Laws of motion

First law of motion

A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force or if the sum of the external forces is not zero.

Second law of motion

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body (F = ma).

Third law of motion

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.