T he equation of motion for a spring-mass system excited by a harmonic force is
$ M\ddot x+Kx=F\;c\mathrm{os}\left(\omega t\right) $,
where $ M $ is the mass, $ K $ is the spring stiffness, $ F $ is the force amplitude and $ \omega $ is the angular frequency of excitation. Resonance occurs when $ \omega $ is equal to