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Window Washer w/ Bucket-Pulley Apparatus

  1. Oct 12, 2015 #1
    1. The problem statement, all variables and given/known data
    A window washer pulls herself upward using the bucket-pulley apparatus shown in the figure. The mass of the person plus the bucket is 65 kg.
    a) How hard must she pull downward to raise herself slowly at constant speed?
    b) If she increases this force by 10%, what will her acceleration be?

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    This is where I got confused. I tried to get help from other websites, but their free-body diagrams had forces like normal force on the end the person was pulling on.
    Would the force she needs to pull down on simply the gravitational force acting on her? Or would it be something else!
    Thanks everyone :).
     
  2. jcsd
  3. Oct 12, 2015 #2

    haruspex

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    Never mind about other websites, draw you own FBD.
    First, decide whether you want to treat the person and the bucket as separate objects or one.
    List the forces acting on each object, and their directions. Assign symbols to unknown forces as necessary.
     
  4. Oct 12, 2015 #3
    Sorry for the late reply, I was away.
    So would the FBD look something like this? (attached below) Or am I missing some forces?
     

    Attached Files:

  5. Oct 12, 2015 #4
    Is there also a tension force on the left side?
     
  6. Oct 12, 2015 #5

    haruspex

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    My only access is via iPad, and .docx pictures don't display properly.
    You mention "pulling force". How is this different from tension?
     
  7. Oct 12, 2015 #6
    I'm probably wrong, but tension force is upwards while the "pulling force" is downwards?
     
  8. Oct 12, 2015 #7

    haruspex

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    Tension is not so much a force as a pair of equal and opposite forces. (Likewise compression.) If you pull on a rope, the rope pulls on you equally. At any point along the rope, if we consider the lengths of rope each side of that point as separate bodies, each pulls on the other - that is tension.

    In drawing a free body diagram for a body, you consider all the forces acting on that body. If the body is a person pulling on a rope, there is the force the rope exerts on the person, but not the force the person exerts on the rope - that force is acting on the rope, not on the person.
     
  9. Oct 12, 2015 #8
    Ah, so the "pulling force" is actually the tension?
     
  10. Oct 12, 2015 #9

    haruspex

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    Yes.
     
  11. Oct 12, 2015 #10
    Are the two tensions the same? In other words would the tension equal the gravitational force on the window washer?
     
  12. Oct 12, 2015 #11

    haruspex

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    Those are two different questions.
    Let the tension have magnitude T. List the external forces acting on the washer+bucket system.
     
  13. Oct 12, 2015 #12
    Um tension and gravitational force? Or is there something else.
     
  14. Oct 12, 2015 #13

    haruspex

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    The washer is holding one end of the rope. Where is the other end?
     
  15. Oct 12, 2015 #14
    Attached to the washer+bucket side I think.

    (Also sorry if I'm being really slow and frustrating, I haven't learned much physics yet)
     
  16. Oct 12, 2015 #15

    haruspex

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    right. So what force does that exert on the washer+bucket system?
     
  17. Oct 12, 2015 #16
    ...tension...? not so sure
     
  18. Oct 12, 2015 #17

    haruspex

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    Is the tension the same all along the rope?
     
  19. Oct 13, 2015 #18
    I think it's the same :confused: correct me if I'm wrong
     
  20. Oct 13, 2015 #19

    haruspex

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    Yes, it's the same. If the tension is T, what is the total of the upward forces on washer+bucket?
     
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