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Homework Help: Tricky torque problem

  1. Nov 14, 2007 #1
    1. The problem statement, all variables and given/known data

    A flywheel of mass 181 kg has an effective radius of 0.63 m (assume the mass is concentrated along a circumference located at the effective radius of the flywheel).

    (a) What torque is required to bring this wheel from rest to a speed of 118 rpm in a time interval of 29.4 s?
    _____ N·m

    (b) How much work is done during the 29.4 s?
    ______ kJ

    2. Relevant equations


    3. The attempt at a solution

    How is this problem done when time is involved?? I am confused.

    I have done I = mr^2 and thats it.
  2. jcsd
  3. Nov 14, 2007 #2
    they give you 118 rpm. You have to convert that to angular velocity using factor label.

    You have time and final angular velocity, so you can find angular acc. using Vf = Vi + at where Vf=final angular vel, Vi = initial angular vel (which is 0 in this case), a=angular acc, and t=time.

    then just use your I = mr^2, but remember that T=Ia (torque = I * alpha) and remember that torque=F*d.

    Good luck!
  4. Nov 14, 2007 #3
    Bump...I am working on a similar problem and having trouble with part b. I have found the torque and I thought to find work I could use w=td where t is torque and d is rotational distance. What am I missing?
  5. Nov 14, 2007 #4
    Work is the change in kinetic energy. KEf - KEi, but since its at rest in the beginning, its KEf - 0, which is just w=KEf. Remember to consider rotational kinetic energy, because it is rotating. Good luck to you both!
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