I am having problem with a relatively simple problem that I probably have done before but because my Physics have become so rusty recently I just can't get the right answer. You have a disc that sitting about some point P on the ground - so its upright - and there is a hole cut out on the top of the disc so that the top of the hole touches the top of the disc. The hole has a radius of a/2 and the disc and radius of a. The center of mass of the disc c is a/6 from the center of the disc. And the disc has a uniform mass density of v. What is the rotational inertia of the disc about the point P that it touches on the ground? So parallel axis theorem right? Ip = Icm + ML^2. My problem is what is Icm? 1/2M*L^2 by any means. But what is L? Usually if the disc' center of mass is about its center then its just the radius but here the center of mass is a/6 from the center of the disc.