Newton's Laws of Motion/First Law

Physical Law
Newton's first law of motion is one of three physical laws that forms the basis for classical mechanics.

Statement of Law

 * Every body remains in a state of constant velocity unless it is acted upon by an external unbalanced force.

This tendency is sometimes called inertia.

Hence this law is also sometimes called the principle of inertia.

Also see

 * Newton's Second Law
 * Newton's Third Law

It is worth pointing out that Newton's first law of motion is in fact a special case of Newton's Second Law, putting $\mathbf F = 0$.

As he put it:
 * A body remains at rest or, if already in motion, remains in uniform motion with constant speed in a straight line, unless it is acted on by an unbalanced external force.