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Homework Help: Torque required to spin a disk

  1. Nov 23, 2008 #1
    1. The problem statement, all variables and given/known data
    A 250 g , 20.0-cm-diameter plastic disk is spun on an axle through its center by an electric motor.
    2. Relevant equations
    What torque must the motor supply to take the disk from 0 to 2000 rpm in 4.50 s?

    3. The attempt at a solution
    I know that Torque = Inertia times angular acceleration or T = Ia.
    I tried using the formula for inertia of a solid disk, I = 1/2Mr^2. So with that I get 1/2(.25)(.1^2) = .00125. Then i found angular acceleration: 2000 rpm = 209.44 rad/s divided by 4.5 s so 209.44 rad/s / 4.5s = 46.54 s^2. I multiply these two number together and get .00125*46.54 = .06 Nm. However, this is wrong. What am I doing wrong?
  2. jcsd
  3. Nov 23, 2008 #2
    Your working seems correct to me...
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