Two particles which are initially at rest, move towards each other under the action of their interna

Question

Two particles which are initially at rest, move towards each other under the action of their internal attraction. If their speeds are $v$ and $2 v$ at any instant, then the speed of centre of mass of the system will be\

Professor PC Thomas Classes · Answer

(b) : As no external force is acting on the system, the centre of mass must be at rest i.e. $v_{\mathrm{CM}}=0$.