Definition: it is either a push or pull that one object exerts on the other object.
SI Unit: Newton (N)
(1) Start an object moving.
(2) Stop a moving object.
(3) Change the direction and speed of a moving object.
(4) Change the size or shape of an object.
(1) Contact-forces which exist between objects when they are in contact. e.g friction, tension
(2) Non-contact forces which act at a distance => objects are not in contact with each other. e.g magnetic force, electric force, gravitational force