The visual path of action from one extreme to another is always described
by an arc. Arcs in nature are the most economical routes by which a form
can move from one position to another. In animation, such arcs are used
extensively, for they make animation much smoother and less stiff than a
straight line for the path of action. In certain cases, an arc may resolve
itself into a straight path, as for a falling object, but usually,, even in a
straight line action, the object rotates.