Hooke's law
Consider the vertical motion of a particle (mass m) suspended from a fixed support by a spring (stiffness k, natural length l). Let x be the length of the spring at time t. By using Hooke's law, the equation of motion 
is obtained. The equilibrium position is given by 0 = mg−k(x−l); that is, x = l + mg/k. Letting x = l + mg/k + X gives 
, where ω2 = k/m. Thus the particle performs simple harmonic motion about the equilibrium.
http://webphysics.davidson.edu/applets/animator4/demo_hook.html
• An interactive illustration of Hooke's law.
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