When an oscillating external force is applied on mass nature of vibration will change dramatically. Here oscillating frequency will be same external force frequency, it is not related natural frequency of the system. Amplitude of the oscillation is not the initial stretch given by the user, but it will be strong function of external frequency and natural frequency. Amplitude of oscillation will become very high when external frequency and natural frequency values are very close.If external force frequency is same as natural frequency of material it will result in a phenomenon called resonance, where amplitude of oscillations become so high and it will eventually result in damage of material.