- INTRODUCTION
Phycical Quantity :-A quantity in terms of which law of physics can be expressed and can be
measured directly or indirectly is called physical quantity.
Types of Physical Quantities:
(i) Fundamental Quantities.
(ii) Derived Quantities
(i) Fundamental Quantities:- The quantities which cannot be derived from other
quantities called fundamental quantities. - e.g. mass, length and time.
(ii) Derived Quantities:- The quantities which can be derived from fundamental physical
quantities are called derived physical quantities. E.g. velocity, acceleration, force etc
Unit:- Unit of a physical quantity may be defined as the standard of its measurement.
If Q = quantity
u = unit of quantity
Then Q = nu
Where n = no. of times the unit u is contained in Q.
(I) Fundamental Units:- The units of those quantities which can not be derived from
other physical quantities are called fundamental units. e.g. units of mass, length, time
etc.
(ii) Derived Units:- The units of those quantities which can be derived from other
physical quantities are called derived units. e.g. units of velocity, acceleration., force etc.
Characteristics of a Unit :
A unit should have the following character
1. It should be well defined.
2. It should be a suitable size.
3. It should be easily accessible.
4. It should be easily reproducible.
5. It should not change with time.
6. It should not change with change in physical conditions like temperature, pressure etc.
System of Units :
A complete set of fundamental and derived units for all kinds of the quantities is called system of units.
Some common system of Units:
1. F.P S. System: - It is a British Engineering system based upon foot, pound, and second an the
fundamental unit of length, mass, and time.
2. C.G.S. System :- It is based on centimeter, gram and second as the fundamental unit of length, mass
and time.
3. M.K.S. System: - It is based on meter, kilogram and second as the fundamental
Unit of length, mass and time
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