Stress and Strain:

When an external force or a system of forces is applied on a body, restoring forces are developed due to displacements of molecules from their respective positions of equilibrium. The restoring force opposes the external (deforming) force. In equilibrium, the restoring force is equal in magnitude and opposite in direction to the external deforming force. Stress is described as the restoring force per unit area of cross-section of the body. As per the Mathematically equation, we write:

Stress = Restoring Force (F) /Area (A)

The unit of stress in SI system is Nm^-2. In the cgs system, stress is measured in dyne cm^{- 2}. The dimensional formula for stress is ML^-1 T^-2.

Figure: Longitudinal Stress