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The SI has special names for 22 of these coherent derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m 2), the SI derived unit of area; and the kilogram per cubic metre (kg/m 3 or kg⋅m −3), the SI derived unit of density.
Each variant of the metric system has a degree of coherence—the derived units are directly related to the base units without the need for intermediate conversion factors. [18] For example, in a coherent system the units of force , energy , and power are chosen so that the equations
Conversion of units is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property.
A combination of base and derived units may be used to express a derived unit. For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa) – and the pascal can be defined as one newton per square metre (N/m 2). [6]
In addition to the unit one, the SI defines 7 base units and associated symbols: The second (s) is the unit of time. The metre (m) is the unit of length. The kilogram (kg) is the unit of mass. The ampere (A) is the unit of electric current. The kelvin (K) is the unit of thermodynamic temperature. The mole (mol) is the unit of amount of substance.
1 km 2 means one square kilometre, or the area of a square of 1000 m by 1000 m. In other words, an area of 1 000 000 square metres and not 1000 square metres. 2 Mm 3 means two cubic megametres, or the volume of two cubes of 1 000 000 m by 1 000 000 m by 1 000 000 m, i.e. 2 × 10 18 m 3, and not 2 000 000 cubic metres (2 × 10 6 m 3).
The SI system after the 2019 definition: Base units as defined in terms of physical constants and other base units. Here, means is used in the definition of . The SI system after 1983, but before the 2019 redefinition: Base unit definitions in terms of other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants ...
A hand is a unit of length used to measure the height of horses. One hand is four inches, and a value of 12.3 hands represents 12 hands + 3 inches (51 inches). Output name