1. The problem statement, all variables and given/known data Hi, I've got a Dynamics question on moments of inertia and I'm not really sure what the question is trying to get at. The question is let R=(O,B) be a rest frame of a rigid body and let the entries in J(R) be Jij. Show that the moment if inertia about an axis through O in the direction of the unit vector with components xi is Jijxixj. 2. Relevant equations The definition that we have been given is that the diagonal entries of the matrix J(R) A,B and C are the moments of inertia about the x, y, z axes respectively. 3. The attempt at a solution I know that if you have two rest frames R(O,B) and R'(O,B') and H transition matrix from B' to B then Jij=HipJpq'Hjq. Am I supposed to use this? I'm really confused..