# S.I. Units: Systeme InternationaleS.I. Units: Systeme Internationale

S.I. Units: These are a set of base units and also a scientific method of expressing the magnitudes
of quantities of important natural phenomena.
 S.I. Units are used to reduce confusion between different units that are used to measure the
same natural phenomena. By using the SI units, time is saved converting between units, and it
also makes scientific data trading with other countries efficient.
The SI base quantities and units that you need to learn are:
o Mass – Kilogram (kg)
o Length – Metre (m)
o Time – Second (s)
o Current – Ampere (A)
o Temperature – Kelvin (K)
o Amount of Substance – Moles (mol)
Derived units: Derived units are units which may be expressed in terms of base units by means
of mathematical symbols of multiplication and division.
E.g. Momentum = kgms-1 and Density = kg/m3
Homogeneity of physical equations: It is said that an equation is homogenous if both sides of
the equations can be simplified to the same base unit or set of base units. This means that the
equation could be correct, as long as there are no numerical errors made.
If an equation is not homogenous then it is incorrect.

Unit Prefixes: Prefixes are used to form decimal multiples and submultiples of SI units. Use the
following table to learn the prefixes that are to be learnt.