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Pulley problem!

  1. Oct 7, 2011 #1
    1. The problem statement, all variables and given/known data

    Need some serious help with this one :O

    A box with a weight WA hangs in pulleys ,(see figure 1). Find the force to pull A up with constant speed and calculate the work which the tractive force F produces ( with the movement of s) and also the work and tractive force T1 produce .
    Pulleys and strings are frictionless and both without mass!.

    pulley.jpg

    2. Relevant equations




    3. The attempt at a solution
     
  2. jcsd
  3. Oct 7, 2011 #2

    ehild

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    What have you tried so far?

    ehild
     
  4. Oct 14, 2011 #3
    taikng tensions ,T1=4F
    and taking constrained motion,
    acc. of pulling end=4 times acc. of wA
    differentiating that we get s=4s1
    therefore work done by F=Fs
    work done by T1=T1s1=4F x s/4=Fs

    correct me if i m wrong,i just tried.
     
  5. Oct 15, 2011 #4

    ehild

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    The result is correct. Balance of forces gives that WA=T1=4F. The question is the force needed to move the load with constant speed, so write out that F=WA/4.
    The mass moves with constant speed so there is no acceleration. The constraints apply for the displacements and changes of length of the pieces of the ropes, but not for the non-existent accelerations. You can say that moving the end of the left rope by s will cause the other two pieces shorten by s/2 so the middle pulley raises by s/2. In the same way, raising the end of the right rope by s/2 lifts the right pulley by s/4.

    ehild
     
  6. Oct 17, 2011 #5
    thank you echild.
     
  7. Oct 18, 2011 #6
    sorry didn't see the name
    properly,very sorry.thanks ehild!
     
  8. Oct 18, 2011 #7

    ehild

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    You are welcome :smile:

    ehild
     
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