A system for which the net external force is zero is described as:

1. Momentum conservation needs a condition on the outside world. 2. An isolated system is defined to be one for which the net external force is zero. 3. Seeing why momentum is conserved for such a system follows from Newton's second law in terms of momentum. 4. With no net external force there is no change in the system's total momentum. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_