• A force is an action that changes, or tends to change, the state of rest or motion of a body.
• When we kick a ball at rest, it starts moving.
• When we press a spring, it is deformed.
• In both cases a force is the cause of the change.
• The unit of force in the international system is the newton, written N.
• One newton is the force that gives a body of mass 1 kg an acceleration of 1 m/s², that is:

1 N = 1 kg × 1 m/s²

• In problems involving weight we usually take g = 10 m/s², unless the problem states otherwise.