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Homework Help: 1 Billard Ball Into Middle of 2 Others

  1. Nov 30, 2008 #1
    Two identical billiard balls are initially at rest when they are struck symmetrically by a third identical ball moving with velocity v0 = v0i (vector)


    express final velocity (vf) of third ball immediately after the elastic collision

    express veloicty of the other two after elastic collision.

    conservation of momentum or kinetic energy

    My work
    Momentum, and kinetic energy, is conserved in elastic collisions so lets use momentum -

    m1v1i + m2v2i = m3v1f + m4v2f
    but all masses being equal, it cancels v1i + v2i = v1f + v2f
    and the other balls are at rest to begin with, so

    v1i = v1f + v2f where the balls transfers momentum to two masses such that
    v1i = v1f + (.5)v2f

    I feel like Im on thin ice, can anyone help me here? I know that the third ball is going to
    roll back wards and the other two balls will shoot out with opposite signs at angle theta off
    the axis through their meeting point.
  2. jcsd
  3. Dec 1, 2008 #2


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    Homework Helper

    You need to construct the two equations that are relevant to the elastic collision in this situation.

    m1Vo = m1V1 +m2V2 + m3V3

    1/2m1Vo2 = 1/2m1V12 + 1/2m2V22 + 1/2m3V32

    Because of the symmetry of the collision I think you can further say that V3 = V2
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