When a deforming force is applied to a body to modify its shape and size, as per Newton’s 3rd Law, a restoring force will be generated inside the body which is equal to but opposite of the deforming force. Say, the deforming force is F and the restoring force is F1.
Then, F1 = – F. Here F is the action and F1 is the reaction force.
Definition of Stress in Physics
The restoring force per unit area is called stress.
Formula of Stress in Physics | Equation of Stress
Considering only the magnitude of the restoring force, we can write the expression of stress as:
Stress = F / A, where A = area of cross-section of the body where action-reaction forces are being applied.
The unit of stress in SI is Newton/meter2 or Pascal.
Related study: Mechanical properties of Solid class 11 – class Notes