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-kg block is sitting at rest and is then pushed horizontally with an initial acceleration of 3 m/s2, the force that sets the block in motion is 15 newtons (5 kg × 3 m/s2).