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Homework Help: Torque: Flywheel problem

  1. Nov 12, 2011 #1
    1. The problem statement, all variables and given/known data
    A flywheel in the form of a uniform disk (I = ½ MR2) 5.0 ft in diameter, weighs 650 lb. What will be its angular acceleration if a net force of 225 ft-lb acts it upon? If the disk is rotating at 1200 rev/min, what torque is required to stop it in 30 minutes?

    2. Relevant equations


    3. The attempt at a solution

    I need help in the first question, because apparently I arrive with a different answer from the solution that my teacher gave me. I just wanted to know if I got it wrong or my teacher forgot something

    So using the definition of torque, I solve for alpha which is

    alpha = FR/I

    Defining I in the equation gives

    alpha = FR/(1/2)MR2

    Since the given is weight, I still have to define mass in the equation

    alpha = FR/(1/2)(W/g)R2

    Substituting the values, I'll have

    alpha = (225)(2.5)/(1/2)(650/9.8)(2.5)2
    alpha = 2.71 rad/s2

    The solution in my note was

    alpha = 2g(Torque)/WR2

    which results to 3.54rad/s2

    Am I missing something? Or is everything just right?
  2. jcsd
  3. Nov 12, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Two problems:
    (1) Even though it says 'net force', what's really given is the net torque.
    (2) Careful with units: g ≠ 9.8
  4. Nov 12, 2011 #3
    Oh I see, one question though, How can I tell whether its net force or net torque?
  5. Nov 12, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The units given (ft-lb) are for torque, not force. Check the problem statement; I suspect there was a typo somewhere.
  6. Nov 12, 2011 #5
    Thanks! I got my english units rambled up, I usually think of pounds as mass, so I was thinking of what the english unit is for force. Nevermind, I understand it now. :D
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