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Contact force problem

  1. May 10, 2007 #1
    1. The problem statement, all variables and given/known data
    Given: The surface is frictionless, two blocks in contact with each other move to the right across a horizontal surface.
    Note: one block is 2.1kg with a 58N force applied to it, and the other block is 5.7kg with a 13N force applied to it.

    2. Relevant equations

    Determine the magnitude of the force exerted on the block with mass 2.1kg by the block with mass 5.7 kg

    3. The attempt at a solution

    First I subtracted 58N-13N=45 to get the net force, then I divided the net force by the mass of both blocks [45N/7.8kg]=5.769m/s^2 to get acceleration.
    Then I multiplied the acceleration by the 2.1kg block to get 12.115N, but this was not the right answer. So thinking that I maybe did it wrong I multiplied the acceleration by the 5.7kg block (32.884N)and this was also incorrect, finally not knowing what I was doing wrong still, I subtracted 32.884-12.115 and of course this was wrong as well. I can't figure out what I'm missing here. Any help would be appreciated.
  2. jcsd
  3. May 10, 2007 #2


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    Assumption I'm going to make: 2.1 kg block is on the left, pushed on with force 58 N from the left side, in the right direction. 5.7 kg block is on the right, pushed on with force 13 N from the right side, in the left direction.

    Envisioning a FBD for the first block, if it's only moving with the acceleration you calculated to the right, then the net force on it must be 12.115 N (assuming you calculated that right) to the right, which means that there must be a force of magnitude 58 N - 12.115 N on it to the left, counteracting the 58 N force to the right partially. So, is 45.885 N the correct answer?
  4. May 10, 2007 #3
    Okay, I was thinking I had to subtract from the net force. Thank you so much, I got it right this time.
  5. May 10, 2007 #4


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    No problem! :smile: Always draw a free body diagram to check your thinking.
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