An SDOF system (m = 20 kg, k = 350 N/m) is given an initial displacement of 10 mm and initial velocity of 100 mm/s. (a) Find the natural frequency; (b) the period of vibration; (c) the amplitude of vibration; and (d) the time at which the third maximum peak occurs. Ans: 0.666 Hz; 1.502 sec; 25.91 mm; 3.285 sec. Which Equation should I use to obtain the amplitude of vibration and time for the third maximum pick occur? Part a and b I got the answer.