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Homework Help: Pulley Question

  1. Jan 16, 2017 #1
    1. The problem statement, all variables and given/known data
    In the picture below.

    2. Relevant equations
    Velocity on a pulley = (velocity on left rope + velocity right rope)/2
    Pulley A and C have 0 velocity.

    3. The attempt at a solution
    By inspection the right sided rope on pulley A is 120 mm/s going up.
    After that is when I get confused. Pulley B and D are in motion, so I don't see what else to do from here.

    Attached Files:

    Last edited: Jan 16, 2017
  2. jcsd
  3. Jan 16, 2017 #2


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    Looks like down to me.
    So apply that to each pulley in turn, working from the left.
    What does it tell you about the velocity of the second rope segment (the one on the right of pulley A)?
  4. Jan 16, 2017 #3
    Yeah, I meant the rope to the right.

    But still, we don't know vB, meaning we'd have:
    vB = (150+vBR)/2
    Where vBR is the velocity on the right of pulley B.
    By inspection vCL = -vBR

    But from here, I'm not sure how to proceed since vB is not 0
  5. Jan 16, 2017 #4


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    I feel your notation is going to get a bit confusing since you will have two names for the same rope.
    I'll just label them and their velocities a to e across the top, and the segment left of the lowest pulley is f.
    You already have a=-b.
    Next, look at pulley C. What equation can you write for that? Then move on to pulley B.
    You don't need to solve these equations, determining velocities, as you go along. Just get all the equations written first.
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