What principle states that the momentum of a system remains unchanged in the absence of net external forces?

The principle that states the momentum of a system remains unchanged in the absence of net external forces is known as the conservation of momentum. When no external forces act on a system, the total momentum before an event (like a collision) will equal the total momentum after the event. This principle is fundamental in physics, particularly in analyzing collisions and explosions.

Understanding this concept is crucial for problems that involve interactions between objects, as it allows for predictions about their movement post-interaction. The conservation of momentum is a cornerstone of mechanics, helping to bridge concepts between various areas such as mechanics, particle physics, and even fluid dynamics.