1)what is the definition of moment of inertia?

Moment of inertia is the inertia of a rotating body with respect to its rotation. So basically it's the object's resistance to a rotational acceleration. This relates to Newton's first law! What does that exactly mean? Let's check out the explanation.

One formula that it is written in is I= mr

Explanation:

As Bill Nye says, "Inertia is a property of matter. Objects that are not moving don't move unless they get pushed or pulled. Moving objects keep moving unless they get pushed or pulled. This feature of objects and materials is what we call inertia."

I would check out Dan Fullerton's concept

and Organic Chemistry

it is a negative acceleration