A moving railway compartment has a spring of constant k is fixed to its front wall. A boy stretches this spring by distance x and in the mean time the compartment moves by a distance s. The work done by boy w.r.t. earth is : 

\(\frac{1}{2}kx^2\)

By Conservation of Energy :

Work done by the boy = Energy stored in spring

   = \(\frac{1}{2}kx^2\)