Final answer:
The conservation of momentum occurs in an isolated system where the net external force is zero, ensuring internal forces within the system cancel each other out and do not change the total momentum.
Step-by-step explanation:
The total momentum of a system of objects is conserved under specific conditions. The principle of conservation of momentum states that the total momentum of an isolated system is conserved when the net external force acting on the system is zero. This means that even as objects within the system collide or move, their internal forces are balanced and cancel each other out, leaving the total momentum unchanged. However, if any external forces, such as friction or gravity, act upon the objects in the system, the momentum can change. Therefore, for the total momentum to remain conserved, we must have an isolated system with no net external forces acting on it.