A force as you defined in secondary school is a push or pull. Forces may fail to change an object’s motion if they are cancelled by other forces, e.g. the force of gravity pulling you down right now is being cancelled by the force of the chair pushing up on you. When nothing is moving, forces are balanced. The metric unit of force is the Newton (N). Forces may be internal (forces acting within the structure itself) or external (forces acting on the object from the outside) In simple terms; when you take a force and divide it over a particular crosssectional area you get stress.