The Law of Conservation of Momentum states that Momentum between two moving objects is conserved when those two objects collide.
Consider the following:
Two cars are moving in the same direction, one behind the other. The rear car is moving
with a momentum of 50 kg*m/s, and the front car moves with a momentum of 100
kg*m/s. The cars have a combined momentum of 150 kg*m/s.
The rear car HITS the car in front of it, but no brakes or other stopping forces are
applied. The rear car's momentum changes to 40 kg*m/s. What is the new momentum
front car?