# force

A force is the cause of a change in motion. Force is defined by Newton's
second law as the rate of change of momentum or, equivalently, as the product of mass and acceleration (*F* = *m* × *a*). It is a vector quantity,
in the direction of the acceleration it brings about. Forces always occur
in equal and opposite action-reaction pairs between bodies, though it is
often useful to think of one body being in a force field.

The SI (international system) unit of force is the newton.
For example, if a 5-kilogram block is sitting at rest and is then pushed horizontally
with an initial acceleration of 3 meters per second per second (m/s^{2}), the force that sets the
block in motion is 15 newtons (5 kg × 3 m/s^{2}).