A mass of 10 kg is suspended by a spring having a stiffness of 10000 N/m. The viscous damping causes the amplitude to decrease to one-tenth of the initial value in four complete oscillations. If a periodic force of 150 cos 50t is applied to the mass in vertical direction, (a) Find the amplitude of the forced vibration. (b) What is its amplitude at resonance? (c) Comment on the results obtained in part (a) and (b). (15 markah/marks)