MEASUREMENT AND SYSTEM OF UNIT (English) GSEB/CBSE

1.   UNIT: The standard measure of any quantity is called unit of that physical quantity.

              

2.   Fundamental quantities : The physical quantities which do not depend on any other physical quantity for their measurements i.e., they can be directly measured are called fundamental quantities.

Fundamental units :  The units used to measure fundamental quantities are called fundamental units.

            Examples: mass, length, time

3.   Derived quantities: Physical quantities other than fundamental quantities which depend on one or more fundamental quantities for their measurements are called derived quantities.

Examples:   ,   acceleration, momentum, force, etc.

Derived units : The units of derived quantities which depend on fundamental units for their measurements are called derived units.

Examples:   

4.   Classification of unit system

1.      British system (FPS system) → foot – pound – second system

2.      CGS system → centimetre – gram – second system

3.      MKS system → metre – kilogram – second system

4.      MKSA system → metre – kilogram – second – ampere  system

5.      SI system →  system of international (seven base units)

SI system – SI unites and their symbol

5.   The rules for writing the S.I. units  of physical quantities

a.      Unit of every physical quantity should be represented according to its symbol.             For example : m for meter not M for meter, N for newton not n for newton,

b.      Full stop and any other mark should not be written after the symbol.

For example :  kg and not kg. Or k.g. , N and not N.

c.      Symbols for units do not take plural form.

For example m is used to denote many metres also.   

d.      The units of physical quantities in numerator and denominator should be written as one ratio only.

For example the SI unit of acceleration should be written either as m/s² or m s^-2; but not as m/s/s.

e.      Full name of a unit, when it is named after a scientist, is not written with a capital letter, but the symbol for that unit has a capital letter. For example, the unit of force should be written as newton but in symbol it is written as N. The unit of pressure is Pascal in symbol it is written as Pa.