The equation of motion for a spring-mass system excited by a harmonic force is
\(M\ddot x + kx = F\cos \left( {\omega t} \right),\)
Where M is the mass, K is the spring stiffness, F is the force amplitude and ω is the angular frequency of excitation. Resonance occurs when ω is equal to1
\(\sqrt {\frac{M}{k}}\)
2
\(\frac{1}{{2\pi }}\sqrt {\frac{k}{M}}\)
3
\(2\pi \sqrt {\frac{k}{M}}\)
4
\(\sqrt {\frac{k}{M}}\)