If resultant external force on a system of interacting particles is zero, its kinetic energy

For a system, net work done by all external and internal forces is equal to the change in kinetic energy.

$\mathrm{i}.\mathrm{e}., {\mathrm{W}}_{\mathrm{ext}}+{\mathrm{W}}_{\mathrm{int}}={(\mathrm{K}.\mathrm{E})}_{\mathrm{f}}-{(\mathrm{K}.\mathrm{E})}_{\mathrm{i}}$

$\mathrm{Here} {\mathrm{W}}_{\mathrm{ext}}=0 \mathrm{but} {\mathrm{W}}_{\mathrm{int}}\ne 0$