Oscillations occur when a system that has been disturbed from stable equilibrium experiences a force that tends to restore the equilibrium; if it didn’t, its equilibrium wouldn’t be stable. When the restoring force is directly proportional to the displacement from equilibrium, the resulting oscillation is called simple harmonic motion (SHM). When a system is subjected to forces that vary at or near its natural oscillation frequency, resonance results in the buildup of large-scale oscillations, which can be damped by friction and related forces.