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Homework Help: Inertia matrix and centre of gravity of a set of rods

  1. Mar 1, 2008 #1
    1. The problem statement, all variables and given/known data
    Three uniform rods OA, OB, and OC, each of mass 3m and length a, lie along the x, y and z axes, respectivley. Show that the inertia matrix relative to Oxyz is

    Io = 2ma^2 \left(\begin{array}{ccc}1&0&0\\0&1&0\\0&0&1\end{array}\right)

    Determine the inertia matrix with respect to a parallel frame through the centre of mass G.

    2. Relevant equations

    3. The attempt at a solution

    The proof was straightforward enough, but finding Ig has proven difficult. The book lists it as

    Ig = 1/4ma^2 \left(\begin{array}{ccc}6&-1&-1\\-1&6&-1\\-1&-1&6\end{array}\right)


    Our current problem is finding the centre of mass of the system. I thought it would be G(a/2, a/2, a/2), as each rod is of length a, but the answer lists this as G(a/6, a/6, a/6). Can anyone point out where I've gone wrong? Thanks.
  2. jcsd
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