A block of mass $\mathrm{M}$ is placed on a smooth horizontal surface and it is pulled by a light spring as shown in the diagram. If the ends A and B of the spring are moving with $4 \mathrm{m} {\mathrm{s}}^{-1}$ and $2 \mathrm{m} {\mathrm{s}}^{-1}$ respectively and the rate at which spring energy is increasing is $20 \mathrm{J} {\mathrm{s}}^{-1}$. then what is the value of spring force (in $\mathrm{N}$)?