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Homework Help: Moment of Torque; Am I solving this correctly?

  1. Mar 15, 2016 #1
    1. The problem statement, all variables and given/known data
    A metal rod of length 90cm has a disc of radius 24cm fixed rigidly at its centre(refer to picture attached). The assembly is pivoted at it's centre.
    Two forces, each of magnitude 30N, are applied normal to the rod at each end so as to produce a turning effect on rod. A rope is attached to the edge of the disc to prevent rotation
    1-Calculate the torque of couple produced by the 30N forces
    2-the tension T in the rope
    2. Relevant equations
    Torque= moment arm * force applied perpendicular to pivot
    torque of couple=distance between two forces * one of the force
    (I assume the above equations will be relevant)

    3. The attempt at a solution
    For the rod the torque of couple would be
    30 * 90 =2700

    Now since the rotation is to be prevented by the disc in center it has to produce an equal torque in the opposite direction(anti-clockwise direction) so the torque produced by disc should also be 2700
    and since
    torque=moment arm*force applied
    =radius*tension in string
    2700= 24 * T

    Have I solved the above numerical correctly? Since the book doesn't provide answer to these questions and I have no tutor I have no other way to confirm my calculations.
  2. jcsd
  3. Mar 15, 2016 #2


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    That looks real good. Well done.
  4. Mar 15, 2016 #3


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    That's all fine, except that you really should state the units everywhere.
  5. Mar 15, 2016 #4
    Thanks, everybody. That's a relief.
    Secondly, I'll make sure to mention the units from here on.
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