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Dynamics - Another collision question

  1. Mar 24, 2008 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    Conservation of momentum
    Conservation of Kinetic Energy

    3. The attempt at a solution

    Ive done the first part of the question. With the second part, i thought about it this way:

    If the ram is seen to have zero velocity just after impact then all of the rams kinetic energy must have been transphered to the pile, no?

    In which case, having worked out the gravitational potential energy in the ram initially to be 5886J. The kinetic energy in the pile must be:

    5886 = 0.5 x 300 x V^2
    39.24 = V^2
    6.26 = V

    However apparently the answer *should* be 7.23 m/s.

    Can someone tell me where i have gone wrong?

  2. jcsd
  3. Mar 24, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your mistake is in assuming that KE is conserved.
  4. Mar 24, 2008 #3
    Ok. I worked out that the block would be travelling at 5.42m/s when it strikes the pile (using V^2= U^2 + 2as).

    Now i know original momentum of the ram = 400 x 5.42 = 2168
    Now if momentum is conservered: 2168 = 300 x V
    V = 7.23 m/s

    Oh, thats right....whoops. Thank you.
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