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Homework Help: Pushing two blocks

  1. Mar 26, 2008 #1
    1. The problem statement, all variables and given/known data

    There are two blocks touching each other, block A, and block B. Block A is being pushed by a force, which in turn moves block A and B at the same time. Block A has a mass of 4kg, block B has a mass of 1kg. (so the force is pushing the larger block, which is pushing the smaller block).

    When the hand starts to push, the blocks are moving with a speed of 2.0 m/s. Suppose that the work done by the hand on block A during a given displacement is 10 J. Determine the final kinetic energy of each block

    2. Relevant equations

    Change in work (f * d) = change in kinetic energy ( 1/2m(vf)^2 - 1/2m(vi)^2)

    3. The attempt at a solution

    10J = Kf - 1/2(4kg)(2m/s)^2, Kf = 18J

    10J = Kf - 1/2(1kg)(2m/s)^2, Kf = 12J

    Now to me, this seems like a pretty straight forward problem. However, my fellow students are getting a different answer. Perhaps I am overlooking something? Any help is appreciated.
  2. jcsd
  3. Mar 26, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Since the blocks move together at the same speeds, you've got to first look at the change in KE of the system (both blocks together) before you calculate the individual final KE of each.
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