Can someone explain this please. From what I understand momentum is often the more fundamental as it is spatially invariant, whereas energy is time invariant and as more real world cases fall into the former category momentum is often more fundamental. What is meant by spatial invariance? Is it that it is independent of position - like the acceleration of a mass in a uniform gravitational field in a polar vector space at a constant radius r. Is time invariance then the same for time? Can someone provide an example of a system that is time invariant where energy is conserved and momentum is not and why? Thanks in advance.