ABSTRACT
When a system is isolated from the outside world, its momentum, angular momentum, and energy are all conserved. Conserved means they do not change. The constant value of these quantities allows one to understand a wide range of phenomena. For mechanical systems, the conservation laws for all three follow from Newton’s laws, but their conservation extends beyond Newtonian mechanics to quantum mechanics and relativity. In this more general context, conservation laws follow from basic symmetries of space and time. The results of a physics experiment (isolated from the outside world) are unchanged if the equipment is moved to a new place, aimed in a different direction or done on a different day. These uniformities of space-time yield the conservation laws for momentum, angular momentum, and energy. Other symmetries, such as time reversal and reflection invariance, are associated with additional conservation laws.
