# Feynman's explanation of parallel axis theorem

In the book Feynman's Lectures on physics volume 1 chapter 19, He explains prallel axis theorem as follows.

Suppose we have an object, and we want to find its moment of inertia around some axis. That means we want the inertia needed to carry it by rotation about that axis. Now if we support the object on pivots at the center of mass, so that the object does not turn as it rotates about the axis (because there is no torque on it from inertial effects, and therefore it will not turn when we start moving it), then the forces needed to swing it around are the same as though all the mass were concentrated at the center of mass, and the moment of inertia would be simply $I_{1}=MR_{cm}^2$, where $R_{cm}$ is the distance from the axis to the center of mass. But of course that is not the right formula for the moment of inertia of an object which is really being rotated as it revolves, because not only is the center of it moving in a circle, which would contribute an amount $I_{1}$ to the moment of inertia, but also we must turn it about its center of mass. So it is not unreasonable that we must add to $I_{1}$ the moment of inertia $I_{c}$ about the center of mass.

I could see the first half of his explanation, but I could not understand last half of it. Why the forces needed to swing it are the same as if all the mass were concentrated at the center of mass and why we should turn the object about its center of mass?