单位系统由7个基本单位构成，包括千克、秒、米、安培、坎德拉、摩尔和开尔文。这些基本单位在质量、时间、长度、电流、光强、物质的量和温度方面互不依赖。衍生单位如牛顿或瓦特则是基于基本单位的组合。例如，SI单位系统就是一种包含这7个基本单位的系统，同时还有22个衍生单位。衍生单位如牛顿（kgm/s²）是为了表达方便而引入的。
Base units must be independent in terms of the dimensions mass, time, length, electric
current, luminous intensity, amount of substance, and
temperature. Therefore, a unit system has up to 7 base units. As
long as the independence is satisfied, any unit can be a base
unit, including units such as newton or watt.
A unit system can have less than 7 base units. For example, mechanical systems need base units
only for the dimensions length, mass, and time.
Derived units in a unit system must have a representation in terms of the products of powers of the base units for that system. Unlike base units, derived units do not have to be independent.
Derived units are optional and added for convenience of
representation. For example, kg m/s2
is abbreviated by newton.
An example of a unit system is the SI unit system, which has 7 base units: kilogram, second,
meter, ampere, candela, mol, and kelvin. There are 22 derived
units found by calling
derivedUnits('SI').
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