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Homework Help: Finding the number of revolutions

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

    Starting from rest, a 12cm diameter compact disk takes 2.7s to reach its operating angular velocity of 2005rpm. Assume that the angular acceleration is constant. The disk's moment of inertia is 2.5 x 10-5 kgm^2.
    a.) How much torque is applied to the disk?
    b.) How many revolutions does it make before reaching full speed?

    2. Relevant equations


    3. The attempt at a solution
    For part a I used t = I(delta w/delta t). I found w by finding the frequency (33.417rev/s), and then I plugged f into w = (2pi radians)f to get 209.965. I then found torque to be 1.944 x 10-3 Nm. I am unsure of how to solve for part b.
  2. jcsd
  3. Oct 14, 2007 #2


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  4. Oct 14, 2007 #3
    I tried using delta theta = wi (delta t) + 1/2(alpha)(delta time)^2. I got 283.453, but that wasn't right.
  5. Oct 18, 2007 #4
    Divide by 2(pi) to put your answer in radians
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