Chapter 9, Problem 4CQ

A uniform ring of mass M and radius R and a point mass M a distance R from the axis of the ring both rotate about that axis, which is perpendicular to the ring at its center. Both objects have the same formula for their moment of inertia, I = MR², about this axis. (a) Is there a good physical reason that they have the same formula? (b) If the ring were replaced by a solid uniform disk of mass M and radius R, would the disk and point mass then have the same moment-of-inertia formula? Why or why not?