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Homework Help: Need help/varify on a torque/moment of inertia problem

  1. Feb 25, 2007 #1
    1. The problem statement, all variables and given/known data
    Hi, this is my first time on this forum, so don't flame me for not doing things right. I'm a junior in high school and currently taking AP physics, this problem in my hwk has been bothering me for a while. I've never recieved the answer to this question and i jsut need to know if i did this right, since this is really the first moment of inertia problem i encountered...
    the problem is:
    A force of 15N is applied tangentially to the edge of a 0.88kg solid disc initially at rest. The radius of the disc is 0.55m. How fast will the disc be spinning after it has gone 3.0 complete rotations? (disregard all the friction/air resistance etc.)

    2. Relevant equations
    Torque = I (alpha) from F=ma
    W^2= Wo^2 + 2 (alpha)(theta)
    Torque= Lever arm x Force applied

    3. The attempt at a solution

    The moment of Inertia of a solid disc is I=1/2mr^2 >> I=1/2(0.88)(0.55^2) >> I=0.1331
    Torque= Lever arm x Force applied Torque= 15N x 0.55m = 8.25Nm
    Torque = alpha x I >>> 8.25= alpha x 0.1331 >>> alpha = 62 (this doesnt look right )
    Then just use Kenematic Equations
    since 3 rotations is 6pi
    W^2=0(62)(6pi)>>> W= 48.35radians/sec (this doesn't look right either )

    Feel free to correct me on watever...i admitt, i suck/hate these problems.
  2. jcsd
  3. Feb 25, 2007 #2
    Looks okay to me, but it is always easy to follow someone's work and make the same simple mistake they did.
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