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# Types of units in System International (SI)

The System International (SI) is build from three kinds of units :

a) base units
b) supplementary units
c) derived units

Base units are fundamental units and can not be derived from any quantity.

There are seven base quantities
1) Length ; its unit is meter denoted by 'm'
2) Mass ; its unit is kilogram denoted by 'kg'
3) Time ; its unit is second denoted by 's'
4) Electric current ; its unit is ampere denoted by 'A'
5) Thermodynamic temperature ; its unit is kelvin denoted by 'K'
6) Intensity of light ; its unit is candela denoted by 'cd'
7) Amount of substance ; its unit is mole denoted by 'mol'

There are two supplementary units :
Both are dimensionless

Radian is the plane angle between two radii of a circle when the arc between them have length equal to radius of circle.
There are 2 pi radians in a complete circle and pi radians in half circle. complete circle have 360 angle. So, 2 pi rads = 360
the angle is denoted by theta
theta = arc length / radius of circle

Steradian is the solid angle subtended at the center of the sphere (circle is two dimensional shape while sphere is a three dimensional shape) by an area on the surface of sphere equal to square of radius
there are 4 pi steradians in sphere
solid angle = area on surface of sphere / (radius of sphere)^2

Derived quantities are those quantities which are derived from the fundamental units example are force, energy, momentum, acceleration.. they all are product or division of two or more base quantities.

Provided by Junaid Ashraf.

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