In a two-car collision, the changes in the two cars' momenta:

1. Newton's third law makes the two forces equal and opposite. 2. They also act for the same duration, since a collision involves both cars at once. 3. Because the changes in momentum add to zero, the total momentum of the two-car system is constant. 4. One car slows down and loses momentum while the other speeds up and gains it. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_