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Homework Help: Double Ball Drop

  1. May 31, 2009 #1
    1. The problem statement, all variables and given/known data

    Hi everyone. Hope i'm not breaking any forum rules, because i've alread wrote this one in another thread by mistake. Here is where it should be.

    Two balls of different mass are dropped one in top of the other, with the light one on top. After they collide with the floor (elastic collision), what is the velocity of both balls?

    m is the mass of the small ball
    M is the mass of the heavier one
    v is the velocity of both balls before the collision with the ground
    v1 and v2 are the velocities of the lighter and heavier ball respectively.

    2. Relevant equations


    3. The attempt at a solution

    (m + M)v = mv1 + Mv2

    What do i assume now?

    I first assumed that mv1 = -(m+M)v + Mv2
    but it doesn't look right to me because v1 turns out 2v2.

    What am i doing wrong?

    Thanks in advance.
  2. jcsd
  3. May 31, 2009 #2


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    Last edited by a moderator: Apr 24, 2017
  4. May 31, 2009 #3
    Re: Welcome to PF!

    Thank you very much.

    Linear momentum is also conserved.

    So i get two equations and two unknowns and a very ugly quadratic equation jumps out of there. Should i solve it or am i wasting my time? This looks like a very simple problem and i'm just making it more complex...
    Last edited by a moderator: Apr 24, 2017
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