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Another torque problem

  1. Apr 17, 2005 #1
    A uniform solid disk with a mass of 28.3 kg and a radius of 0.414 m is free to rotate about a frictionless axle. Forces of 90.0 N and 125 N are applied to the disk, as the drawing illustrates.
    http://www.webassign.net/CJ/p9-31.gif

    What is the net torque produced by the two forces? (Assume counterclockwise is the positive direction.)

    What is the angular acceleration of the disk?

    For the first question, I thought maybe to do:
    90.0 N * r + 125 * r = 89.01
    That is wrong.

    I know of the eqn.
    torque = I * alpha
    but I dont know how to use it in this problem.
     
  2. jcsd
  3. Apr 17, 2005 #2

    OlderDan

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    The two torques are in opposition. Use the sign conventions specified to find the result. Use the net torque to find the acceleration using your last equation. Look up the moment of inertia of a disk rotating about its axis of symmetry.
     
  4. Apr 17, 2005 #3
    thank you. how can i see that the forces are in opposition? i am confused b/c the arrows are pointed in the same direction.
     
  5. Apr 18, 2005 #4

    OlderDan

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    The direction of the forces are indeed the same, but in your figure the lower force would tend to make the disk rotate counterclockwise. The upper force would tend to make the disk rotate clockwise. The bracket holding the disk would have to exert a force to the left equal to the sum of the forces pulling to the right. That force is applied to the axis of rotation and produces no torque.

    Torque is technically a vector. The direction is the direction of the axis about which the torque is calculated. The torque associated with the lower force would be directed out of the page toward you. The torque associated with the upper force would be directed into the page away from you. In your problem it is sufficient to just call one of the torques positive, and one negative to recognize their opposition.
     
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