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Physics has uncovered a number of types of quantities that remain unchanged through physical actions, and the physics term conservation law indicates the unchanging nature of these values. Such a constant value (per such a law) is often used in analyzing or predicting the results of a physical event, serving as a hint toward what must be happening. They are commonly used in dynamics (e.g., orbits, the motions within galaxies, clouds, etc.) and in quantum mechanics which has some.
Some relations that are not absolutely universal can still be treated as conservation laws for many purposes: for example, for many purposes, mass and energy can each be treated as constant, but in other cases, they must be treated as interchangeable, i.e., under a single conservation law.
Conserved quantities include momentum, angular momentum, energy, electric charge, and several other quantum numbers.