__Physical Quantities:__In Physics we deal with different physical quantities. We need to measure them. For the measurment we compare the physical quantity with its some standard value called 'Unit'. In any measurement we have two things (i) Numerical Value n and (ii) and its unit u. Numerical value tells that how many times the unit comes in particular measurement. The numerical value and unit of a physical quantity are related with each other as

__n α 1/u i.e. nu=constant__................................very Important

**System of Units:-**There are different systems used for the measurement of physical quantities like FPS, CGS and MKS.

At present the system which is used SI System i.e. International Standard of Units. There are seven fundamental quantities and their units in SI Systems

(1) LENGTH METRE

(2) MASS KILOGRAM

(3) TIME SECOND

(4) TEMPERATURE KELVIN

(5) ELECTRIC CURRENT AMPERE

(6) LUMINOUS INTENSITY CANDILA

(7) AMOUNT OF SUBSTANCE MOLE

In addition to this, there are two more fundamental units i.e. radian (Unit of Plane angle) and Steredian ( Unit of solid angle)

**Fundamental Quantities and their Units**:- Fundamental means independent whether it is quantity or units.

One can not be converted into other .

**Derived Quantities and thier Units:-**The other physical quantites depend on fundamentals, called Derived quantities. So we can say that derived quantities are those which are derived from fundamental quantities and same for the Derived Units.

Example :- Area, Speed , velocity. acceleration. Force etc.

**Dimensions:-**Dimensions are the powers which are raised on fundamenal quantities to express derived quantities.

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