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Homework Help: Some review problem

  1. Jul 31, 2009 #1
    1. The problem statement, all variables and given/known data
    I got this problem from K&K. A pedagogical machine is illustrated in the sketch above. Refer link : http://www.slideshare.net/brigittperalta/sol-maquina-pedagogica-1546585 . What force F must be applied on M1 to keep M3 from moving up or down. All surfaces frictionless.

    2. Relevant equations

    3. The attempt at a solution
    If I go and sit on the M1 block, I can mark a fictitious force on M2 to the left with magnitude (M2.F)/M1. On M2 there is also tension from the string (T) to the right. If M3 is to remain in equilibrium, M3.g = T and T = (M2.F)/M1 to prevent block 2 from moving which will cause block 3 to move.

    Therefore, I get my answer to be F = (M3.g.M1)/M2. I am making a mistake here but cannot see where. Can someone please tell me where?
  2. jcsd
  3. Jul 31, 2009 #2


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    Homework Helper

    Hi anirudh215,

    I believe this is incorrect here. According to this expression, the overall horizontal acceleration would be given by F=(m1) a, which is not true.

    Instead, begin by considering the entire setup as one mass to find the horizontal acceleration, and then continue from there as you already have done.
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