The principle of momentum conservation
As mentioned in the last chapter, there is a conservation principle for momentum just like the one for energy. Let’s state the principle first and then explain it:
For any system of interacting particles, the total momentum of all the particles remains constant, as long as no external forces act on the system.
“Interacting particles” means that the particles influence each other by exerting forces on each other. The forces can be of any type; for example, gravitational forces between stars in a galaxy. The principle is true regardless of the nature of the forces between the particles. Once you define your system, the particles in that system may be mutually subjected to any number of internal forces between ...