<P> Restoring force, in a physics context, is a force that gives rise to an equilibrium in a physical system . If the system is perturbed away from the equilibrium, the restoring force will tend to bring the system back toward equilibrium . The restoring force is a function only of position of the mass or particle . It is always directed back toward the equilibrium position of the system . The restoring force is often referred to in simple harmonic motion . The force which is responsible to restore original size and shape is called restoring force . </P> <P> An example is the action of a spring . An idealized spring exerts a force that is proportional to the amount of deformation of the spring from its equilibrium length, exerted in a direction to oppose the deformation . Pulling the spring to a greater length causes it to exert a force that brings the spring back toward its equilibrium length . The amount of force can be determined by multiplying the spring constant of the spring by the amount of stretch . </P> <P> Another example is of a pendulum . When the pendulum is not swinging all the forces acting on the pendulum are in equilibrium . The force due to gravity and the mass of the object at the end of the pendulum is equal to the tension in the string holding that object up . When a pendulum is put in motion the place of equilibrium is at the bottom of the swing, the place where the pendulum rests . When the pendulum is at the top of its swing the force bringing the pendulum back down to this midpoint is gravity . As a result gravity can be seen as the restoring force in this . Restoring force of a spring: (f = - kx) </P>

In simple pendulum motion restoring force is provided by