Strain in Physics:
Under the influence of external forces, when a body gets deformed, the different parts of the body suffer relative displacements. As a result, the body changes length, volume or shape. This deformation or change in dimension is called strain. It generally relates to three aspects – length, volume and shape of an object. Strain is always represented as a ratio of two identical quantities and hence it has no unit. Starin is a dimensional quantity.
Stress in Physics:
When a body gets deformed under the influence of external forces, a reaction force develops in the body because of elasticity. This force tries to resist the external forces and helps to bring the body back to its unstrained condition after the deforming forces are withdrawn. The reaction force acting on per unit area of the cross-section of the body is called stress.
Since action and reaction are opposite but equal, stress is equal in magnitude to the force applied per unit area of the deformed body.
Stress = applied force/area of cross-section of the body
Unit and Dimension of Stress:
In SI, Unit of stress is N.m-2.
Dimension of stress = dimension of force / dimension of area
= MLT-2 / L2