Two small spheres of mass $1 \mathrm{~kg}$ each are connected to each other by means of an unstretched spring of force constant $k=100 \mathrm{~N} / \mathrm{m}$ and natural length $2.4 \mathrm{~m}$. The system is placed on a smooth horizontal surface and the two spheres are given velocities $v_{u}$, as shown in the figure. The maximum elongation in the spring is found to be $0.4 \mathrm{~m}$ during the subsequent motion. Calculate $v_{0}$ in $\mathrm{m} / \mathrm{s}$. Take $\sqrt{31}=5.6$