Question

Why don't we consider both the forces while calculating the magnitude of stress in an elastic body?

Answer 1

Because the expression for stress is,

Stress = Deforming force / Area

This expression says that stress is dependent only on the deforming force which is stored in the body in opposite direction when the force is applied, and the area of contact. Value of stress is not dependent on the force given, so we don't have to consider that force too.

Deforming force has the same magnitude as that of the applied force. So if applied force is also considered, then the value of the deforming force is doubled.

So on finding value for stress, only deforming force and area are needed. We don't have to consider both the forces.