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Moment of inertia help

  1. Oct 4, 2004 #1
    My problem is trying to calculate the moment of inertia for a rod with two weights on the each end (like the type weightlifters lift) turning in a horizontal plane with a wire fixed to the middle of the rod.

    How do you calculate this? I suppose it could be seen as two cylinders, one small radius big thickness, one large radius small thickness, but how do i sum these together and how do i calculate this.

  2. jcsd
  3. Oct 6, 2004 #2
    moment of inertia

    Try using the parallel axis and perpendicular axis theorems.

    Parallel axis theorem: The moment of inertia (MoI) of a body about any axis parallel to an axis passing through its center of mass (CoM) is equal to the sum of the MoI about the CoM and its mass*(the distance between the two axes ).

    Perpendicular axis theorem. The sum of MoI's of a plane body about two perpendicular axes through its plane is equal to the MoI of the body about an axis perpendicular to the plane and passing through the point of intersection of the two previous axes.

    tell me if that helps.

  4. Oct 7, 2004 #3
    Yes it does spacetime, the parallel axis theorem seems to be what i required because it appears to allow me to just regard the masses as two cylinders with the axis of rotation through their centres. This allows me to simply add mr^2 to the equation for a cylinder, (i think). That also works damn well with the results i have picked up from my investigation.

    Thanks for your help.
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