Chapter 4.4, Problem 6P

(a) A mass of $100$ g stretches a spring $5$ cm. If the mass is set in the motion from its equilibrium position with a downward velocity of $10$ cm/s, and if there is no damping, determine the position $y$ of the mass at any time $t$. When does the mass first return to its equilibrium position ?

(b) Draw a phase portrait of the equivalent dynamical system that includes the trajectory corresponding to the initial value problem in part (a).