Measurement :-
Measurement is the process of comparison of the given physical quantity with the known standard quantity of the same nature.
Unit :-
Unit is the quantity of a constant magnitude which is used to measure the magnitude of other quantities of the same nature.
Characteristics of a unit :-
The characteristics of a unit are -
1) It should be of convenient size.
2) It should be possible to define the unit without any error.
3) It should be reproducible.
4) Its value should not change with place and time.
Fundamental Units :-
A fundamental or basic unit is that which is independent of any other unit or which can neither be changed nor can be related to any other fundamental unit.
Example :The units of mass (kg), length (m), time (s) etc.
Derived units :-
Derived units are those which depend on the fundamental units or which can be expressed in terms of the fundamental units.
Example : The units of area (m2), volume (m3), speed (ms-1) etc.
Common systems of units :-
1)
C.G.S.System - centimetre (cm), gramme (g), second (s)
2)
M.K.S.System - metre (m), kilogramme (kg), second (s)
3)
F.P.S.System - foot (ft), pound (lb), second (s)
4)
M.K.S.A.System - metre (m), kilogramme (kg), second (s), ampere (A)
5)
S.I.System - Systeme Internationale d’ Unites
Please Note : To express the measurement of a physical quantity we must know–
- The unit in which the quantity is to be measured.
- The numerical value which expresses how many times the above selected unit is contained in the given quantity ,ie, physical quantity = (numericalvalue) x (unit)
Eg : Length of a piece of cloth is 10m. It means that the length is measured in the unit metre and this unit is contained 10 times in the length of that piece of cloth.