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Homework Help: Two Blocks and a pulley (Equation derivation)

  1. Sep 19, 2014 #1
    1. The problem statement, all variables and given/known data

    Look at the figure below. Derive the formula for the magnitude of the force F exerted on the large block (mC) in the figure such that the mass mA does not move relative to mC. Ignore all friction. Assume mB does not make contact with mC.


    2. Relevant equations

    F = ma

    3. The attempt at a solution

    So mB is pulling the block mA to the right. Therefore if the block mC were to not move, mA would move to the right. So the acceleration of mA needs to be equal to the acceleration of mC to make sure the blocks stay at the same relative position. mA is accelerating at the rate of g since mB is accelerating at the rate of g.

    To accelerate mC at the rate of g, you'd need to move both the mass of mC and mB therefore I thought the force would be (mB + mC)g.

    This is wrong and I'm not sure why.
  2. jcsd
  3. Sep 19, 2014 #2


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    The flaw is at "since mB is accelerating at the rate of g" : the force that accelerates is mB * g alright, but the mass that has to be accelerated is mA+mB .

    However, it is much better to use the floor (or table, or what is it) as a reference frame and set up free body diagrams for each of the three masses. Note that the connection bar between the pulley and block C exercises a force on C as well.
  4. Sep 19, 2014 #3


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    The way the situation is shown in your picture, this is impossible. As mass mC accelerates downward, mass mB will have an acceleration relative to the pulley. But the pulley is attached to mass mC so any movement relative to the pulley is motion relative to mass mC. Accelerating mC also accelerates the pulley and so adds more acceleration to mA.
  5. Sep 20, 2014 #4


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    Well, have to get used to the new environment.
    Ivy makes things complicated. Let's assume no swinging of block B.
    Make the three drawings. All have the same acceleration a, as a resultant of different forces.
    ##\vec a## is horizontal.
    On block A:
    On block B:
    On block C:
    Fill in the blanks!
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