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Homework Help: Another Angular Acceleration Problem

  1. Nov 6, 2005 #1
    The problem states: when the play button is pressed, a CD accelerates uniformly from rest to 450 rev/min in 3.0 revolutions. If the CD has a radius of 6.0-cm and a mass of 17-g, what is the torque exerted on it?

    I used the the forumla tau = I * alpha; I = .5(0.017)(0.06)^2 = 3.06E-5;
    in order to calculate alpha, I used wf^2 = wo^2+2alpha(theta); from this I get alpha = square root (wf^2)/(2*theta). I realize some converting is necessary so I convert 450 rev/min into 47.124 rad/s, and 3.0 revolutions into 18.50 rad. Applying these figures to solve for alpha, I get 60.018 rad/s^2. Finally, tau = I*alpha = 0.00184 N*m.

    This is incorrect. Can someone direct me in the right direction? Thanks in advance.
  2. jcsd
  3. Nov 6, 2005 #2
    You have two equations, omega = alpha * t, and fi = ½ * alpha * t^2, these equations are like v = at and s = ½at^2 in linear motion. You also have two unkwons, so you can solve both.

    Then use M = J*alpha.
  4. Nov 6, 2005 #3


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    alpha = wf^2/(2 theta) , no sqrt.
    you see Work and Energy hiding in this kinematic equation?
  5. Nov 7, 2005 #4
    I still don't have a clue where to go with this. I was in error, there is no sqrt., but the solution I'm getting is incorrect. The answer to this problem is 0.0072-N*m. How is that? What am I doing wrong?
  6. Nov 7, 2005 #5


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    I get the same answer as you. Maybe the given answer is incorrect.
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