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Homework Help: Two masses colliding

  1. Jun 27, 2011 #1
    1. The problem statement, all variables and given/known data
    A mass m with initial velocity v0 collides head on with a mass 2m initially at rest. What is the final velocity of the smaller mass, as a ratio to v0?

    What is the final velocity of the larger mass, as a ratio to v0?



    2. Relevant equations

    Pi=Pf

    KE=.5mv^2

    3. The attempt at a solution

    for part a I am confused as to what I am doing wrong:

    mvo=mv1+2mv2
    it wants to find v1 in terms of vo so:

    v2=.5(vo+v1)

    then plug into KE equation:

    vo2=v12+.25(v12+vo2)
    and simplify to get

    (3/4)vo2=(5/4)v12

    then v1=(3/5)1/2vo
     
  2. jcsd
  3. Jun 27, 2011 #2

    ehild

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    That + should be minus.


    ehild
     
  4. Jun 27, 2011 #3

    tiny-tim

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    hi kraigandrews! :smile:
    hmm …

    i] v2=.5(vo minus v1)

    ii] you need to put m or 2m in the KE equation

    iii] (vo+v1)2 is not v12+vo2 :redface:
     
  5. Jun 27, 2011 #4
    yeah wow that was a dumb mistake not foiling, for the KE part I just cancelled the masses:

    but now im left with
    (3/4)Vo^2=(5/4)V1^2-(1/2)Vo*V1
    and I am not quite sure how to solve this for V1
     
  6. Jun 27, 2011 #5

    tiny-tim

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    erm :rolleyes: … it's a quadratic equation! :smile:

    (and anyway, v0 = v1 is obviously going to be a solution, since that corresponds to the masses not colliding!)
     
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