Harmonic oscillator

A system which, when moved from its position at rest, experiences a force that tends to restore equilibrium; that force is usually proportional to the displacement, as, for example from a spring. Without other forces at work, the system is called a simple harmonic oscillator. In real life a frictional force (damping) may be present and proportional to the velocity, for example a viscous damper such as a shock absorber. The system may be driven by an external time-dependent force. If the external force is in phase with the natural frequency of the system (resonance), the oscillations will expand.