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Easy rotational problem

  1. Apr 4, 2008 #1
    [SOLVED] Easy rotational problem

    1. The problem statement, all variables and given/known data
    A string is wound around a uniform disk of radius 0.25m and mass 8.00kg. The end of the string is pulled with a constant force of 50.0 N, and there is a frictional torque acting at the axle of 3.00 Nm.
    A) Find the total torque exerted on the disk

    3. The attempt at a solution
    I think I have it solved... just wanted to make sure and I have one question. It says that the frictional torque is acting on the axle, do I still use the radius r=0.25 to find the torque for that part? Here is what I did:

    [tex]\Sigma \tau = \Sigma Fd[/tex]
    [tex]\Sigma \tau = (50)(0.25) + (-3)(0.25)[/tex]
    [tex]\Sigma \tau = 11.75 Nm [/tex]

    So is that how I would find the net torque?
     
  2. jcsd
  3. Apr 4, 2008 #2

    Doc Al

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    Staff: Mentor

    Careful. For friction they gave you the torque, not the force. So you don't have to use any radius to find the frictional torque.
     
  4. Apr 4, 2008 #3
    Oh yeah I just realized that. so I guess what i meant to say would the net force end up being 9.5 Nm?
     
  5. Apr 4, 2008 #4

    Doc Al

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    Staff: Mentor

    The net torque is 9.5 Nm.
     
  6. Apr 5, 2008 #5
    oops again...
    Thanks.
     
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