Classification of physical quantity

How a physical quantity can be classified?


A physical quantity can be classified into two broad categories:

(i)Scalar quantity                     (ii) Vector quantity


Scalar quantity: The physical quantities which have only magnitude but no direction are called scalar quantity.


Description: The rod is 2 metres long’, the vessel contains 200 gms. Of water’ and we  waited there for five hours’. In these expressions , 2 metre long , five hours , 200 gms , we have some ideas of magnitude  of a length , mass , and a time . But length mass and time has nothing  to do with any direction. Such quantities which gives the idea of some magnitude but are not associated with any direction are called scalar quantities.

Example : Volume, density, energy, , speed , mass, distance, work, time, power, electric current , mechanical advantage , pressure, etc are the examples of  scalar quantities.



Vector quantity : The physical quantities which have magnitude as well as direction is called vector quantity.


Description: The car is moving is moving with a velocity of 40 km/hr ‘east’ 

here east is representing the direction of the car and 40 represents the magnitude of the car .So in this example both magnitude and direction are coming so definitely it is a vector quantity, because it fulfils the condition to be a vector . A very important thing with regard to vector is that it is represented by an arrow sign (→ ). If in a problem vector word is not mentioned but the arrow sign (→ ) is there  it is a vector quantity.


Example : Displacement, velocity, acceleration, momentum, force, weight, temperature, electric field , magnetic field, retardation ,moment of force etc.


What observation should be taken for the choice of unit ?


The selection of unit to measure   a  physical quantity should have the following properties.

1.The unit should be convenient size.

2.The unit should be well defined .

3.The unit should not be changed with space , time and temperature.

4. It should be reproducible.



Fundamental quantities and derived quantities

Each of the various physical quantities is expected to have its own unit and a standard. But is found that nearly all physical quantities(except temperature,

Electric current and luminous intensity)can be derived from the units of three

physical quantities , namely, length, mass , time.



Fundamental quantity : The quantities which are independent of any other quantity  or which can neither be changed nor be related  to any other quantity

are called  fundamental quantities.


Example : Length, mass, time, temperature, current, luminous intensity atc are the examples of fundamental quantities.



Derived quantities : The quantities which depend upon the fundamental quantities  or which can be expressed in terms of the fundamental quantities i.e derived from the fundamental quantities are called derived quantities.


Example : velocity, acceleration, energy, force, momentum, etc are the exaples of derived quantities.

Discussion :The velocity of a moving body is obtained by dividing the distance traveled by the body with time taken to travel the distance . When distance is expressed in the unit of length as centimetres and time is expressed in seconds then,

,unit of velocity = unit of length / unit of time = cm/second=cm/sec.