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**:

a) radian

b) steradin

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

or pi rads = 180 .. or pi/2 rads = 90

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.__

nyc...

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