Units “ To measure or represent any physical quantity we need units”

Exception: Although there are few physical quantities which do not

need any unit like refractive index, Relative density or specific gravity etc.

Physical quantity            

 nu = Constant


Example: In a particular system, the unit of length,

mass and time  are chosen to be 10cm, 10gm and 0.1 sec

respectively. The unit of force in this system will be

equivalen to:      

  1. 0.1 N
  2. 1 N
  3. 10 N
  4. 100 N

Sol. Unit of force Þ F = ma = Kg metre /sec2


F = 0.1 N     So the Correct option is (a)

Example: The density of a material in CGS System of unit

is 4gm/cm3. In a system of units in which unit

of length is 10cm and unit of mass is 100 gm the

value of density of material will be     

(a) 0.04

(b) 0.4

(c) 40

(d) 400

Solution. Given n1 u1 = 4 gm/cm3

n2 u2 = n1 u1


n2 = 40                                            Answer (a)



Derived Units: The units which may derive by

fundamental or Supplementry units

Ex. Force= ma = Kg mt/sec2

Conventions adopted while writing a unit

1. Even if a unit is named after a person it should

not be Capital initial letter.

Newton (no)      Joule (no)            Ampere (no)  

     or                      or                            etc.

newton (yes)     joule (yes)          ampere (yes)    

2. In symbol for a unit named after a person

Newton ® N

Ampere ® A

Joule ® J

Watt ® w

3. The symbol or units are not expresse

d in plural form : 50 m (yes) , 10 joule (yes)

                          50 ms (no)  , 10 joules (no)

4. Not more than one Solidus is used

1 poise = 1g/s  cm or 1 gs-1