Which equation best describes the law of conservation of momentum?

pi = pf

pi > pf
pi < pf
pi + pf = 0

pi = pf

Explanation:

The law of conservation of momentum states that in any isolated system (=no external forces acting on it), the total momentum is conserved. This is also valid, for example, for a system consisting of many particles colliding with each other: during each collision between two particles, the total momentum of the two particles is always conserved, even if the momentum of each particle can change. In formulas, this can be written as

where pi is the total momentum of the system before the collision and pf is the total momentum of the system after the collision.