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Homework Help: Moments of Inertia of a rod

  1. Nov 14, 2006 #1
    The moment of inertia about an axis along the length of a rod is zero, correct?
     
  2. jcsd
  3. Nov 14, 2006 #2

    OlderDan

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    Almost. If the rod has a measurable radius, it is a cylinder.
     
  4. Nov 15, 2006 #3
    In all the other parts ot the problem though I considered it a rod.

    Here's the problem:

    Find the moment of inertia about each of the following axes for a rod that is 0.280 cm in diameter and 1.70 m long, with a mass of 5.00×10−2 kg.

    A. About an axis perpendicular to the rod and passing through its center.
    I = (1/12)ML^2
    I = .012 kgm^2

    B. About an axis perpendicular to the rod passing through one end.
    I = (1/3)ML^2
    I = .048 kgm^2

    C. About an axis along the length of the rod.
     
  5. Nov 15, 2006 #4

    OlderDan

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    The rod has a diameter. It has a moment of inertia about the long axis.
     
  6. Nov 15, 2006 #5
    So, I = (1/4)MR^2 + (1/3)ML^2 ?
     
  7. Nov 15, 2006 #6

    Doc Al

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    The length of the stick--or cylinder--parallel to the axis doesn't matter, only the radius.
     
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