回复力方程

Equation for the restoring force:

If the system is perturbed from this equilibrium state (i.e., if the mass is moved, so that the simple harmonic oscillator becomes extended) then the mass experiences a restoring force given by Hooke's law or the equation of restoring force: $$f = -k x$$

Here, $k>0$ is the force constant of the spring. The negative sign indicates that $f$ is indeed a restoring force. Note that the magnitude of the restoring force is directly proportional to the displacement of the system from equilibrium (i.e., $f\propto x$). Of course, Hooke's law only holds for small spring extensions. Hence, the displacement from equilibrium cannot be made too large.