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Homework Help: Moment of inertia help, please

  1. Dec 1, 2006 #1
    This should be simple but I'm not sure whether I'm doing it right or not...
    I have a rod rotating about its edge #1. Rod's length is L and its mass is M. There is a point mass m on the rod, 3/4 L away from the edge #1, and on the other edge (#2) there is a sphere of radius R and mass m1. I need to find the moment of inertia of the whole system. I know that I need to use parallel axis theorem for the point mass, but what should I do with the sphere?
    Is it I = (1/3) ML^2 + m (0.75L)^2 + m1*(L)^2 + (2/5)m1*R^2
    or I = (1/3)ML^2 + m (0.75L)^2 + (2/5)m1*R^2?
    I think it is the first option, but I'm not sure.
    Thank you!
  2. jcsd
  3. Dec 1, 2006 #2


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    Option 1 loooks correct, but actually, you used the parallel axis theorem for the sphere, not the point mass, which is correct.
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