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System of Quantity Kinds

A system of quantity kinds is a set of one or more quantity kinds together with a set of zero or more algebraic equations that define relationships between quantity kinds in the set. In the physical sciences, the equations relating quantity kinds are typically physical laws and definitional relations, and constants of proportionality. Examples include Newton’s First Law of Motion, Coulomb’s Law, and the definition of velocity as the instantaneous change in position. In almost all cases, the system identifies a subset of base quantity kinds. The base set is chosen so that all other quantity kinds of interest can be derived from the base quantity kinds and the algebraic equations. If the unit system is explicitly associated with a quantity kind system, then the unit system must define at least one unit for each quantity kind. From a scientific point of view, the division of quantities into base quantities and derived quantities is a matter of convention.