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Need help on a difficult pulley problem

  1. Mar 17, 2012 #1
    1. The problem statement, all variables and given/known data
    m(1) is 20kg
    m(2) is 30kg
    gravity use 10
    U static is .6
    u kinetic is .4


    2. Relevant equations
    Basically my professor gave the answer to the question in the link along with the picture. The only problem is that i am not sure he did case three right and i am after spending hoursl ooking at it i am not sure why there is a reaction force in the upper pulley and not in system 1. I am confused about how the sytems are different and how that changes my answer.


    3. The attempt at a solution
    Well i already had the answer with me so i just kept doing and trying to get the same answer but although i managed to get case 1 and 2 i dont know fi i should treat the pulleys as object or whatever else is necessary for the second system.
     

    Attached Files:

  2. jcsd
  3. Mar 17, 2012 #2

    PeterO

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    I too was concerned with talk of reaction force from the upper pulley.

    It looked to me that the vertical string would reduce the reaction force force between the big block and the ground, making it easier to move? (due to things happening at the LOWER pulley)
     
  4. Mar 18, 2012 #3
    But how does the lower pulley effect everything else? I did a free body for the lower one but i don't know how that would concern the upper pulley
     
  5. Mar 18, 2012 #4

    PeterO

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    In Option B, there are two cords going up from the lower pulley, but only one thread going down from the upper pulley. Once you provide tension in the cord that indicates twice the upward force compared to the downward force [just considering effects of Tension] so I expect a net lifting force equal to the Tension.

    With no tension, the (upward) Reaction Force will be equal to the (downward0 weight force of the two blocks.

    With Tension, the sum of the (upward) Tension and Reaction force should equal the (downward) weight force, so the Normal force would be reduced?
     
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