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Physics has uncovered a number of types of quantities that remain unchanged through physical actions, and the 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 a number.
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.