Figure 39.
In the case of the two cars, the momentum in the “before” situation will equal the momentum in the “after” situation, referring to the momentum of the entire system. An isolated system is one in which there is no external force on the system. This relies on knowing the system and making sure that the system doesn’t have any external forces that can be applied. In the case of the two cars, the velocity changes are the only things that change the momentum in reality. Friction is a force that is not included in the example; it exists, however, in the real situation.
SUBATOMIC COLLISIONS AND MOMENTUM CONSERVATION This idea of conservation of momentum applies to atomic and subatomic particles as well. Momentum is a property of all subatomic particles, including particles that have no specific mass, such as photons. What it means is that the idea of momentum may not be as closely related to mass as it seems to be. Momentum also applies to wave properties as you will see later in the course as well as to issues related to particle physics. The concept of momentum and conservation of momentum is used to identify the mass of things that are otherwise impossible to weigh.
96