We know that whenever a body is deformed a restoring force originates and tries to restore the shape of the body and according to newtons third law of motion whenever a force acts on a body an equal and opposite force acts on the other body . So is the restoring force an opposite reacton to the deforming force and if it is not then why do we consider its direction equal and opposite to the deforming force while solving problems .
(Asked Akshat )
Answer: Restoring force comes into play only when a deforming force is exerted. When no external force acts on a body, the molecules are in equilibrium. No net force acts on it. If an external force is applied on it, it will try to increase or decrease the intermolecular distance giving rise to a state of inequilibrium to teh molecules and they will tend to go back to their original positions to retain their state of equilibrium. This gives rise to the RESTORING FORCE.
Restoring force the the internal force that comes into play whenever an external force tries to change the inter-molecular distance. The restoring force at any instant (within limits) is equal in magnitude and opposite in direction to the deforming force