The momenta of individual objects changes. The total momentum remains constant. I have to disagree. If you have two cars that collide head on, the momentum of both vehicles stops. The ENERGY created by the impact causes usually, some reverse momentum but the momentum is lost.
As the SNS continues to trigger physical reactions, it causes a wear-and-tear on the body. It's not so much what chronic stress does to the nervous system, but what continuous activation of the nervous system does to other bodily systems that become problematic.
The parasympathetic nervous system (PNS) controls homeostasis and the body at rest and is responsible for the body's rest and digest function. The sympathetic nervous system (SNS) controls the body's responses to a perceived threat and is responsible for the fight or flight response.