In 1960 an international committee agreed on a set of definitions and standard to describe the physical quantities. The system that was established is called the system International (SI).
The system international (SI) is build up from three kinds of units: base units, supplementary units and derived units.
BASE UNITS:- There are seven base units for various physical quantities namely: length, mass ,time ,temperature, electric current,luminous intensity and amount of substances (with special reference to the number of particles).
The names of base units are listed in table 1.1
Table 1.1
physical quantity SI units Symbol
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Length meter m
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Mass kilogram kg
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Time Second s
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Electric current ampere A
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Thermodynamic
temperature kelvin k
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Intensity of light Candela cd
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Amount of
substances mole mol
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Supplementary unit:-
The general conference on weights and measures has not yet classified certain units as base units or derived units.These SI units are called supplementary units.For the time being this class contains only two units of purely geometrical quantities,which are plane angle and solid angle.
Plane angle:-
The radian is the plane angle between the two radii of a circle which cuts off on the circumference an arc,equal in length to the radius.
Solid angle:-
The steradian is the solid angle subtended at the centre of a sphere by an area of its surface equal to the square of radius of sphere.\
Derived units:-
SI units for measuring all other physical quantities are derived from base and supplementary units.Some of the derived units are:
physical quantity Unit Symbol In term of base units
Force newton N kg ms
