Rotation about CM -- Can I use Moment of Inertia? 1. The problem statement, all variables and given/known data I have a program I coded to simulate the movement of a some point masses connected to each other rigidly. Each mass is the same. I am trying to code in the correct equations for rotation, but I am having some difficulty with a nonzero product of inertia. 3. The attempt at a solution I have the x, y, and z coordinates to these masses where x y and z are n by 1 matrices (where I have n masses). I calculated the center of mass as: xcm = average(x) ycm = average(y) zcm = average(z) Then I defined new coordinates, xx yy and zz: xx = x - xcm yy = y - ycm zz = z- zcm The following Wikipedia article then says, "With respect to a coordinate frame whose origin coincides with the body's center of mass, they can be expressed in matrix form as:" and then it shows that the rotation depends only on the moments of inertia. http://en.wikipedia.org/wiki/Newton–Euler_equations However, I then compute the product of inertia: sum(yy.*zz), which does not equal 0! where .* means element-wise multiplication. What is going on here? Am I computing something incorrectly, is Wikipedia wrong, or am I just misunderstanding something?