In
physics, a
conservation law states that a particular measurable property of an isolated
physical system does not change as the system evolves over time. Exact conservation laws include
conservation of energy,
conservation of linear momentum,
conservation of angular momentum, and
conservation of electric charge. There are also many approximate conservation laws, which apply to such quantities as
mass,
parity,
lepton number,
baryon number,
strangeness,
hypercharge, etc. These quantities are conserved in certain classes of physics processes, but not in all.