According to the equation of motion of the centre of mass

$M {\mathbf{a}}_{\mathrm{C}\mathrm{M}}={\mathbf{F}}_{\mathrm{e}\mathrm{x}\mathrm{t}}$

If ${\mathbf{F}}_{\mathrm{e}\mathrm{x}\mathrm{t}}=0,\mathrm{ }{\mathbf{a}}_{\mathrm{C}\mathrm{M}}=0$

$\therefore$ ${\mathbf{v}}_{\mathrm{C}\mathrm{M}}=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}$

$\mathrm{i.}\mathrm{e.}$, if no external force acts on a system the velocity of its centre of mass remains constant. Thus, the centre of mass may move but not accelerate.