If two objects collide in an isolated system, what principle describes the total momentum after the collision?

In the context of an isolated system where two objects collide, the principle that governs the total momentum after the collision is indeed the conservation of momentum. This principle states that the total momentum of a system remains constant if no external forces are acting upon it.

During a collision, while the individual momenta of the colliding objects may change due to the interaction, the vector sum of their momenta before the collision will be equal to the vector sum of their momenta after the collision. This is true regardless of whether the collision is elastic or inelastic, meaning that kinetic energy may not be conserved, but momentum will always be.

In isolated systems, where external forces are negligible, momentum is conserved, making it a fundamental principle in analyzing collisions in physics. This principle is crucial for understanding a wide range of phenomena, from simple collisions in everyday life to complex interactions in particle physics.