The amplitude become large when the frequency of the driving force (ω) is near the natural frequency of oscillation or when ω=ω0. This frequency is known as resonance frequency. Amplitude of oscillation for a forced, damped oscillator is
A=f0/m√(ω2−ω20)+(bω/m)2
where b is constant related to the strength of the resistive force, ω0=√k/m is natural frequency of undamped oscillator (b=0).