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.
1 Note that in this regard both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions.[3] The sympathetic nervous system also affects salivary gland secretions indirectly by innervating the blood vessels that supply the glands.