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Homework Help: 3d equilibrium problem

  1. Apr 17, 2013 #1
    1. The problem statement, all variables and given/known data

    See attachment

    2. Relevant equations

    Let u be the angle between AC and AO and v the angle between BE and BD.
    Ta is the tension at point A and Tb is the tension at point B.

    3. The attempt at a solution

    Tried starting with the OAB body and got:

    -mg + Ta sin(u) + Tb sin(v) = 0
    -Oz + Ta cos(u) + Tb cos(v) = 0

    for the force equilibrium. For the moment equilibrium, I chose point A as reference. The moment due to mg is -mg*0.75, and the moment due to the vertical component of Tb should be Tb sin(v)*1.5.

    But doesn't the horizontal component of Tb cause a moment about point A here too? If so, what's preventing the beam OAB from rotating about point O?

    Attached Files:

  2. jcsd
  3. Apr 17, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You've left out cable BE. Because u > v this will be under tension. Take moments about O.
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