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.

Also known as
This law is also sometimes called the principle of inertia.

It is also often referred to as just Newton's first law.

Also see

 * Newton's Second Law of Motion
 * Newton's Third Law of Motion

It is worth pointing out that Newton's first law of motion is in fact a special case of Newton's Second Law of Motion, 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.