The spring constant = the measure of stiffness = 2000Nm−1\mathrm{Nm}^{-1}Nm−1
Energy stored in a spring = Work done to stretch it through a distance = change in potential energy
= kx22\;\frac{kx^{2}}{2}2kx2 = 2000×(0.04)22\;\frac{2000 \times (0.04)^{2}}{2}22000×(0.04)2 (converting 4cm to metres)
= 3.22=1.60 J\;\frac{3.2}{2} = 1.60\,\mathrm{J}23.2=1.60J
