What is the definition of rotational inertia?
The moment of inertia is a measure of how difficult it is to change the angular velocity of an object. To use an analogy, objects with large mass require large forces to cause them to accelerate. Objects with smaller mass require smaller forces to cause them to accelerate the same amount. The moment of inertia plays the same role in rotational dynamics as the mass plays in linear dynamics. Just remember, large moments of inertia mean you need large torques to change the angular velocity. Small moments of inertia mean you need smaller torques to change the angular velocity.
