Physics · Mechanical Properties Of Solids · NEET
Yes, while the body is in equilibrium and within the elastic limit, the internal restoring force is equal in magnitude and opposite in direction to the applied deforming force. This balance is why the stretched wire stays still instead of moving. If the deforming force grows, the restoring force grows to match it, until the material reaches its elastic limit.
The restoring force is internal. It comes from the forces between the molecules of the solid. When you pull the molecules apart (or push them together), they resist and try to return to their normal spacing. The deforming force, on the other hand, is external — it is applied from outside by a load, hammer, or fluid.
Stress is defined as the internal restoring force per unit area of cross-section. Because the restoring force equals the deforming force at equilibrium, you can calculate stress using either value, so numerically stress = applied force / area. But the physical meaning of stress is the restoring force acting inside the material.
Below the elastic limit the restoring force fully balances the deforming force and the body returns to its original shape when the load is removed. Beyond the elastic limit the material cannot generate enough restoring force to recover fully, so a permanent (plastic) deformation is left behind. If loading continues, the material eventually fractures.
An ideal rigid body does not change shape or size at all, so its molecules never move from their normal positions. Since restoring force appears only when molecules are displaced, an ideal rigid body would show no deformation and no restoring force. Real solids are not perfectly rigid, so they always deform a little and develop a restoring force.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the external force applied on a solid that changes its length, shape, or volume — for example the weight that stretches a wire or the pressure that squeezes a ball.
It is the internal force that a deformed solid develops on its own. It is equal and opposite to the deforming force and tries to bring the body back to its original shape and size.
They are equal and opposite like an action-reaction pair, but strictly they act on the same body (external vs internal), so in equilibrium they simply balance each other and keep the body still.
The restoring force per unit area is called stress, and the fractional change in dimension caused by the deforming force is called strain. Within the elastic limit, stress is proportional to strain (Hooke's law).
Every problem on stress, strain, Young's modulus, and elasticity starts from this idea. Knowing that stress uses the internal restoring force (= applied force at equilibrium) helps you set up numericals correctly and avoid direction mistakes.
