<P> 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 . </P> <P> A local conservation law is usually expressed mathematically as a continuity equation, a partial differential equation which gives a relation between the amount of the quantity and the "transport" of that quantity . It states that the amount of the conserved quantity at a point or within a volume can only change by the amount of the quantity which flows in or out of the volume . </P>

What is the meaning of conservation in physics