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Homework Help: Moment of inertia for composite objects

  1. Dec 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the moment of inertia of these composite objects.

    I've attached two different composite objects; each rod has length L and mass M.

    2. Relevant equations
    I = Icm + MD2
    Long thin rod with rotation axis through centre: Icm = 1/12ML2
    Long thin rod with rotation axis through end: Icm = 1/3ML2

    3. The attempt at a solution
    (a) For the image on the left:
    I first found the total inertia of the objects: Icm total = 1/12ML2
    Then applied the Parallel Axis Theorem, because the composite object is rotating parallel to the actual axis of rotation: I = 5/12ML2 + 2MD2 (where D is the distance between the object's centre of mass and axis of rotation).
    Feedback on this?

    (b) For the image on the right:
    Since one of the rods are going through the axis of rotation, I don't think I need to apply the Parallel Axis Theorem... So I just found the total inertia for both objects based on where their axis of rotation is.

    I = ICM axis through centre + ICM axis through end
    = 1/12ML2 + 1/3M(L/2)2
    = 1/12ML2 + 1/3ML2/4
    = 1/6 ML2

    Attached Files:

  2. jcsd
  3. Dec 8, 2015 #2


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    Staff: Mentor

    Can you give some more details about your determination of the ##I_{cm}## for the first image? I don't see how it turns out to be the same as that of a single rod about its center.
  4. Dec 9, 2015 #3
    You shouldn't need to use the parallel axis theorem for either situation.
    For part (b), aren't all points on the vertical thin rod at a distance of L/2 from the axis of rotation;
    or in other words where is the center of mass of the vertical rod in part (b)?
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