Let us imagine that there is a Massless spring with fixed at one end. Unloaded length = L Spring constant: = k Mass = 0 The spring is at static Equilibrium Force: F Displacement: d F = k d, elongated length: L+d If I remove F, what will happen ? 1. Spring return to the original length: L and Stop moving 2. Spring length vibrate between "L - d" and "L+d" Note: there is no mass, damping, friction.