I am currently reading about moment of inertia. Now, I know inertia is an important concept; with it, we can determine how difficult something is to move. For linear motion, the difficulty of altering an object's position is dependent upon the mass of the object being moved. For rotational motion, the difficulty in rotating an object around an axis is dependent upon the mass, and the radius of that mass to the the axis of rotation?
Why do we need to different measures of inertia?
And for rotational motion, why is the inertia dependent upon mass, and its radial distance from the axis of rotation?