Introduction to Units

 UNITS AND MEASUREMENTS
Introduction:

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 in all walks 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 d’ 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

Fundamental quantities

Physical quantity                         Unit           Symbol
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