We know that momentum of a closed or isolated system is always conserved . Now for INELASTIC collision, lets make It simple,consider a system with only two moving particles. If they collide inelastically, we know that energy is surely lost! now the system is no longer a closed system right?? The total momentum of d system must be decreasing right? However we could still use d principle of conservation of momentum equation is because during collision the impulsive force is totally exerted onto both colliding objects and is not to external factor right??