Originally Posted by

**iva** Hi everyone, hope this is the right forum to put this question.

I've just read the theorem in my study guide called: Perpendicular Axes Theorem

It says that if X, Y and Z are mutually perpendicular axes of rotation, and assuming a rigid body forms a lamina which likes on the XY-plane then

Iz = Ix + Iy (I being moments of inertia about each respective axes)

It looks so simple BUT not every situation is it clear what axes is X, Y, or Z, so say you have 2 out of 3, how do you know that those 2 are the Z and X axes and you need to then subtract rather than add values. Is it always going to be obvious? Is there a way to know?

Thanks!