The spring constant = the measure of stiffness = 2000\(Nm^{-1}\)

Energy stored in a spring = Work done to stretch it through a distance = change in potential energy

= \(\frac{kx^{2}}{2}\) = \(\frac{2000 \times (0.04)^{2}}{2}\) (converting 4cm to metres)

= \(\frac{3.2}{2} = 1.60J\)