The law that says that the tension in a spring or a stretched elastic string is proportional to the extension. Suppose that a spring or elastic string has natural length l and actual length x. Then the tension T = (λ/l)(x−l), where λ is the modulus of elasticity, or T = k(x−l), where k is the stiffness.
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.