Harmonic oscillators, for instance a coil spring or a pendulum, as sketched in figure2.5, require a mass. After being deflected from its resting position, it returns to its equilibrium position. The restoring force can, for instance, be a spring or gravity.One simple harmonic oscillator consists of a linear spring with spring constant, κ , loaded with a mass, m. The natural frequency, f n , of such an oscillator is defined as: