcsd
  3. Dec 15, 2008 #2

    LowlyPion

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    Homework Helper

    Welcome to PF.

    mv +2m(v/2) = Total Momentum = 2*mv

    After collision you have a total mass of 3m and you have total momentum = 2mv

    So ... 3m*V = 2mv

    V = 2v/3
     
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