What is physics? (special Article)


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In Physics, we study about a large number of quantities, which can be broadly classified into two categories : scalars and vectors.

Scalar Quantities Physical quantities

which have magnitude only. e.g. Mass,

speed, volume, work, time, power, energy,

etc. are scalar quantities.

Vector Quantities Physical quantities,

which have magnitude and direction both,

e.g., Displacement, velocity, acceleration,

force, momentum, torque, etc.

For a quantity to be a vector, it is necessary

that it follows the triangle rule of addition of

two vectors.



First Law Every body maintains its initial

state of rest or uniform motion on a straight

line unless any external force acts on it. It is

also called Galileo’s law of inertia.

Example While jumping from a slowly

moving train/bus one must run for a short

distance, in the direction of motion.

Second Law The force acting on an object

is directly proportional to the product of the

mass of the object and the acceleration

produced on it.

Third Law To every action, there is an

equal and opposite reaction.

Example Bogies of the trains are provided

with buffers to avoid severe jerks during

shunting of trains.

Rocket moves up due to reaction of

downward ejection of gas.


● When an object moves along a circular

path, its motion is called circular


● The direction of motion at any point in

circular motion is given by the tangent

to the circle at that point.

● The external force required to act

radially inward over the circular motion

of the body is called centripetal force.

● In the death well, the walls of well exert

an inward force over the motorcycle

and as a reaction, the motorcycle exert

an outward force on the walls of the


● Centrifugal force is such a pseudo

force that is equal and opposite to

centripetal force.

● Cream separator, centrifugal dryer work

on the principle of centrifugal force.


It is the opposing force that is set-up

between the surfaces of contact of two

bodies when one body slides or rolls or

tends to do so on the surface of another


● Due to friction we are able to move on

the surface of Earth.

● On applying brakes in automobile


● Work is said to be done, if force

acting on a body is able to actually

move it through some distance in the

direction of the force. Its SI unit is


Work = FS cos θ

where, F = force, S = displacement

and θ is the angle between the

direction of force and displacement.

● If θ > ° 90 , then work will be negative.

● If θ < ° 90 , then work will be positive.

● If θ = ° 90 , then work will be zero.

If a coolie carrying a load on his head is

moving on a horizontal platform, then

theoretically he is not doing any work

because θ = ° 90 ,W FS = ° = cos 90 0


Capacity of doing work by a body is

called its energy. Energy is a scalar

quantity and its unit is joule.

Mechanical energy is of two types.

● Kinetic Energy (K) Energy

possessed by a body due to its motion.

K =



2 mv

where, m is mass and v is the


● Potential Energy (U) The capacity

of doing work developed in a body

due to its position or configuration.

U = mgh

where, m is mass, g is acceleration

due to gravity and h is height.

● The sum of all kinds of energies in an

isolated system remains constant at

all times. This is the law of

conservation of energy.


Rate of doing work is called power. Its

unit is watt.

Power =

Work done

Time taken

■ 1 watt hour = 3600 joule

■ 1 kilowatt hour = × 3 6 10 .



■ 1 HP = 746 watt


● Everybody in the universe attracts other

body by a force called force of gravitation.

● The gravitational force of the earth is called


● The acceleration produced in a body due to

force of gravity is called acceleration due to

gravity (g) and its value is 9.8 m s/ 2


● Acceleration due to gravity is independent of

shape, size and mass of the body.

● Escape velocity is the minimum velocity

with which an object just crosses the Earth’s

gravitational field and never returns. Escape

velocity at the Earth’s surface is 11.2 km/s.

● Escape velocity at the Moon’s surface is

2.4 km/s. Due to low escape velocity there is

no atmosphere on the Moon.

● Value of g decreases with height or depth

from Earth’s surface.

● g is maximum at poles.

● g is minimum at equator.

● g decreases due to rotation of Earth.

● g decreases if angular speed of Earth

increases and increases if angular speed of

Earth decreases.

● The acceleration due to gravity at the Moon

is one-sixth that of the Earth. So, the weight

of a person on the surface of the Moon will

be one-sixth of his actual weight on the



● Satellites are natural or artificial bodies

revolving around a planet under its

gravitational force of attraction.

● Moon is a natural satellite, while INSAT-B is

an artificial satellite of Earth.

● The period of revolution of satellite revolving

near the surface of Earth is 1 hour

24 minutes (84 minutes).

● Geo-stationary satellite revolves around the

Earth at a height of 36000 km (approx). The

orbit of geo-stationary satellite is called

parking orbit. Geo-stationary satellite revolves

in equatorial plane from West to East. Time

period of rotation of geo-stationary satellite is

24 h.

● The Earth rotates on its axis from West to

East. This rotation makes the Sun and the

stars appears to be moving across the sky

from East to West.

● Geo-stationary satellite is used to telecast TV

programmes from one part of the world to

another, in weather forecasting, in

predictions of floods and droughts.

● Polar satellite revolves around the Earth in

polar orbit at a height of 800 km (approx).

Time period of these satellites is 84 min.

● These are used for weather forecasting,

mapping, etc.




Elasticity is the property of material of a body by

virtue of which the body acquires its original

shape and size after the removal of deforming


● A force, which changes the configuration of a

body, is called a deforming force.

● Steel is more elastic than rubber.


Pressure is defined as force acting normally on

a unit area of the surface.

Pressure =



Its unit is N/m2

. It is a scalar quantity.

● Atmospheric pressure is measured by an

instrument called the barometer.

● Sudden fall in barometric reading is the

indication of storm.

● Slow fall in barometric reading is the indication

of rain.

● Slow rise in the barometric reading is the

indication of clear weather.

● The pressure exerted by liquid column at the

surface given as p hdg = , where d is the

density of liquid and h is height of liquid

column. In a static liquid at same horizontal

level, pressure is same at all points.

Pascal’s Law of Pressure

● If gravitational attraction is

negligible in equilibrium condition,

(approx) pressure is same at all

points in a liquid.

● The pressure exerted anywhere at a

point of confined liquid is

transmitted equally and

undiminished in all directions

throughout the liquid.

● Hydraulic lift, hydraulic press and

hydraulic brakes are based on the

Pascal’s law of pressure.

Archimedes’ Principle

When a body is immersed partly or

wholly in a liquid, there is an apparent

loss in the weight of the body, which is

equal to the weight of liquid displaced

by the body.

● The weight of water displaced by an

iron ball is less than its own weight

whereas water displaced by the

immersed portion of a ship is equal

to its weight. So, small ball of iron

ball sink in water, but large ship float.

● A fat person will quickly learn the

swimming as compared to a slim

person because he will displace more

water. So, he will be more balanced.

● Hydrogen filled balloon float in air

because hydrogen is lighter than air.

● A person can lift more weight in


Laws of Floatation

A body floats in a liquid if

● The density of material of the body is

less than or equal to the density of


● When the density of material of the

body is equal to density of liquid, the

body floats fully submerged in liquid

in neutral equilibrium.

● When body floats in neutral

equilibrium, the weight of the body is

equal to the weight of displaced

liquid. The centre of gravity of the

body and centre of gravity of the

displaced liquid should be in one

vertical line for this condition.


● Density is defined as mass per unit


● Relative density is measured by hydro-


● The density of sea water is more than that

of normal water.

● When a ship enters in a sea from a river, it

raises a bit because the density of saline

water (salt water) is higher.

● The density of iron is more than that of

water, but less than that of mercury. So, a

solid chunk of iron sink in water but float

in mercury.

● If ice floating in water in a vessel melts, the

level of water in the vessel does not


Surface Tension

● It is the force (F) acting normally on unit

length (l) of imaginary line drawn on

the surface of liquid i.e., T



= , its unit

is N/m.

● The property of a liquid by virtue of which

it tries to minimise its free surface area is

called the surface tension.

● Surface tension decreases with rise in

temperature and becomes zero at the

critical temperature.

● The surface tension of clean water is

higher than that of a soap solution.

● Formation of lead shots, spraying result in

coldness, floatation of needle on water,

dancing of camphor on water, are based on

surface tension.

● Rain drop form spherical shape due to

surface tension.

● When kerosene oil is sprinkled on water,

its surface tension decreases, due to which

the excess of mosquitoes floating on the

surface of water die due to sinking.

Cohesive and Adhesive Forces

Force of attraction applied between molecules of

same substance is called cohesive force

while attractive force between molecules of

different substances is called adhesive



The phenomenon of rise or depression of

liquids in a capillary tube is called


■ A piece of blotting paper soaks ink because

the pores of the blotting paper serve as

capillary tubes.

■ The oil in the wick of a lamp rises due to

capillary action of threads in the wick.

■ The root hairs of plants draws water from

the soil through capillary action.


Viscosity is the property of a fluid by

virtue of which an internal frictional

force acts between its layers, when it is in


Bernoulli’s Theorem

When an incompressible and

non-viscous liquid (or gas) flows in

streamlined motion from one place to

another, then at every point of its path

the total energy per unit volume

(pressure energy + kinetic energy +

potential energy) is constant.

Venturimeter, Atomizer, filter pump,

motion of aeroplane are based upon the

Bernoulli’s theorem.




● Heat is a form of energy, which

measures the sensation or perception of

warmness or coldness of a body or


● Its units are calorie, kilocalorie or joule.

● 1 calorie = 4.18 joule.


● Temperature is the measurement of

hotness or coldness of a body.

● When two bodies are placed in contact,

heat always flow from a body at higher

temperature to the body at lower



A wave is a disturbance, which propagates

energy from one place to the other without

the transportation of matter.

Waves are broadly of two types

(i) Mechanical wave (longitudinal wave

and transverse wave)

(ii) Electromagnetic wave

Longitudinal Waves

In this wave, the particles of the medium

vibrate in the direction of propagation of


Waves on springs or sound waves in air are

examples of longitudinal waves.

Transverse Waves

In this wave, the particles of the medium

vibrate perpendicular to the direction of

propagation of wave.

Waves on strings under tension, waves on

the surface of water are the examples of

transverse waves.

Electromagnetic Waves

● The waves, which do not require medium

for their propagation i.e., which can

propagate even through the vacuum are

called electromagnetic waves.

● Light radio waves, X-rays, etc. are the

examples of electromagnetic waves.

These wave propagate with the velocity of

light in vacuum.

Sound Waves

Sound waves are longitudinal mechanical

waves. Based on their frequency range

sound waves are divided into following


● The sound waves which lie in the

frequency range 20 Hz to 20000 Hz are

called audible waves.

● The sound waves having frequencies less

than 20 Hz are called infrasonic waves.

● The sound waves having frequencies

greater than 20000 Hz are called ultrasonic


● Ultrasonic waves are used for sending

signals, measuring the depth of sea,

cleaning machinery parts located in hard

to reach places, such as spiral tubes, etc.

Speed of Sound

■ Speed of sound is maximum in solids and

minimum in gases.

■ When sound goes from one medium to

another medium, its speed and wavelength

changes, but frequency remains unchanged.

■ The speed of sound remains unchanged by

the increase or decrease of pressure.

■ The speed of sound increases with the

increase of temperature of the medium.

■ The speed of sound is more in humid air than

in dry air because the density of humid air is

less than the density.

Echo The repetition of sound due to

reflection of sound waves, is called echo.

Intensity It is defined as the amount of

energy passing per unit time through a

unit area that is perpendicular to the

direction in which sound waves are


Pitch The sensation of a frequency is

commonly referred to as the pitch of a


SONAR It stands for sound navigation

and ranging. It is used to measure the

depth of a sea, to locate the enemy

submarines and shipwrecks.

Doppler’s Effect

If there is a relative motion between source

of sound and observer, the apparent

frequency of sound heard by the observer is

different from the actual frequency of

sound emitted by the source. This

phenomenon is called Doppler’s effect.

■ When the distance between the source and

observer decreases, then apparent

frequency increases and vice-versa.


● Light is a form of energy, which

is propagated as electromagnetic wave.

● It is the radiation which makes our eyes

able to see the object. Its speed is

3 108

× m/s. It is the form of energy. It is

a transverse wave. It takes 8 min 19 s to

reach on the Earth from the Sun and

the light reflected from Moon takes

1.28 s to reach Earth.

Reflection of Light

When a ray of light falls on a boundary

separating two media comes back into the

same medium, then this phenomenon is

called reflection of light.

Laws of Reflection

● The incident ray, reflected ray and the

normal to the reflecting surface at the

incident point all lie in the same plane.

● The angle of reflection is equal to the

angle of incidence.

Reflection from Plane Mirror

● The image is virtual and laterally


● The size of image is equal to that of


● If an object moves towards a plane

mirror with speed v, relative to the object

the image moves towards it with a speed


● To see his full image in a plane mirror, a

person requires a mirror of atleast half of

his height.

● The number of images formed by

two plane mirrors, inclined by an angle θ,

n =


360 1



Spherical Mirror

● Spherical mirrors are of two types

(i) Concave mirror (ii) Convex mirror

● Image formed by a convex mirror is

always virtual, erect and diminished.

● Image formed by a concave mirror is

generally real and inverted.

Uses of Concave Mirror

(i) As a shaving glass. (ii) As a reflector for the

headlights of a vehicle, search light. (iii) In

ophthalmoscope to examine eye, ear, nose by

doctors. (iv) In solar cookers.

Uses of Convex Mirror

(i) As a rear view mirror in vehicle because it

provides the maximum rear field of view and

image formed is always erect. (ii) In sodium

reflector lamp.

Refraction of Light

The bending of the ray of light passing from

one medium to other medium is called

refraction. When a ray of light enters from

one medium to other medium, its

frequency and phase do not change, but

wavelength and velocity change. Due to

refraction from Earth’s atmosphere, the

stars appear to twinkle.

Refractive index (µ)


Speed of light in vacuum

Speed of light in the medium

Critical Angle

The angle of incidence in a denser medium

for which the angle of refraciton in rarer

medium becomes 90°, is called the critical


Total Internal Reflection (TIR)

If light is travelling from denser medium to

rarer medium and the angle of incidence is

more than the critical angle, then the light

is reflected back into the denser medium.

This phenomenon is called total internal


Sparkling of diamond, mirage and looming,

shinning of air bubble in water and optical fibre

are examples of total internal reflection.

Optical Fibre

It works on the principle of TIR. It is used

for telecommunication and various

medical purposes like endoscopy.


● Lens is generally of two types

(i) Convex lens (ii) Concave lens

● When lens is dipped in a liquid of higher

refractive index, the focal length

increases and convex lens behave as

concave lens and vice-versa.

● An air bubble trapped in water or glass

appears as convex, but behaves as

concave lens.

Dispersion of Light

● When a ray of white light is passed

through a prism, it gets splitted into its

constituent colours. This phenomenon

is called dispersion of light.

The different colours appeared in the

spectrum are in the following order,

violet, indigo, blue, green, yellow, orange

and red (VIBGYOR).

● Rainbow is formed due to dispersion of

sunlight by water droplets.

● Wavelength of red colour is maximum

and for violet colour is minimum.

● Red, green and blue are primary colours.

Green and magenta, blue and yellow, red

and cyan are complementary colours.

Scattering of Light

● When light passes through a medium in

which particles are suspended whose

sizes are of the order of wavelength of

light, then light striking on these particles

deviated in different directions. Scattering

of light is maximum in case of violet

colour and minimum in case of red


● Blue colour of sky is due to scattering of

blue and violet light. The brilliant red

colour of rising and setting sun is also due

to scattering of light.


● It is an optical instrument like camera. It

forms the real image of the object on

retina of the eye. Least distance of distinct

vision is 25 cm.

Myopia (Short


A short-sighted eye can see

only nearer objects. Distant

objects are not seen clearly.

This defect can be removed

by using concave lens of

suitable focal length.




A long sighted eye can see

distant objects clearly but

nearer object are not clearly

visible. This defect can be

removed by using a convex


Presbyopia In this defect both near and

far objects are not clearly

visible. It can be removed by

using bi-focal lens.

Astigmatism In this defect eye cannot see

horizontal and vertical lines

clearly. This defect can be

removed by using suitable

cylindrical lenses.


● Simple microscope is a convex lens of

small focal length.

● Compound microscope is a

combination of two convex lenses,

called objective lens and eyepiece,

separated by a distance.

● Astronomical Telescope is also a

combination of two lenses in which

objective lens is a convex lens of large

aperture and large focal length while

eye-piece is a convex lens of small

aperture and small focal length.




Charge is the basic property associated with

matter due to which it produces and

experiences electric and magnetic effects.

Similar charges repel each other and

opposite charges attract each other. The SI

unit of charge is coulomb.

Conductor Conductors are those materials,

which allow electricity to pass

through themselves. Metals like

silver, iron, copper and earth acts

like a conductor. Silver is the best


Insulator Insulator are those materials which

do not allow electricity to flow

through themselves. Wood, paper,

mica, glass, ebonite are insulators.

Electric Current

● Electric current is defined as the rate of

flow of charge or charge flowing per unit

time. Its unit is ampere. It is a scalar


● A lightning conductor is fixed on tall

buildings to protect them from the

destructive effects of the lightning.

● An electric bulb produces a bang when it

is broken because there is a vacuum

inside the electric bulb, when the bulb is

broken air rushes at great speed from all

sides to fill the vacuum. The rushing of

air produces a noise generally referred

to as the bang.


● Radioactivity was discovered by Henry

Becquerel, Madame Curie and Pierre

Curie for which they jointly won Nobel


● The nucleus having protons 83 or

more are unstable. They emit α β, and

γ particles and become stable. The

elements of such nucleus are called

radioactive elements and the

phenomenon of emission of α β, and γ

particles is called radioactivity.

● Robert Pierre and his wife Madame

Curie discovered a new radioactive

element radium.

● The end product of all natural

radioactive elements after emission of

radioactive rays is lead.

● With the emission of an α-particle,

atomic number is decreased by 2 and

mass number is decreased by 4.

● With the emission of a β-particle,

atomic number is increased by 1 and

mass number does not change.

Nuclear Fission

● The nuclear reaction, in which a

heavy nucleus splits into two nuclei of

nearly equal mass is nuclear fission.




1 U + → + n 56 36 Ba Kr 141 92+ 30n


+ energy

● Atom Bomb is based on nuclear

fission. U

235 and Pu239 are used as

fissionable material.

● Nuclear fission was first

demonstrated by Hatin and Fritz


Nuclear Fusion

● When two or more light nuclei

combined together to form a heavier

nucleus is called as nuclear fusion.

● For the nuclear fusion, a temperature

of the order of 108 K is required.

● Hydrogen Bomb was made by the

American Scientist in 1952. This is

based on nuclear fusion. It is

1000 times more powerful than atom


Nuclear Reactor or Atomic Pile

● Nuclear reactor is an arrangement, in

which controlled nuclear fission reaction

takes place.

● First nuclear reactor was established in

Chicago University under the supervision

of Prof Enrico Fermi.

● Heavy water, graphite and beryllium oxide

are used to slow down the fast moving

neutrons. They are called moderator.

● The cold water, liquid oxygen, etc. are used

as coolant to remove heat generated.

● Cadmium or boron rods are good absorber

of neutrons and called the control rods.

Uses of Nuclear Reactor

(i) To produce electrical energy from the

energy released during fission.

(ii) To produce different isotopes, which

can be used in medical, physical and

agriculture science.

There are several components of nuclear reactor

which are as follows

■ Fissionable FuelU

235 orU

239 is used.

■ Moderator Moderator decreases the energy

of neutrons, so that they can be further used for

fission reaction. Heavy water and graphite

are used as moderator.

■ Control Rod Rods of cadmium or boron are

used to absorb the excess neutrons produced in

fission of uranium nucleus, so that the chain

reaction continues to be controlled.

■ Coolant A large amount of heat is produced

during fission. Coolant absorbs that heat and

prevents excessive rise in the temperature. The

coolant may be water, heavy water or a gas like

He or CO2

LASER (Light Amplification by Stimulated Emission of Radiation) 

It is a device that produces an intense,

coherent and highly directional beam of the

single frequency. It can be transmitted over a great distance without being spread. 

LASER Technology in India

In 1964, the first laser as Gallium Arsenide

(GaA) semi-conductor laser was designed and

fabricated by Bhabha Atomic Research Centre (BARC).

Various Institutions as CAT (Centre for Advanced Technology), DRDO (Defence

Research and Development Organisation) and Indian Institute of Science work on the

laser plasma, quantum optics, etc., are going to work with American collaboration.

MASER (Microwave Amplification by Stimulated Emission of Radiation)

It was invented by three American scientist Gordon, Gieyer and H Townes in 1952.

It uses microwaves in amplified form of longer wavelength of the light, while ordinary laser uses light.

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