Amass of 100 g stretches a spring 5 cm. if the mass is set in motion from its equilibrium position with a downward velocity of 30 cm/s, and if there is no damping, determine the position u of the mass at any time t. (use g = 9.8 m/s2 for the acceleration due to gravity. let u(t), measured positive downward, denote the displacement in meters of the mass from its equilibrium position at time t seconds.)