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What is Physics?

            Physics is associated with those aspects of nature which can be understood in most fundamentals way in terms of elementary principles and laws.

General Physics:

Quantity and measurement:

Quantity :

            Anything which is expressed in number or whose representation is totally numerical is called quantity.

Examples: Population, the age of men or women, a weight of objects etc.

Physical quantity:

            Quantities expressed in terms of laws of physics are called physical quantity.

Examples: Mass of an object, length, force, speed, distance, displacement, momentum, electric current etc. their physical quantities are two types those are scalars and vectors.


            Those physical quantities which have magnitude only and whose direction is not taken into the consideration are called scalars.

Examples: Mass, temperature, density and volume etc.


            Those physical quantities which have both magnitude and direction and which are represented by the direction by line segment() obeying the triangle law of vectors or parallelogram law of vectors are called vectors.

Examples: Displacement, linear momentum, angular velocity, torque, magnetic field, integrity, electric displacement, current density etc.

Units of measurement:

            To measure any quantity, a definite and a substantial amount of that quantity is assumed to be a standard which is called the unit of the quantity and when any given quantity is measured in the terms of this unit, the process is called measurement there is usually two types of units.

  1. Fundamental Units.
  2. Derived Units.

Fundamental Units:

            If a physical quantity is expressed in terms of units which are used as standard and their standards are independent of each other, then these units are called fundamental units.

            During the early stage or research and development the units of length, mass and time were assumed to be fundamental but later the units of electric current, temperature, luminous, intensity and the amount of substance were added and this at present there are seven fundamental units.

Derived Units:

            If a physical quantity is expressed in terms of two or more fundamental units then these units are called derived units. Their units have no independent existence like fundamental units. the unit of force momentum, work, potential energy, density etc. are derived units.

The system of units:

Usually, physical quantities are measured in four system of units.



            In this system of units length, mass and time are measured in centimeter, gram and second respectively, CGS system in also called “Metric or French system of Units”.


            In this system of units length, mass and time are measured in Foot, Pound and Second. it is also called as “British system of units”.


            In this system of units length, mass and time are measured in Meter, Kilogram and Second.


            In the international conference of weights and measures hold Geneva in 1960 SI System of units was obtained on the basis of comprehensive consensus. In fact, SI system is extended and modified form of MKS system.
            There are 7 fundamental units and two supplementary units in SI system. is the list of 7 fundamental units of SI system.

Here is the list of 7 fundamental units of SI system.


            In this system, length is measured in meters and it is defined as traveled by light in vacuum 1/299792458 seconds is called 1 meter.


            In this system, mass is measured in kilograms and it is defined as amount of the mass of cylindrical alloy of Platinum- iridium international Bureau of weights and measures at Severus in France is called one kilogram.


            In this system, time is measured in second and it is defined as in the transition of two hyperfine levels of energy in the ground state of an atom Cesium- 133 by means of radiation between and intervals of 9192631770 time-period is called 1 Second.

Einstein in his special theory of Relativity used time as the fourth coordinate in space – Time coordinate system.

Electric current:

            In SI system electric current is measured in Ampere and is defined as if two long parallel wires are kept 1 meter apart through which an electric current is passed in such a way that is produced  a magnetic force of 2 x  ,N, then the magnitude of the electric current is called 1 ampere.


            In SI system temperature is measured in kelvin and is defined as three phases of water-solid(ice), liquid and vapor coexist at 273.16 K which is called the triple point of water and this temperature is called critical temperature and its 1/273.16th part is called 1 Kelvin.

Luminous intensity:

            In SI system luminous intensity is measured in candela which is defined as if any monochromatic source of light produces a frequency of 540 x  HZ. In a definite direction and if its intensity is 1/683 watt/steradian, then is luminous intensity is of 1 candela. If 1-joule energy is emitted in 1 sec within any solid angle, then it is called 1 watt/steradian.

The amount of substance:

            In SI system amount of substance is measured in mole which is defined as if this number of molecules, atoms or ions in any substance or element is 6.023 x  , then it is also called Avogadro’s number.

1 mole = Avogadro’s number.

supplementary units of SI System:

There are two supplementary units in SI System.


            The plane angle made by an arc of a circle of equal radius is called 1 radian. all plane angles are measured in radian.


            The solid angle made on the center of a sphere by the area formed as a square on the surface of the sphere where the side of the square is equal to the radius of the sphere is called 1 steradian. All the solid angles are measured in steradian.

Fundamental Units

S.noPhysical QuantitySI UnitsSymbol
4Electric currentAmpereA
5TemperatureKelvinθ or K
6Luminous intensityCandelacd
7Amount of SubstanceMoleMOl

Supplementary Units

S.noPhysical QuantitySI UnitsSymbol
1Plain AngleRadianrad
2Solid AngleSteradiansr

Some Important Derived Units

S.noPhysical QuantityDefinition of Quantity SI Units
1AreaLength Square
2VolumeLength Cube
3DensityMass Per Unit VolumeKg
4VelocityDisplacement Per Unit time
5ForceMass X AccelerationKg or Newton
6Linear MomentumMass X Velocity
7PressureForce per Unit Area or Pascal
8Work or EnergyForce X DisplacementN-M or Joule
9Magnetic Field IntensityForce/(electric current X Displacement)N- or Tesla or weber/

Units for Astronomical distance:

Astronomical unit(A.U):

            It is a unit of distance it means distance between The Sun and The Earth

1 A.U=1.495 x  meters.


            It is also a unit of distance and it is the distance traveled by light in vacuum in one year.

1 Lightyear =9.46x meters.

Per sec:

            It is the largest unit of distance among all the astronomical units of distance.

1 per sec = 3.08 x  meters.

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