# Newton's Laws of Motion/First Law

(Redirected from Principle of Inertia)

## 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 of course includes it being stationary, that is, with a constant velocity of zero.

This tendency is sometimes called inertia.

As Isaac Newton himself 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.

## 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

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 $\mathbf F = 0$.

## Source of Name

This entry was named for Isaac Newton.