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Homework Help: Momentum question asking ratio of final 2 velocities

  1. Dec 4, 2016 #1
    1. The problem statement, all variables and given/known data
    A cart mass of M moving at 3V (right) collides with a stationary mass of M, Determine the ratio of the two final velocities if the collision is elastic

    2. Relevant equations

    3. The attempt at a solution

    Since the masses are all equal and v1 is 3V the equation becomes
    3 = v1' + v2'

    Since the collision is elastic the total energy afterwards equals the total energy afterwards


    so Ek1=Ek1'Ek2'
    and since every term is over 2,and the masses equal the equation simplifies to

    After rearranging the first equation you get v1' = v2'-3, and then plug that into the energy equation to get
    9= 2v2'^2 -6v2-9
    0=2v2'^2 -6v2

    after applying the quadratic equation you get 3 or 0, and since the velocity has to change v2' is 3, subsequently v1' is 0 so the ratio is 3:0, or 0:3

    3:0 doesn't make sense to me for some reason but I can't find any mistake in what I did, is this right?
  2. jcsd
  3. Dec 4, 2016 #2


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    I can't find a mistake either.
    Check it by sliding a coin against a similar coin on a flat table (aim for a central collision)

    By the way: if an initial speed of 3V is given, your calculations should be in units of V, e.g.
    3 MV = MV1'+ MV2'
  4. Dec 4, 2016 #3


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    It's quite correct. Look for a video of Newton's cradle.
    With equal masses and elastic collision, swapping over the two initial speeds to get the final speeds must be right. It would have the same momentum and the same energy.
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