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Homework Help: Force Question

  1. Aug 25, 2009 #1
    1. The problem statement, all variables and given/known data

    Three blocks on a frictionless horizontal surface are in contact with each other as shown in Fig. 4-5. A force F is applied to block 1 (mass m1). (a) Draw a free-body diagram for eeach block. Determine (b) the acceleration of the system (in terms of m1, m2, m3), (c) the net force on each block, and (d) the force of contact that each block exerts on its neighbor. (e) If m1 = m2 = m3 = 12.0 kg and F = 90 N, give numerical answers to (b), (c), and (d). Do your answers make sense intuitively?

    2. Relevant equations

    def of net force = ma

    3. The attempt at a solution

    ok i followed this problem my only problem with it is that were only given the force applied how are you suppose to find the acceleration of the system...

    because net force = ma

    were only given one force

    applied the net force in the x direction there would be more than just the applied force... yes were ignoring friction and air resistance but still there would be others like the contact force opposing each block

    so I don't see how simply taking 90 N and the total mass allows us to find the acceleration when 90 N is not the net force
  2. jcsd
  3. Aug 25, 2009 #2


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    Gold Member

    why don'y you draw the free-body diagram for each block first
  4. Aug 25, 2009 #3
    Once you draw the free body diagram, remembering that every force has an equal and opposite force, you can effectively prove that you can treat the 3 blocks as one single block. therefore the F=mA become a pretty simple equation to solve. (I am assuming the blocks are arrange linearly horizontally on a table)
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