UNITS AND MEASUREMENT
The word physics comes from the Greek word meaning “nature”. Today physics is treated as the most fundamental branch of science and finds numerous applications of life. Physics deals with matter in relation to energy and the accurate measurement of the same. Thus physics is inherently a science of measurement. The fundamentals of physics form the basis for the study and the development of engineering and technology.
Measurement consists of the comparison of an unknown quantity with a known fixed quantity. The quantity used as the standard of measurement is called ‘unit’. For example, a vegetable vendor weighs the vegetables in terms of units like kilogram.
Fundamental physical quantities
Fundamental quantities are the quantities which cannot be expressed in terms of any other physical quantity.
(eg) length, mass and time.
Derived quantities
Quantities that can be expressed in terms of fundamental quantities are called derived quantities.
(eg) area, volume, density.
Unit
Unit of a physical quantity is defined as the accepted standard used for comparison of given physical quantity.
The unit in which the fundamental quantities are measured are called fundamental unit and the units used to measure derived quantities are called derived units.
SI Units
SI unit is the abbreviation for System International de units and is the modern form of metric system finally agreed upon at the eleventh International conference of weights and measures, 1960. This system of units is now being adopted throughout the world and will remain the primary system of units of measurement. SI system possesses features that make it logically superior to any other system.
There are seven fundamental units (base units) and two supplementary units.
SI system of units
Physical quantity
Unit
Symbol
Fundamental quantities
1.Length
metre
m
2.Mass
kilogram
kg
3.Time
second
s
4.Electric current
ampere
A
5.Temperature
kelvin
K
6.Luminous Intensity
candela
cd
7.Amount of substance
mole
mol
Supplementary quantities
1. Plane angle
radian
rad
2. Solid angle
steradian
sr
Physical quantity 
Unit 
Symbol 
Fundamental quantities 


1.Length 
metre 
m 
2.Mass 
kilogram 
kg 
3.Time 
second 
s 
4.Electric current 
ampere 
A 
5.Temperature 
kelvin 
K 
6.Luminous Intensity 
candela 
cd 
7.Amount of substance 
mole 
mol 
Supplementary quantities 


1. Plane angle 
radian 
rad 
2. Solid angle 
steradian 
sr 