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Homework Help: Masses and pulley

  1. Sep 16, 2009 #1
    1. The problem statement, all variables and given/known data


    Two masses, A and B, lie on a frictionless table. They are attached to either end of a light rope of length l which passes around a pulley of negligible mass. The pulley is attached to a rope connected to a hanging mass, C.

    We are supposed to find the accelerations of A, B and C.

    3. The attempt at a solution

    I'm sorry that I don't have the time to write out my detailed working. I didn't get the answer required. But I notied that the solution given (page 5 from http://hep.uchicago.edu/cdf/frisch/p141/ps4_solutions.pdf) simply states

    [tex]M_A a_A = T[/tex]

    and so on. My other equations, like the kinematic constraint (relation between ac, aa, and ab) and the equation of motion for mass C is ok. We need 4 equations to solve. So 2 are ok.

    My equations for MA and MB are different from the solution here. I have assumed that M_A is in a frame that is accelerating at a rate equal to a_C, in addition to the tension imparted on mass A. Same for B. Can I do it like I have done here, or is this totally wrong?
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Sep 16, 2009 #2

    Doc Al

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    Staff: Mentor

    It's not clear to me what you've done. The equations for Ma and Mb are just Newton's 2nd law (which applies in an inertial frame). The tension is the only horizontal force on those masses. (The fact that the pulley is accelerating will end up affecting the tension, but that will come out of solving the equations for all three masses.)
  4. Sep 16, 2009 #3


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    I don't understand what you mean by this. Tension T alone is the net force acting on MA, therefore its acceleration is T/MA. There is no other acceleration to be added to this. MA doesn't care how tension T is generated. All it knows is that, if tension T acts on it, its acceleration will be T/MA.
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