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Homework Help: Question about momentum (elastic collision)

  1. Jun 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A block with mass 4 kg and velocity of 3 m/s collides elastically with block B with mass 2 kg which is originally at rest. Find the velocities of the two blocks after the collision.

    2. Relevant equations
    P= m * v

    3. The attempt at a solution
    This is the first time I try to solve an elastic collision problem, not sure if its the right way or no.

    (M1*V1) + (M2*V2)= (M1+M2) Vc
    (4*3) + (2*0) = (6) Vc
    Vc= 12/6 = 2 m/s
  2. jcsd
  3. Jun 1, 2013 #2

    Doc Al

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    Staff: Mentor

    You cannot assume that both blocks end up with the same velocity. That would be true for an inelastic collision.

    Hint: What two quantities are conserved in an elastic collision? (What does elastic collision mean?)
  4. Jun 1, 2013 #3
    In elastic collision, momentum before equals to momentum after (Pi=Pf).

    (M1*V1) + (M2*V2) = (M1 * V1') (M2*V2')
    1) 12= (4 * V1') + (2 * V2')

    V1 - V2 = -(V1' - V2')
    2) 3-0 = -(V1' + V2')

    3 + V1' = V2'

    By substitution to get V1'
    12 = (4*V1') + (2 * (3+V1'))
    Therefore, V1'= 1 m/s

    And for V2'
    12= 4+ (2*V2')
    V2'= 4 m/s
  5. Jun 1, 2013 #4

    Doc Al

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    Staff: Mentor


    Two comments:
    (1) Momentum is conserved in any collision, not just elastic ones.
    (2) Your second equation, that the relative velocities are reversed, is only true in elastic collision.

    Good job.
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