sentence: External forces are either surface forces or body forces.
sentences_before: Stress is a measure of the internal forces in a body between its particles. These internal forces are a reaction to the external forces applied on the body that cause it to separate, compress or slide.
sentences_after: Stress is the average force per unit area that a particle of a body exerts on an adjacent particle, across an imaginary surface that separates them.
concept_name: external force
headings: Stress (mechanics)
categories: Materials science Elasticity (physics) Mechanics Engineering