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Homework Help: Basic Statics Mechanics - moments

  1. Sep 8, 2007 #1
    I think I am getting the wrong answer here, but I don't know why. Would someone please be so kind to go through this basic stuff.

    Question: The force F exerts a 200 ft-lb counterclockwise moment about A and a 100 ft-lb clockwise moment about B. What are F and theta?

    [tex] M_A = 2 F \cos \theta + 9 F \sin \theta = 200 [/tex]
    [tex] M_B = -7 F \cos \theta + F \sin \theta = -100 [/tex]

    A little bit of algebra yields:
    [tex] \frac{\sin \theta}{\cos \theta} = \frac{12}{-11} [/tex]

    [tex] \tan \theta = \frac{12}{-11} = -47.49 (deg) [/tex]

    Would someone please tell me what I am doing wrong. Thanks!

  2. jcsd
  3. Sep 8, 2007 #2


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    You missed a minus sign somewhere, tan theta = +12/11; otherwise, your work looks good to me, theta = +47.5 degrees, (and F = 25 pounds).
    (there is a typo in the figure at joint A, I think: should read (-5,5))
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