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Need verify on a momment of inertia/torque problem

  1. Feb 25, 2007 #1
    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.)

    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 :cry: )
    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 :mad: :cry: )

    Basicly my thoughts through the problem ^^^^^^^^^
    Feel free to correct me on watever...i admitt, i suck/hate these problems.
    Thanks :biggrin:
  2. jcsd
  3. Feb 25, 2007 #2


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    Staff Emeritus
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    Gold Member

    I have not verified your numbers, but the approach is correct.

    I will suggest that you make it a habit to carry the units along in your calculations. Properly used, units can be a real aid in arriving at the correct solution.

    Note: I have moved this post to the homework help section. In the future please post such problems here.... Welcome to Physics fourms.
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