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Tension Problem

  1. Sep 30, 2007 #1
    A man standing on a platform with an attached frictionless pulley is supporting himself at a fixed position in midair (zero acceleration). What force F is required to do this? Solve for F for a man of arbitrary mass M, and a platform plus pulley of arbitrary mass m.

    So, since a = 0 : Fnet = 0
    Fnet = Fu - Fg = 0
    Fu = Fg
    Fu being the upward force exerted by the man

    I'm not sure if I can use Fu as an upward force since there will be 2 tensional forces

    How can I apply the T forces between the man and the pulley, and the pulley and the roof
    could i do
    Fnet = T1 + T2 - Fg = 0
    ?

    Thanks
     
  2. jcsd
  3. Sep 30, 2007 #2

    Doc Al

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    Staff: Mentor

    You're on the right track. Consider the man+platform as a system. What forces act on it?

    Hint: What can you say about the tension in each section of the rope?

    Hint 2: How does the force the man exerts relate to the tension in the rope?
     
  4. Sep 30, 2007 #3
    for the man + platform we have force of gravity, the normal force of the platform on the man (although that could be irrelevant if we are thinking of the man+platform as a system), and the tensional force of the string the man is holding

    would the tension of each section of the rope be equal?
    t1 = t2 ?

    the force the man exerts should be equal to the tension of the rope?
     
  5. Sep 30, 2007 #4

    Doc Al

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    All correct. Keep going.
     
  6. Sep 30, 2007 #5
    ok
    Fnet = 0
    t1 = t2
    Ttotal = t1 + t2 = Fman

    im not sure how to relate the force of gravity to these other forces

    would the force of gravity be opposite to the total tension forces, being equal to the force the man exerts

    or, would the force the man exerts be equal to a single tension force:
    t1 = t2 = Fman

    making the force the man exerts twice the force of gravity?
     
  7. Sep 30, 2007 #6

    Doc Al

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    OK.

    Not OK.

    Yes!
    No!

    Yes.

    No.

    (1) What upward forces act on the system?
    (2) What downward forces act on the system?
     
  8. Sep 30, 2007 #7
    ok
    Fnet = 0
    t1 = t2 = Fman

    t1 + t2 = -Fg
    is this correct?

    upward forces:
    Force of the man on the string
    Force of the roof on the string

    downward forces:
    force of gravity on the platform + man
    force of the string on the man
    force of the string on the roof

    Fms + Frs = Fg + Fsm + Fsr

    Fms = Fsm
    Frs = Fsr

    Do I have to account for any normal forces?
     
  9. Sep 30, 2007 #8

    Doc Al

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    OK.
    Almost. From Fnet = 0 we know that: t1 + t2 -Fg = 0
    Or: t1 + t2 = Fg

    That's really all you need to solve this one. (Combine these equations together to solve for Fman.)

    The upward force on the system is just the pull of the rope. The rope attaches twice--once to the man's hand and once to the platform--so the total upward force is twice the tension.

    The downward force is just the weight of the system.
     
  10. Sep 30, 2007 #9
    Fnet = 0
    t1 = t2 = Fman
    t1 + t2 - Fg = 0
    t1 + t2 = Fg

    so

    Fman + Fman = Fg
    2Fman = Fg
    Fman = (1/2)Fg
     
  11. Sep 30, 2007 #10

    Doc Al

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    Perfecto! Now just express Fg (and then Fman) in terms of the given information: M and m.
     
  12. Sep 30, 2007 #11
    ah yes

    Fnet = 0
    t1 = t2 = Fman
    t1 + t2 - Fg = 0
    t1 + t2 = Fg

    Fman + Fman = Fg
    2Fman = Fg
    Fman = (1/2)Fg

    Fman = (M+m)a
    Fg = (M+m)g

    Fman = (1/2)(M+m)g
     
  13. Sep 30, 2007 #12

    Doc Al

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    Staff: Mentor

    Excellent.
     
  14. Sep 30, 2007 #13
    thanks very much for your help
     
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