Mechanical Properties of Matter

Elasticity The property of a material to regain its original shape and size after the deforming force is removed. When a force is applied to an elastic body, it deforms. If the force is removed and the body returns completely to its original dimensions, the material is perfectly elastic. Rubber bands and steel springs are practical examples, though no material is perfectly elastic — there is always some permanent deformation beyond a certain limit. Stress (σ) The restoring force per unit area set up inside a deformed body, equal in magnitude to the applied force per cross-sectional area. Stress is a measure of the internal forces that neighbouring particles of a material exert on each other. It is defined as σ = F/A, where F is the force applied perpendicular to the area A. For example, a 100 N force acting on a cross-sectional area of 2 × 10⁻⁴ m² produces a stress of 5 × 10⁵ Pa. Stress has the same units as pressure (Pa or N m⁻²).