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Homework Help: Question on torque exerted

  1. Nov 25, 2009 #1
    1. The problem statement, all variables and given/known data

    A trap door, of length and width 1.65 m, is held open at an angle of 65.0° with respect to the floor. A rope is attached to the raised edge of the door and fastened to the wall behind the door in such a position that the rope pulls perpendicularly to the trap door. If the mass of the trap door is 11.8 kg, what is the torque exerted on the trap door by the rope?

    2. Relevant equations


    3. The attempt at a solution
    I can't seem to figure this out. I am not sure how to get exact values for the radius and Force.. any help would be great, thanks
  2. jcsd
  3. Nov 25, 2009 #2
    Draw the door as a bar (no z dimension). Make a free body diagram. Use the hinge as a pivot.
  4. Nov 25, 2009 #3
    I don't think you need the length of the string.

    This system is in equilibrium, which means that the sum of the torque must be equal to 0.
    The things that cause torque are the tension in the string and the force of gravity. The tension is applied at the end of the rope and gravity pulls from its center of mass. I assume this is a uniform body which means the center of the trapdoor is its center of mass.


    Now break down the torque of gravity into its definition.
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