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Pulley Traction in Equilibrium

  1. Sep 14, 2013 #1
    1. The problem statement, all variables and given/known data
    Sum of downward forces exerted at A and B is 41lbs by the leg, what is the weight attached?
    Kpvcb5E.jpg

    2. Relevant equations
    None, really...


    3. The attempt at a solution
    Tension (T) throughout the system must be constant and equal to the weight (W).
    I first assumed that the leg was entirely pulled up by B since I thought the force passing A
    is going down (wrong since this is in equilibrium) and claimed W = 41sin60, but it's not that simple.

    Tension pulls both ways, so there is an upward force at A as well as B.
    Since
    [itex]ƩF_y = 0[/itex]
    [itex]Tsin25 + Tsin60 - 41lb = 0[/itex]
    [itex]T = W = 41 / (sin25 + sin60) = 31.0lb[/itex]

    But using this value of T in checking horizontal forces, they don't balance out.
    What am I missing here, or are my calculations correct and there just is a net horizontal force
    on the leg that the leg has to resist?

    Thanks in advance :).
     
  2. jcsd
  3. Sep 15, 2013 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    The horizontal forces balance if you take into account the friction between the patient and the bed.

    AM
     
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