1.6:
Base Quantities and Derived Quantities
Physical quantities can be divided into base quantities and derived quantities. Base quantities can be expressed through a measurement process, and their units are known as base units.
For example, the distance between two points is measured in terms of meters. Here the base quantity is the distance, and the unit used to measure it is the meter, which is the base unit.
The physical quantities obtained from the combinations of base quantities are known as derived quantities, and the units used to define these quantities are known as derived units.
For example, the speed of a moving car from one point to another is defined as the distance traveled divided by travel time. Here, speed is a derived quantity obtained from the base quantities distance and time.
The unit of speed is meters per second, the derived unit from the base units, meters, and seconds. All the derived quantities and derived units are defined from base quantities and base units, respectively.
1.6:
Base Quantities and Derived Quantities
In any system of units, the units for some physical quantities must be specified through a measurement process. These measurements are the base quantities of the system, and their units are the base units of the system. The algebraic combinations of the base values can then be used to express all other physical quantities. Each of these physical quantities is then referred to as a derived quantity, with each unit being referred to as a derived unit.
The International Organization for Standardization recommends using seven base quantities, which form the International System of Quantities (ISQ). All the other physical quantities can be expressed as combinations of these seven base physical quantities. For example, in geometry, the concept of area is always calculated as the product of two lengths. Thus, area is a derived quantity that can be expressed in terms of SI base units using square meters. Meanwhile, density is defined as mass divided by volume, which is expressed as kilograms per cubic meter (kg/m3) in terms of SI base units. In summary, all physical quantities can be derived from the seven base quantities, and the units of all the physical quantities can be derived from the seven SI base units.
This text is adapted from Openstax University Physics Volume 1, Section 1.2 Units and Standards.