Newton's Laws of Motion

The three laws that found classical mechanics.

First law (inertia)

A body remains at rest, or in uniform motion in a straight line, unless acted upon by a net external force.

The net force on a body equals the rate of change of its momentum:

\vec{F} = \frac{d\vec{p}}{dt} = m\vec{a}

For constant mass this reduces to the familiar $\vec{F} = m\vec{a}$ .

A $2\,\text{kg}$ block pushed with a net force of $10\,\text{N}$ accelerates at

a = \frac{F}{m} = \frac{10}{2} = 5\,\text{m/s}^2

For every action there is an equal and opposite reaction:

\vec{F}_{AB} = -\vec{F}_{BA}