Question

A bar that has weights attached to its ends is more difficult to rotate than one with weights attached close to the centre of the bar.Why?​

Answer 1

Explanation:

You presumably already know that in the absence of external forces, the center of mass of any collection of particles moves at a constant velocity. This is true whether they are stuck together in a single body or are just a bunch of separate bodies with or without interactions between them. We now move to a frame of reference moving at that velocity. In that frame the CofM is stationary.

Now suppose that the particles are indeed stuck together to form a rigid body. We see that the body is moving so that: 1) the CofM remains fixed, 2) all the distances between the particles are fixed. (This second condition is what is meant by a rigid body after all).

A motion with these two properties, (1) and (2), is precisely what is meant by the phrase ``a rotation about the CofM''