| A distinction
is made between base quantities, for which no definition exists in
the sense of basing them on quantities already known, and derived
quantities, which can be developed from the base quantities. The base
quantities are defined by base units, from which the units that apply
to derived quantities are obtained.
Since the definition of base quantities and base units is
in principle arbitrary, a whole series of different quantity and unit
systems have been established in physics over the years ( History of the length
units).
Therefore, the recommendation by the General Conference for Measurements and
Weights in 1960 to make general use of the International System of Units (Système
International d’Unités, metric system) ( Definition
of all SI units) was considered a big step forward.
The International System of Units is based on the following seven base
quantities and base units (see on the right):
Derived quantities, such as the speed, can be
defined by means of these base quantities. (Speed = Distance / Time) The
units of these derived quantities are products of powers of the base units
without numeric factors. (Unit of the speed = meter / second) |
|
Base quantity |
Name |
Symbol |
