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Torque problem

  1. Oct 18, 2005 #1
    Trying to describe this problem in words would be rather tedious, dull and very difficult for all involved, so without further ado:


    My problem: I don't have a clue. I know torque is basically radius (lever arm) times force and that it is also moment of inertia times angular acceleration. I realize there are a lot of "PLZ DO MY HOMEWARKZ!!!11one" questions here, but I am really at a loss.

    I don't expect anyone to do my homework, I just need a path to follow.
  2. jcsd
  3. Oct 18, 2005 #2
    First think about what have to be equal in order for that pulley to stay still (omega = 0 rad/s, alpha = 0 rad/s^2)? And then work you problem form there.
  4. Oct 18, 2005 #3
    So is it ok to assume the two torques would have to be equal?
  5. Oct 18, 2005 #4


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    As is frequently the case with problems like this, a free body diagram is an excellent place to start. When you're dealing with the pulleys - in particular with the pulley attached to the truck - remember that each line of the rope provides an independent pull on the pulley.

    Be sure you list all of the forces involved, remember that the pulley by itself does nothing except redirect the tensions, and remember Newton's Second Law. You should be fine.
  6. Oct 18, 2005 #5
    Newton's second law, ΣM = ΣI*α, if alpha is equal to 0, thus net moment is 0. Two torques have equal magnitude but opposite direction.

    As Diane said, draw FBDs of both pulleys and use Newton's second Law (F=ma).
  7. Oct 18, 2005 #6
    The problem as I have it now:

    first pulley torque=second pulley torque

    I am not sure if the radius on the second pulley's torque equation would simply be r/2 (2nd pulley radius) or the sum of the radii of the inner part of pulley 1 and pulley #2 r+r/2 as above.

    Clarification requested if at all possible and is this setup right? Thanks again :approve:
  8. Oct 19, 2005 #7


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    The belts don't transmit torque, they transmit force - specifically, the tension in the belt/rope. The differing radii of the pulleys will cause that tension to give different torques. Does that make any sense?
  9. Oct 19, 2005 #8

    Figured it out, thanks.
    Last edited: Oct 19, 2005
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