SI Units- الوحدات العالمية  SI Units- الوحدات العالمية

SI Units

In the Système Interntional d’Unitès, SI,  a unit is chosen for a particular purpose and larger and smaller are obtained by applying a prefix to this unite and multiplying or dividing by 10 or power of 10.

Larger units, obtained by multiplying by 1000, etc., are called ‘multiples’, smaller units, obtained by multiplying by 0.01, etc., are termed ‘sub-multiples’.

SI multiples and sub-multiples

 Prefix Symbol Factor by which unit is multiplied Example mega M 1 000 000 = 106 meganewton, MN kilo k 1 000 = 103 kilometer, km The unit centi c 0.01   = 1/102 centimeter, cm mili m 0.001   = 1/103 millimeter, mm micro µ 0.000   001 = 1/106 micrometer, µm

SI Symbols

Only one prefix can be applied to a unit at a time. For example, 1000 kilonewton must not expressed 1 kilokilonewton but as meganewton; 0.001 millimeter must not be written as 1 millmillmeter but as 1 micrometer.

The correct use of upper and lower case letters (capital and small letters) is important. For example m- meter, but M- mega; k- kilo, but K- Kelvin.

Symbols must not made plural by adding ‘s’ since ’s’ is symbol of second.

When a prefix is attached to a unit there should be no space between the prefix and the unit; for example, mm- millimeter, kW- kilowatt, MN- meganewton.

When a complex unit is formed by multiplying two units together, however, the symbol should be separated by a space; for example, N m- newton meter (the unit of torque).   