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Homework Help: Conservation of Momentum Problem

  1. Mar 2, 2010 #1
    1. The problem statement, all variables and given/known data
    Objects A and C are made of different materials, with different
    “springiness,” but they have the same mass and are initially at
    rest. When ball B collides with object A, the ball ends up at rest.
    When ball B is thrown with the same speed and collides with
    object C, the ball rebounds to the left. Compare the velocities of
    A and C after the collisions. Is vA greater than, equal to, or less
    than vC?

    2. Relevant equations
    Pi = Pf

    3. The attempt at a solution
    A: mb(Vbi) = mVAf
    B: mbVBi = mbVBf + mvCf

    Now...I can see that:
    VAf= mb(Vbi)/m
    VCf = (mb(Vbi) - mbVBf )/m

    I don't think I can say, from the above equations which is bigger (VAf or VCf)...do I have to sub one into the other now? How do I continue the solution? Thanks for your help!
  2. jcsd
  3. Mar 2, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    First off equation B is incorrect. If the ball rebounds to the left, its final velocity should be negative.

    Change the sign as indicated above and proceed as you did. Then take the ratio VAf/VCf. Is it greater or less than one?
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