The law that states that if no net force acts on a system, then the total momentum of the system does not change is Newton's First Law of Motion, also known as the Law of Inertia.

While Newton's First Law is often stated as "an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force", it can be applied to systems.

Here's how it relates to momentum:

* Momentum is a measure of an object's mass in motion. It's calculated by multiplying the mass of an object by its velocity.

* Net force is the overall force acting on an object.

* When no net force acts on a system, the total momentum of the system remains constant. This means that the total amount of motion within the system stays the same.

Therefore, Newton's First Law is the foundation for the principle of conservation of momentum.