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Concepts about pulleys

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data

    I can do the math work, but I always overthink these multiple choice concepts.

    A block is attached to each end of a rope that passes over a pulley suspended from the ceiling. The blocks do not have the same mass. If the rope does not slip on the pulley, then at any instant after the blocks start moving the rope:

    2. Relevant equations



    3. The attempt at a solution

    Exerts different forces on both blocks, the lesser force exerted on the smaller block?

    I figure that F = ma and the acceleration is the same.
     
  2. jcsd
  3. Oct 10, 2011 #2

    SammyS

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    What are ALL the choices?
     
  4. Oct 10, 2011 #3
    Pulls on both blocks and exerts the same non-zero force.
    Pulls on both blocks, exerting greater force on heavier block.
    Does not pull on either block.
    Pulls on both blocks, exerting greater force on lighter block.
    Pulls only on the lighter block.
     
  5. Oct 10, 2011 #4

    SammyS

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    If you were to solve such a problem, would the tension be the same , or different for the two blocks?
     
  6. Oct 10, 2011 #5

    NascentOxygen

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    Which options can you strike out right from the word go?
     
  7. Oct 10, 2011 #6
    Pulls on both blocks and exerts the same non-zero force.
    Does not pull on either block.
    Pulls only on the lighter block.

    Are the options I would strike out. Agree?
     
  8. Oct 10, 2011 #7
    I've always worked problems assuming pulleys were nothing more than "direction changing" and therefore the tension was always equal.
     
  9. Oct 11, 2011 #8

    SammyS

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    Then, why eliminate: "Pulls on both blocks and exerts the same non-zero force." ?
     
  10. Oct 11, 2011 #9
    Because it was my initial choice, marked incorrect.
     
  11. Oct 11, 2011 #10
    Anyone?
     
  12. Oct 11, 2011 #11

    NascentOxygen

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    Yes. So that leaves just two to choose from .....
     
  13. Oct 11, 2011 #12

    NascentOxygen

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    I don't believe it's possible to overthink this one. It's nasty.
     
  14. Oct 11, 2011 #13

    SammyS

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    Not an ideal mass-less pulley?
     
  15. Oct 11, 2011 #14

    NascentOxygen

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    We are told the rope does not slip on the pulley. We are not told that the pulley bearing is frictionless; so expect energy loss there. Nor are we told the pulley has negligible mass. As SammyS points out, when the pulley has mass it too requires a force to speed up its rotation.

    I presume that these practicalities must have been discussed in class, for them to appear on a multi-choice test paper.
     
  16. Oct 11, 2011 #15
    They will appear in class :P

    Our assignments are posted ahead of time

    So, since the driving force is the heavier block, it preceeds the frictional loss, and pulls on the cord normally, which in turn pulls back on it with a tension.

    Meanwhile, the rope on the other end pulls up, but since the oulley has sapped some efficiency, it is with a lesser force.

    Correct?
     
  17. Oct 11, 2011 #16

    NascentOxygen

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    That's the general picture.
     
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