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Homework Help: Moments of Inertia, almost got it

  1. Oct 27, 2004 #1
    The problem is:

    Three identical thin rods, each of length L and mass m, are welded perpendicular to one another. The assembly is rotated about an axis that passes through the end of one rod and is parallel to another. Determine the moment of intertia of this structure.

    Ok what I know is that the rod which is connected to the axis of rotation has inertia of I = 1/3ML^2
    And I know that the rod which is parallel to the axis has inertia of I = ML^2

    So far I have (1/3ML^2) + (ML^2) and I just need one other moment of intertia which I am having difficulty finding.

    I believe I need to apply the parallel-axis theorem, but I dont understand why.

    If someone could please explain why you would use this theorem it would be very helpful! Thanks!
  2. jcsd
  3. Oct 27, 2004 #2


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    Can you elaborate? There are MANY ways to join three rods at right angles to each other. Are they joined at their midpoints, ends, something in between or other permutations?
  4. Oct 27, 2004 #3
    The three rods are joined at their midpoints.
  5. Oct 27, 2004 #4
    do you mean the assembly looks some what like an xyz coordinate system with all of the centers of mass(midpoints) at the origin and rotating about one of the axes? If thats the case I don't think that the parallel axis theorem applies.
  6. Oct 28, 2004 #5
    Yes the assembly looks some what like an xyz coordinate system with all of the centers of mass (midpoints) at the center.
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