(i) CGS: Centimetre Gram Second system
(ii) MKS: Metre Kilogram Second system
(iii) FPS: Foot Pound Second system.
(iv) SI: System International
The first three systems namely CGS, MKS and FPS were used extensively till recently. In 1971, the 14th International general conference on weights and measures recommended the use of ‘International system' of units. This international system of units is called the SI units. As the SI units use decimal system, conversion within the system is very simple and convenient.
Fundamental Quantities and Units:
The physical quantities which do not depend on any other physical quantities for their measurements are known as fundamental quantities. There are seven fundamental quantities: length, mass, time, temperature, electric current, luminous intensity and amount of substance
Fundamental units:
The units used to measure fundamental quantities are called fundamental units. The fundamental quantities, their units and symbols are shown in the Table
Derived Quantities and Units:
In physics, we come across a large number of quantities like speed, momentum, resistance, conductivity, etc. which depend on some or all of the seven fundamental quantities and can be expressed in terms of these quantities. These are called derived quantities and their units, which can be expressed in terms of the fundamental units, are called derived units.
= kg m/s = kg m s-1 The above two units are derived units
Supplementary units :
Besides, the seven fundamental or basic units, there are two more units called supplementary units: (i) Plane angle dθ and (ii) Solid angle dΩ
(i) Plane angle (dθ) : This is the ratio of the length of an arc of a circle to the radius of the circle as shown in Fig. 1.1 (a). Thus dθ = ds/ris the angle subtended by the arc at the centre of the circle. It is measured in radian (rad). An angle θ in radian is denoted as θc
(ii) Solid angle (dΩ) : This is the 3-dimensional analogue of dθ and is defined as the area of a portion of surface of a sphere to the square of radius of the sphere. Thus dΩ = dA/r2 is the solid angle subtended by area ds at O as shown in Fig. 1.1 (b). It is measured in steradians (sr). A sphere of radius r has surface area 4πr 2 . Thus, the solid angle subtended by the entire sphere at its centre is Ω = 4πr 2 /r2 = 4π sr
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