Units, Physical

specific concrete physical quantities to which numerical values equal to 1 are assigned according to definition. Many units of physical quantities are represented by measures used in measurements (for example, meter, kilogram).

During the early stages of the development of material culture (in slaveholding and feudal societies), units existed for only a small number of physical quantities, such as length, mass, time, area, and volume. Units of physical quantities were selected without any relationship to each other and in a different manner in various countries and geographic regions. This led to a large number of units that were often identical in name but different in magnitude, for example, ells, feet, pounds. As trade between nations increased and science and technology developed, the number of units of physical quantities grew and the need to unify the units and create systems of units became evident. Special international agreements regarding the units of physical quantities began to be concluded. A metric system of measures was proposed in France in the 18th century, which subsequently gained international acceptance. A series of metric systems of units was devised on its basis. Further ordering of the units of physical quantities is presently in progress on the basis of the International System of Units (SI).

The units of physical quantities are divided into systematic, that is, those belonging to some system of units, and nonsystematic units (forexample, mm Hg, horsepower, electron volt). The systematic units of physical quantities are divided into the fundamental units, which are selected arbitrarily (meter, kilogram, second, and others), and the derived units, which are formed from the relationships between the quantities (meterper second, kilogram per cubic meter, newton, joule, watt). Multiple and fractional units are used to conveniently express quantities that are many times larger or smaller than the units of physical quantities. The multiple and fractional units of physical quantities are formed in metric systems (with the exception of units of angle and time) by multiplying the systematic units by 10”, where n is a positive or negative integer. A decimal prefix, adopted for the formation of multiple and fractional units, corresponds to each of these numbers.