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Homework Help: Please help! Statics

  1. Jul 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A mass M=171 kg is suspended from the end of a uniform boom as shown. The boom (mass=83.0 kg, length=2.40 m) is at an angle θ=69.0 deg from the vertical, and is supported at its mid-point by a horizontal cable and by a pivot at its base. Calculate the tension in the horizontal cable.


    2. Relevant equations


    3. The attempt at a solution

    Theta = 69 deg, alpha=21 deg

    mg(L/2)cos(theta) + Mg(L)cos(theta) + T(L/2)sin(alpha) = 0

    When I solved for T I got 4169.25N but that is incorrect please help
  2. jcsd
  3. Jul 5, 2009 #2


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    Homework Helper

    Why are you using cosine for mg(L/2) and sine for the others, instead of the other way round? Think more carefully about which functions you should use.
  4. Jul 5, 2009 #3
    Would I be using sine for the masses because they are vertical and cosine for the tension because it's horizontal? If that's the case, is the rest of my work correct if I switch sine and cosine?
  5. Jul 5, 2009 #4


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    Homework Helper

    Yes! So:

    mg(L/2)sin(theta) + Mg(L)sin(theta) + T(L/2)cos(theta) = 0

    The answer you get will be negative, but that's only because the tension acts to counteract the torque applied by gravity.
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