A mass m is suspended at the end of a spring, its weight stretches the spring
by a length L to reach a static state (the equilibrium position of the system).
Let u(t) denote the displacement, as a function of time, of the mass relative
to its equilibrium position. Recall that the textbook’s convention is that
downward is positive Then, u > 0 means the spring is stretched beyond its
equilibrium length, while u < 0 means that the spring is compressed. The
mass is then set in motion (by any one of several means).