Question

Which concept is difficult in rotational motion?

Answer 1

In rotational motion each and every concept is easy if you study it carefully but if you don't study then each topic will seem to be difficult.

Answer 2

Moment of Inertia
Imagine a small boat and a large boat floating alongside a dock. Push either of them with your foot and it moves. You can easily move, and stop, the small boat. But once you get the big boat moving, you have a heck of a time stopping it. Why? Mass. You can think of mass as the property that makes it difficult to change the motion of an object.