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Homework Help: Where did I go wrong (torque/moment problem)

  1. Sep 28, 2014 #1
    1. The problem statement, all variables and given/known data
    In the figure above, what is the torque about the pendulum’s suspension point produced
    by the weight of the bob, given that the mass is 40 cm below the suspension point,
    measured vertically, and m = 0.50 kg ? (let g = 10 ms^-2 for easier calculations)

    2. Relevant equations
    Torque = force x perpendicular distance to the pivot

    3. The attempt at a solution
    I first got the components of the Weight force:
    And I got L using trigonometry:
    sin60 = 0.4/L
    L = 0.4/sin60
    L = (4root3)/15 which is about 0.463

    Now torque = F x Perpendicular distance =
    5cos60 x L = 5cos60 x 0.463 = 1.15 N.m

    But the answer is 2 N.m in the text book!! where have I gone wrong?
  2. jcsd
  3. Sep 28, 2014 #2


    User Avatar

    Staff: Mentor

    I don't think you've gone wrong, I think the textbook has. It looks like they used the wrong angle or the wrong trig function for the calculation of the force component perpendicular to L. Your result looks okay to me.
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