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Homework Help: Radius of Gyration?

  1. Oct 28, 2012 #1
    1. The problem statement, all variables and given/known data

    3. The attempt at a solution
    Alright, so the solution manual shows that the intertia of the disk is:
    I = m * k2, where k is the radius of gyration​

    However why can't the inertia of the disk be:
    I = 0.5 * m * r2, where r is the radius of the disk​
    This is the formula for the inertia of a rotating disk... However when I plug in all my numbers I get a different inertia value when compared to the inertia value that I got using the radius of gyration.
  2. jcsd
  3. Oct 28, 2012 #2


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    Science Advisor
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    Gold Member

    Because the spool is not a uniform disk. You don't need to know why it's not uniform, just accept that you are given the effective radius.
  4. Oct 28, 2012 #3
    Ah okay, I wish it said that the density is not uniformly distributed... But thanks!
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