Class 9 Science_clone

1. Simple machines multiply force, i.e. a bigger load is lifted
by applying a smaller effort.

2. It transfers force from one point to another.

3. It accelerates the rate of doing work.

4. A simple machine changes the direction of the applied force.

Terms related to simple machine

Effort and effort distance

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effort. It is represented by E and its SI unit is Newton (N). 7KH
distance travelled by the effort while moving the load is called effort
distance. It is denoted by (Ed) and its SI unit is metre (m).

Load and load distance

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is called load. It is represented by L and its SI unit is newton (N).

For example, when we lift a heavy piece of iron with our hands, the
force that we apply is effort and the weight of iron piece is the load.
7KH GLVWDQFH WUDYHOOHG E\ WKH ORDG LV FDOOHG ORDG GLVWDQFH

Mechanical Advantage (MA)

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overcome by the machine to the effort applied.

Mathematically,

Load L
Mechanical advantage = Effort ? MA = Ƨ

Mechanical advantage has no unit because it is the ratio of two
similar physical quantities (i.e. two forces).

Mechanical advantage is the measure of number of times by which
a simple machine multiplies the applied force. For example, if the
mechanical advantage of a machine is 2, it means that the machine
can lift a load two times heavier than the effort applied.

Fact Reason

MA of a simple machine is 3, what does it mean?
It means that load is 3 times the effort in the simple machine.

47 Times' Crucial Science Book - 9

If the mechanical advantage of a machine is 1, it means that the
machine cannot multiply the applied effort. Such machine can lift a
load that is as heavy as the effort applied.

Velocity Ratio (VR)
7KH YHORFLW\ UDWLR RI D PDFKLQH LV GHÀQHG DV WKH UDWLR RI GLVWDQFH
travelled by the effort applied in a machine to the distance travelled
by the load.

Mathematically,

Distance travelled by effort Ed
Velocity Ratio = Distance travelled by load ? VR = Ld

Velocity ratio has no unit as it is the ratio of similar physical
quantities (i.e, two distances). If the velocity ratio of a machine is 2,
it means that the effort has to move twice the load distance to lift a
load up to a particular height.

(IÀFLHQF\ Lj

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work done in the machine is called input work.

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distance, i.e.

Input work (Wi) = Effort × Effort distance.

? Wi = E × Ed
On the other hand, the machine performs some work due to the
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work.
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Output work (Wo) = Load × Load distance.

? Wo = L × Ld

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LQ SHUFHQWDJH LV FDOOHG HIÀFLHQF\

Mathematically, Output work
(IÀFLHQF\ Input work × 100%

Times' Crucial Science Book - 9 48

,I WKH HIÀFLHQF\ RI D VLPSOH PDFKLQH LV ,W PHDQV WKDW
of applied energy (i.e, input work) is converted into useful work
(i.e, output work). Rest 25% of our energy is wasted to overcome
frictional and gravitational force.

In a simple machine, some amount of input work is always wasted
in the form of heat to overcome friction. Hence, the value of output
work is always less than the input work because no machine can be
made frictionless. Due to this reason, a simple machine cannot have
RU PRUH HIÀFLHQF\

Fact Reason

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means?
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the work and remaining 25% is lost to overcome the friction.

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)URP WKH GHÀQLWLRQ RI HIÀFLHQF\ ZH KDYH

Output work

Lj Input work × 100%

Here, Output work = Load × Load distance MA =L ,
E
Input work = Effort × Effort distance
L × Ld VR = Ed

? Lj E × Ed × 100%

[ [It can be rearranged as
/ ( MA Ld
Lj (G /G × 100% ? Lj VR × 100%
7KXV HIÀFLHQF\ RI D PDFKLQH FDQ DOVR EH GHÀQHG DV WKH UDWLR RI
mechanical advantage to the velocity ratio expressed in percentage.
Due to friction, the value of mechanical advantage is always less
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is never 100% or more.

Principle of simple machines

A simple machine does not perform work by itself. Hence, we have
to apply force on a machine to do a particular work.

7KH IRUFH DSSOLHG LQ D PDFKLQH LV FDOOHG HIIRUW RU input force whereas
the force which is to be overcome by the effort is called load or output
force.

49 Times' Crucial Science Book - 9

All simple machines work on a basic principle which can be stated
DV in a balanced condition, the work done on a machine (input
work) is equal to the work done by the machine (output work)
i.e. in a balanced condition,

Input work = Output work
or, E × Ed = L × Ld
Ideal machine and practical machine
According to the principle of a simple machine, the output work
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$ VLPSOH PDFKLQH ZKLFK KDV HIÀFLHQF\ LV FDOOHG DQ ideal
machine or perfect machine. In other words, a machine which can
convert the whole input work into output work with the no wastage
of energy is called an ideal machine.
However, in practice, no machine can convert whole input work
into output work. So, the output work is always less than the input
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that a machine is always affected by friction. 7KH IULFWLRQ ZDVWHV
the kinetic energy of the machine in the form of heat energy and
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of a simple machine can never be 100% or more.
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the input work is converted into useful work and remaining 10%
energy is converted into heat energy while overcoming the friction.
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DSSO\LQJ JUHDVH RU OXEULFDWLQJ RLO 7KLV LQFUHDVHV WKH HIÀFLHQF\ RI
the machine.
Types of simple machines
On the basis of structure and function, simple machines are divided
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1. Lever
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point. 7KH À[HG SRLQW DERXW ZKLFK D OHYHU URWDWHV LV FDOOHG IXOFUXP
A lever consists of three main parts - load, effort and fulcrum. 7KH
load is the object that has to be lifted by the lever whereas the effort
is the force applied to the lever to lift the load.

Times' Crucial Science Book - 9 50

FL E L
E F
F
EL

Scissors Bottle opener Fire tongs

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effort distance.

In a lever, MA = Load , VR = Effort distance Lj MA× 100%
Effort Load distance VR

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RI ORDG HIIRUW DQG IXOFUXP 7KH\ DUH
a. First class lever
7KH OHYHU LQ ZKLFK WKH IXOFUXP OLHV DW DQ\ SRLQW LQ WKH PLGGOH RI
ORDG DQG HIIRUW LV FDOOHG ÀUVW FODVV OHYHU Crowbar, scissors, see-saw,
beam balance, dhiki, pliers, cutting shears, etc are some examples
RI ÀUVW FODVV OHYHU

E

L E L E L
F F

F See-saw Scissors
Crowbar

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the direction of applied force, multiplies force and accelerates the
rate of doing work. When the fulcrum is close to the load, the effort
distance increases and it multiplies the applied force. Similarly,
when the fulcrum is close to the effort, the load distance increases
and it accelerates the work. In the use of a crowbar, if we apply the
effort in downward direction, the load is lifted up. Hence, it changes
the direction of applied force.

b. Second class lever

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and fulcrum is called second class lever. In the second class lever,
the effort distance is always greater than the load distance. Hence,

51 Times' Crucial Science Book - 9

the second class lever multiplies effort more than any other class
of lever.

Wheel-barrow, nut-cracker, bottle opener, paper cutter, etc are
some examples of second class lever.

LE FL
E E

FL

F

Wheel-barrow Onion cutter Bottle opener

Fact Reason

Why is second class lever called force multiplier?
In second class lever, effort distance (Ed) is greater than load
distance (Ld) due to which it always multiplies the effort. So,
second class lever is called force multiplier.

c. Third class lever

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fulcrum is called third class lever. Such lever makes the work safer
and easier. But it cannot multiply effort because the load distance
is always greater than the effort distance.

E F EL
E Fire tongs
L FF L
Broom
Fishing rod

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etc are the examples of third class lever.

Solved Numerical Problem 3.1

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Solution :

Given,

Load (L) =1600N 400N
Effort (E) =400N
Load distance(Ld)=20cm 1600N

Effortdistance(Ed)=100cm 20cm 100cm

Times' Crucial Science Book - 9 52

Here,

Mechanical advantage,

Velocity ratio, MA = Load = 1600N = 4

Effort 400N

VR = Effort distance = 100 =5
Load distance 20

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MA = 4 × 100% = 80%
Lj VR × 100% 5

+HQFH WKH HIÀFLHQF\ RI WKH OHYHU LV

2. Pulley

A pulley is a hard metallic or wooden disc with a grooved rim. A
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of the rope and it is pulled from another end of the rope by applying
effort.

Pulleys are used for different purposes. Some of the uses of pulleys
DUH

1. It is used to draw water from well.
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,W LV XVHG LQ ÁDJ VWDQGV IRU KRVWLQJ WKH ÁDJV

$ SXOOH\ FDQ EH FODVVLÀHG LQWR WKUHH W\SHV VLQJOH À[HG SXOOH\ VLQJOH
movable pulley and compound pulley.

D 6LQJOH À[HG SXOOH\

$ SXOOH\ LQ ZKLFK WKH IUDPH LV À[HG WR D Frame
rigid support and the disc rotates along Wooden disc
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7KH PHFKDQLFDO DGYDQWDJH RI VLQJOH Rope
À[HG SXOOH\ LV L H WKHUH LV QR JDLQ LQ
mechanical advantage. But we use such Load
pulley because it helps to change the
direction of applied force.

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by load is equal to the distance moved by
effort.

53 Times' Crucial Science Book - 9

b. Single movable pulley Effort Rope

A pulley which moves up and down along Wooden disc
with the load is called single movable Frame
pulley. In such pulley, one end of the rope Load
is tied to a rigid support and the load is
pulled IURP RWKHU HQG RI WKH URSH 7KH
circular disc moves along with load. In a
single movable pulley,

VR = No. of rope segments supporting Single movable pulley
the load =2

Hence, the VR of a single movable pulley is 2.

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6LQJOH À[HG SXOOH\ Single movable pulley

,W LV À[HG WR D ULJLG VXSSRUW DQG LW 1. It moves up and down along with
does not move along with load. load.

2. It cannot multiply applied force. 2. It multiplies the applied force
(effort).

3. It changes the direction of applied 3. It does not change the direction of
force to a more convenient direction. applied force.

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À[HG SXOOH\ LV DOZD\V movable pulley is 2.

Combined pulley

A pulley which consists of Fixed pulley
combination of two or more Rope
pulleys is called combined
pulley. A combined pulley is Frame Movable
pulley
also called compound pulley or
block and tackle system.

In case of a compound pulley,

VR = No. of rope segments Load Effort
supporting the load.

Or Combined pulley
VR = No. of pulleys used in the
pulley system

Times' Crucial Science Book - 9 54

Solved Numerical Problem 3.2

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What is the number of pulley used? What effort will be needed to lift a load of
1 LQ VXFK SXOOH\ V\VWHP"

Given,

(IÀFLHQF\ Lj) = 80%

Mechanical advantage (MA) = 4

Number of pulleys = ?

According to formula, 4
80%
Lj MVRA × 100% or, VR = MA × 100% or, VR = × 100% =5
Lj
In a combined pulley system the velocity ratio is equal to the number of
pulleys used in the system.

Hence,
No. of pulleys used = 5

Again,
Load (L) = 2000N

Effort (E) = ?

We know that,

Mechanical advantage = Load or, Effort = Load = 2000N = 500N
? Effort = 500N Effort 4
MA

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3. Wheel and axle

A wheel and axle is a simple machine which consists of two wheels
(cylinder) of different diameters. 7KH ODUJHU F\OLQGHU RU wheel is
À[HG ULJLGO\ ZLWK WKH VPDOO ZKHHO FDOOHG axle in such a way that
both the wheels spin about the same axis. Generally, load is lifted
by the small wheel and the effort is applied on the big wheel.

Wheel
Axle

Wheel

Axle

String roller reel Screw driver Steering of car

Door knob, wheel of a vehicle, steering wheel of a vehicle, screw
driver, etc are the examples of wheel and axle.

55 Times' Crucial Science Book - 9

In a wheel and axle, the circumference of larger cylinder (wheel) is
considered as effort distance and that of smaller cylinder (axle) is
considered as load distance. 7KH 0 $ DQG 95 RI ZKHHO DQG D[OH FDQ
EH FDOFXODWHG DV

MA = Load ? MA = L
Effort E
Similary,

Effort distance Circumfernce of larger cylinder
[ ]VR = load distance = Circumfernce of smaller cylinder
›5 R= Radius of larger cylinder
= ›U r = Radius of smaller cylinder

=R ?VR= R
r r
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1 because the radius of larger cylinder is always greater than that
of smaller cylinder.

Wheel and Axle : A continuous lever

7KH ZKHHO DQG D[OH LV UHJDUGHG D FRQWLQXRXV Fulcrum

lever. It is because wheel and axle move
DURXQG D À[HG SRLQW VLPLODU WR IXOFUXP LQ
WKH ÀUVW FODVV OHYHU 7KH ORDG DQG HIIRUW
OLH RQ WKH HLWKHU VLGH RI WKH À[HG SRLQW Effort

(fulcrum). A lever can move around the Load

fulcrum in less than 90° but the wheel and
axle makes an angle of 360° continuously while working. So it is a
continuous lever.

Solved Numerical Problem 3.3

A wheel of diameter 50cm is connected with an axle of diameter 10cm. If a weight
RI 1 LV OLIWHG E\ DSSO\LQJ 1 IRUFH FDOFXODWH 0$ 95 DQG HIÀFLHQF\ RI
the machine.
Given,

Load (L) = 1000N
Effort (E) = 250N
MA =?

Times' Crucial Science Book - 9 56

According to formula,

MA = L = 1000N =4
E 250N

Again,

Radius of wheel(R)= Diameter (D)
50cm
2

? R = 2 =25cm
Similarly,

10cm
r = 2 = 5cm

Velocity ratio (VR) = ?

Using formula,

Again, R 25cm
VR = r = 5cm = 5

MA 4
(IÀFLHQF\ Lj) = VR × 100% = 5 × 100% = 80%
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respectively.

4. Inclined plane Load Effort Height
A slanted surface over which a load Inclined Plane
can be pulled or pushed is called
inclined plane. In general, it is a
sloping surface. Winding roads on
hills, spiral stair-case, wooden plank
used for loading goods in a truck, etc
are some examples of inclined plane.

In an inclined plane,

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K 7KH DFWXDO KHLJKW WKURXJK ZKLFK WKH ORDG LV UDLVHG IURP WKH
ground.

Now,

Input work = E × l

Output work = L × h

57 Times' Crucial Science Book - 9

MA = Load ? MA = L
Effort E
But, due to the frictional force between the load and the surface
of inclined plane, the mechanical advantage is less than the value
given by the above relation.

7KH YHORFLW\ UDWLR LV JLYHQ E\

VR = Distance travelled by effort on inclined plane
Distance travelled by load through vertical height

= Length of inclined plane = l
Height of the plane h

7KH HIÀFLHQF\ RI LQFOLQHG SODQH LV JLYHQ E\
Lj MVRA × 100%

Solved Numerical Problem 3.4

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of inclined plane is 20m and its height is 5m.
Given, E=600N

Load(L) = 1800N

Effort(E) = 600N 20m 5m
Length of inclined plane(l) = 20m
Height of inclined plane(h) = 5m LLoa=d1800N
Mechanical advantage(MA) = ?

Velocity ratio(VR) = ?

(IÀFLHQF\ Lj) = ?

We know that,

MA = L = 1800N =3
Similarly, E 600N

VR = l 20m
Again, h = 5m = 4

Lj MVRA × 100% = 3 × 100% = 75%
4

Hence, the MA, VR and Lj of the given inclined plane are 3,4 and 75%
respectively.

Times' Crucial Science Book - 9 58

Moment of force F
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a body about its axis is called moment
of force. It is given by the product of d B
magnitude of force and the perpendicular A
distance between the line of action of force and the fulcrum.

Mathematically,
Moment of force = Force × Perpendicular distance
?M=F×d

Since the SI unit of force is newton (N) and that of distance is metre
(m), the SI unit of moment of force is Newton-metre (Nm).

7KH ODUJHU WKH GLVWDQFH RI WKH IRUFH IURP WKH IXOFUXP WKH JUHDWHU LV
the turning effect. It means that a small force can produce a large
turning effect LI LWV GLVWDQFH IURP WKH IXOFUXP LV ODUJH 7KXV LW LV
easier to open a tight bolt using a spanner with long handle than by
with a short handle.

Fact Reason

Why is long spanner used to open the rusted nut?
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LQFUHDVHV WKH PRPHQW 7KH JUHDWHU WKH PRPHQW JUHDWHU LV WKH
turning effect and rusted nut could be easily opened. So, long
spanner is used to open the rusted nut.

When the force is applied in certain angle other than 90°, the moment
of force decreases. If the line of action of force and the fulcrum are
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Moment of force = F × d sin B

If a force produces a clockwise rotation of d
a body, the moment is called clock wise F
moment and it is taken as a positive
moment. But, if a force produces anti- T
clockwise rotation of a body, the moment A
is called anticlockwise moment and is taken as negative.

Fact Reason

7KH SUREDELOLW\ RI EUHDNLQJ GRZQ RI WDOOHU WUHH LV PRUH ZK\"
In taller trees, the moment is greater due to greater
perpendicular distance which increases the turning effect. So,
the probability of breaking down of taller tree is more.

59 Times' Crucial Science Book - 9

Law of moment
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under the action of a number of forces, the sum of clockwise moments
is equal to the sum of anticlockwise moments.
Mathematically,

Sum of clockwise moments = Sum of anticlockwise moments
or, Load× Load distance = Effort × Effort distance
? L × Ld = E × Ed

Learn and Write
1. VR of a machine is always greater than MA. Why?

VR of a machine is not affected by the friction but MA is affected
E\ WKH IULFWLRQ 7KHUHIRUH 95 LV DOZD\V JUHDWHU WKDQ 0$
2. Roads are made winding in hilly regions.
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7KH KHLJKW RI WKH SODQH LV GHFUHDVHG ZLWK UHVSHFW WR OHQJWK
7KXV WKH 0$ RI WKH SODQH LQFUHDVHV ,Q WKLV VLWXDWLRQ PRUH
load can be rolled by applying small effort. But if the roads are
made steep up in hilly areas, the vehicles cannot move up.
7KH SUREDELOLW\ RI EUHDNLQJ D WDOO WUHH LV PRUH WKDQ D VKRUW RQH
during storms. Why?
When storm applies force to the trees, the distance of the line
of action of force from the fulcrum (root) is more in tall tree
than in the short tree. Due to this, more turning effect of the
IRUFH LV FUHDWHG LQ WKH WDOO WUHH WKDQ LQ WKH VKRUW RQH 7KXV
there is more probability of breaking a tall tree .
4. Lubricants are used in machines. Why?
Efficiency of a machine is reduced by friction between the
movable parts of a machine. When lubricants are used, the
friction will be reduced thereby increasing the efficiency
5. Is it possible to have the same value of mechanical advantage
and velocity ratio for a machine?
It is possible in an ideal case. If there is no friction between
moving parts of the machine, its mechanical advantage is
equal to the velocity ratio.

Times' Crucial Science Book - 9 60

6. A long spanner (wrench) is used to open a rusted nut or tight
bolt. Why?
It requires larger moment to open a rusted nut or tight bolt.
Since a long spanner has a longer moment arm, it multiplies
the applied force and produces greater turning effect. Hence, it
is easier to open a tight bolt with a long spanner.

Glossary

Sewing machine a machine with a mechanically driven needle used for
sewing or stitching cloth
Wheelbarrow a small cart with a single wheel at the front and two
supporting legs at the back used for carrying loads
Watermill a mill which works with the energy of flowing of water
Windmill a mill which works with the energy of fast moving wind
Dhiki a traditional device used in countryside for grinding and
beating food grains
Shovel a tool with a broad blade used for moving coal, earth, snow
or other material.

Main points to remember

7KH GHYLFHV ZKLFK DUH VLPSOH LQ VWUXFWXUH DQG PDNH RXU ZRUNV
easier and faster are called simple machines.

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LV FDOOHG PHFKDQLFDO DGYDQWDJH

7KH YHORFLW\ UDWLR RI D PDFKLQH LV GHÀQHG DV WKH UDWLR RI GLVWDQFH
WUDYHOOHG E\ HIIRUW DSSOLHG LQ D PDFKLQH WR WKH GLVWDQFH WUDYHOOHG
by load.

7KHUH DUH VL[ W\SHV RI VLPSOH PDFKLQHV 7KH\ DUH OHYHU SXOOH\
wheel and axle, inclined plane, screw and wedge.

$ OHYHU LV D ORQJ ULJLG EDU ZKLFK LV FDSDEOH RI PRYLQJ DERXW D
À[HG SRLQW FDOOHG IXOFUXP

$ SXOOH\ LV D URXQG PHWDOOLF RU ZRRGHQ GLVF KDYLQJ D JURRYHG
rim along which a rope passes.

%DVLFDOO\ WKHUH DUH WZR W\SHV RI SXOOH\V VLQJOH À[HG SXOOH\ DQG
VLQJOH PRYDEOH SXOOH\

8. Wheel and axle is a simple machine which is made of two
coaxial cylinders of different diameters.

9. Inclined plane is a simple machine because it makes our works
easier and faster

61 Times' Crucial Science Book - 9

Exercise

1. Choose the best alternative in each case.
D 7KH PHFKDQLFDO DGYDQWDJH RI D VLQJOH IL[HG SXOOH\ LV

i. 1 ii. 2 iii. 0 iv. Not fixed
b. A movable pulley

i. Multiplies force ii. Changes the direction of applied force
iii. Multiplies speed iv. All of these
c. Which of the following is not affected by friction?

i. MA ii. VR iii. Efficiency iv. All of these
d. What is the SI unit of moment of force?

i. N/kg ii. N/m2 iii. N/m iv. Nm
H 7KH YHORFLW\ UDWLR RI D ZKHHO DQG D[OH LV JLYHQ E\

i. r/ R ii. L/E iii. R/r iv. l/h

2. Answer these questions in very short.
a. Define the terms load and effort in a simple machine.
b. What are the factors affecting the mechanical advantage of
a machine?
c. What is velocity ratio? What is its unit?
d. What is an ideal machine?
e. Is it possible to have an ideal machine in our daily life?
f. A machine is labelled with 60% efficiency. What does it mean?
g. Write down the formula to calculate the velocity ratio of a
wheel and axle.
h. What is a wheelbarrow?
i. Derive the relationship between MA, VR and efficiency.
j. What is the advantage of using single fixed pulley although
it does not multiply force?

'HILQH

a. Effort b. Load c. Effort distance d. Load distance

H (IÀFLHQF\ I 0HFKDQLFDO DGYDQWDJH J ,QSXW ZRUN

h. Output work i. Lever j. Pulley

'LIIHUHQWLDWH EHWZHHQ

a. Pulley and wheel and axle

b. Screw driver and jack screw

Times' Crucial Science Book - 9 62

*LYH UHDVRQV
D 7KH HIILFLHQF\ RI D VLPSOH PDFKLQH FDQ QHYHU EH
E 7KH YHORFLW\ UDWLR RI D VLQJOH PRYDEOH SXOOH\ LV
F 7KH KLOO\ UHJLRQV KDYH ZLQGLQJ URDGV
d. Wheel and axle is regarded as a continuous lever.
e. It is easier to open a tight bolt with a spanner having a long
handle.
f. A tall tree has a greater possibility of breaking than a short
one in a storm.

$QVZHU WKHVH TXHVWLRQV
a. What is a lever ? Give two examples each of three types of
levers.
b. Give examples of any six simple machines used in your
daily life and classify them.
c. Why is an inclined plane considered as a machine?
d. What is the law of moment?

6ROYH WKH IROORZLQJ QXPHULFDO SUREOHPV
a. In a first class lever of length 1m, a pivot is placed at a
distance of 20cm from a load of 400N. If the effort used is
200N, calculate

L 0$ LL 95 DQG LLL (IÀFLHQF\
b. A block and tackle consisting of 6 pulleys is used to lift a
load of 300N with an effort of 75N. Calculate the MA, VR
and efficiency of the machine.
c. A truck of mass 10,000kg is moving through a sloppy road of
length 20km with the help of 5000N force. If the truck climbs
the vertical height of 200m and its efficiency is 80%, calculate
the additional mass that can be carried by the truck.
d. A wheel and axle arrangement having the wheel of radius
20cm and the axle with radius 10cm can lift a load of 150N
with an effort of 90N. Calculate MA, VR and Lj .
e. If a lever lifts a load four times the effort applied and the
effort distance is 5 times the load distance, calculate its
efficiency.
f. Calculate the value of ‘E’ from the diagram.

63 Times' Crucial Science Book - 9

50 cm 20 cm

E=? 50N

g. Calculate the value of ‘L’ from the following diagram.

50 cm 40 cm

30 cm
L=? 40N 30N

Answers 7. a. 2,4,50% b. 4,6,66.67% c. 30000kg
d. 1.67,2,83.5% e. 80% f. 20 N g. 48 N

Project Work

Visit the carpenter’s workshop near by your home or school. What kinds of
simple machine does he use and for what purpose? Ask him or observe yourself
DQG FRPSOHWH WKH JLYHQ WDEOH
Name of simple machine 7\SH RI VLPSOH PDFKLQH )XQFWLRQ 8WLOLW\

Times' Crucial Science Book - 9 64

Chapter

4 :RUN (QHUJ\
DQG 3RZHU James Watt

+H LV NQRZQ IRU WKH GLVFRYHU\ RI VWHDP HQJLQH
separate condenser, parallel motion, etc.

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ ϲ

Objectives

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗

• GHÀQH ZRUN HQHUJ\ DQG SRZHU ZLWK WKHLU XQLWV
• GHÀQH HQHUJ\ DQG LGHQWLI\ YDULRXV IRUPV RI HQHUJ\
• LQWHUSUHW UHODWLRQV DPRQJ ZRUN HQHUJ\ DQG SRZHU
• GLVWLQJXLVK EHWZHHQ ZRUN HQHUJ\ DQG SRZHU
• VROYH VLPSOH QXPHULFDO SUREOHPV UHODWHG WR ZRUN HQHUJ\ DQG SRZHU

Mind Openers

• Why do you feel hungry when you work hard?
• Are you doing any work while reading books?
• How many forms of energy do you know? What are they? Discuss.

Work
9DULRXV DFWLYLWLHV DUH GRQH LQ HYHU\GD\ OLIH 3ORXJKLQJ ÀHOG FKRSSLQJ
woods, reading books, playing games, writing, walking, carrying
ORDGV HWF DUH VXFK DFWLYLWLHV 7KHVH all activities are called works
in general sense. But, all these activities are not work in science.
In science, for any activity to be work, force is to be applied to any
object and the object should be displaced in the direction of force.
7KXV work is said to be done when a force is applied on a body and
the body moves in the direction of the force applied. It is calculated
as the product of force applied and the displacement of body in the
direction of the force.
Mathematically,

Work = Force × displacement (in the direction of force)
W=F×d

7KH 6, XQLW RI ZRUN LV 1HZWRQ PHWHU (Nm) or Joule (J). In CGS
system, it is measured in erg.

65 Times' Crucial Science Book - 9

From the above formula, it can be said that two conditions are
QHHGHG IRU WKH ZRUN WR EH GRQH 7KH\ DUH

L IRUFH PXVW EH DSSOLHG RQ DQ REMHFW DQG
LL WKH REMHFW PXVW EH GLVSODFHG LQ WKH GLUHFWLRQ RI IRUFH

,I DQ\ RQH RI WKH DERYH FRQGLWLRQV LV QRW IXOÀOOHG QR ZRUN ZLOO EH
done. If a porter carries a heavy load on his head and stands for one
KRXU WKH ZRUN GRQH ZLOO EH ]HUR -RXOH 7KLV LV EHFDXVH WKHUH LV QR
displacement.

One Joule work

We have,

W=F×d

If F = 1 N and d = 1 m then W = F × d = 1 × 1 = 1 Joule

7KXV 2QH -RXOH ZRUN LV GHÀQHG DV WKH DPRXQW RI ZRUN GRQH ZKHQ
Newton force displaces a body through 1 meter in its own direction.

Types of work

,W LV RI WZR W\SHV 7KH\ DUH
i. Work against friction
LL :RUN DJDLQVW JUDYLW\

Work against friction

When a body moves over another Effort
body, a type of force is created
between surfaces of these two
bodies in contact which opposes the
motion. Such opposing force is called
friction. When force is applied to a
body and the body moves opposite
to the direction of the friction, the
work done is called work against
friction. For example, when we push
a body, friction opposes the motion
of the body, i.e. direction of friction Friction

is opposite to the direction of our
IRUFH :KHQ RXU IRUFH LV VXIÀFLHQW WR RYHUFRPH IULFWLRQ WKH ERG\
moves against the direction of friction. Here, work done is the work
against friction.

Times' Crucial Science Book - 9 66

Solved Numerical Problem 4.1

$ PDQ SXVKHV D ORDG RI 1 WKURXJK D GLVWDQFH RI P &DOFXODWH WKH
amount of the work done. Mention the type of work done in this case.

Given,

Force (F) = 500 N

Displacement (d) = 15 m

Work (W) =?

We have,

W = F × d = 500 × 15 = 7500 Joule

7KHUHIRUH WKH DPRXQW RI ZRUN GRQH LV -RXOH 7KH ZRUN GRQH LQ WKLV
case is work against friction.

Work against gravity
Gravity is the force by which everybody is
pulled by the earth towards its centre. When
force is applied to a body against the gravity,
the work done is called work against gravity.
For example, when you lift a load from the
ground, the force is applied against the gravity
and the load is lifted against gravity.

Fact Reason

Why do we get more tired during ascending the hill than
descending?
During ascending the hill, the work is done against the gravity
and more amount of energy is used against gravity whereas
while descending the hill, work is done towards the gravity and
less amount of energy is required. Hence, we we get more tired
during ascending the hill than descending.

Solved Numerical Problem 4.2

A man lifts a load of 50 kg to a height of 20 m. Calculate the amount of
ZRUN GRQH 0HQWLRQ WKH W\SH RI ZRUN GRQH LQ WKLV FDVH >J P V2 ].
Given,

Force (F) = mg = 50 × 10 = 500 N
Displacement (d) = 20m
Work (W) = ?

67 Times' Crucial Science Book - 9

We have,
W =F×d
= 500 × 20
= 10,000 Joule

7KH DPRXQW RI ZRUN GRQH LV -RXOH DQG WKH W\SH RI ZRUN LV ZRUN
against gravity.

Energy

All living things need food to survive. Food is the source of energy.
Energy is used by them for carrying out various activities. Similarly,
YHKLFOHV PDFKLQHV HQJLQHV HWF DOVR QHHG HQHUJ\ WR UXQ 7KH\ JHW
HQHUJ\ IURP IXHOV VXFK DV SHWURO GLHVHO FRDO HWF 7KH DSSOLDQFHV
like lamp, radio, television, telephone, etc also work with the help
of energy. No work can be done without energy.

7KXV HQHUJ\ FDQ EH GHÀQHG DV WKH FDSDFLW\ RI GRLQJ ZRUN Its SI
unit is Joule. It is also measured in calorie. (4.2 J = 1 calorie)

Forms of energy

7KHUH DUH YDULRXV IRUPV RI HQHUJ\ 7KH\ DUH

i. Chemical energy ii. Mechanical energy

LLL (OHFWULFDO HQHUJ\ LY 0DJQHWLF HQHUJ\

Y +HDW HQHUJ\ YL 6RXQG HQHUJ\

YLL /LJKW HQHUJ\ YLLL 1XFOHDU HQHUJ\

i. Chemical energy

'LIIHUHQW VXEVWDQFHV OLNH IRRG FRDO SHWURO GLHVHO ÀUHZRRG HWF VWRUH
energy. When those substances undergo chemical change, such
HQHUJ\ LV UHOHDVHG 7KLV IRUP RI HQHUJ\ LV FDOOHG FKHPLFDO HQHUJ\

7KH HQHUJ\ VWRUHG LQ D ERG\ ZKLFK LV UHOHDVHG GXULQJ FKHPLFDO
change occurring in it is chemical energy. Chemical energy of
SHWURO GLHVHO FRDO ÀUHZRRG HWF LV UHOHDVHG LQ WKH IRUP RI KHDW OLJKW
or sound when they are burnt.

ii. Mechanical energy

7KH HQHUJ\ SRVVHVVHG E\ D ERG\ E\ YLUWXH RI LWV SRVLWLRQ FRQÀJXUDWLRQ
or motion is called mechanical energy. ,W LV RI WZR W\SHV
a. Kinetic energy b. Potential energy

Times' Crucial Science Book - 9 68

a. Kinetic energy (KE)

7KH HQHUJ\ SRVVHVVHG E\ D ERG\ GXH WR LWV PRWLRQ LV FDOOHG NLQHWLF
energy. $ Á\LQJ ELUG D PRYLQJ FDU UXQQLQJ YHKLFOH ÁRZLQJ ZDWHU
D EXOOHW ÀUHG IURP D JXQ D UROOLQJ VWRQH HWF FRQWDLQ NLQHWLF HQHUJ\
Kinetic energy of a body is calculated by using a formula

KE = 1 mv2
Where, m = m2 ass of the body and v = speed of the body

Activity 4 .1 7R VWXG\ WKH IDFWRUV DIIHFWLQJ NLQHWLF HQHUJ\ RI DQ REMHFW

0DWHULDOV UHTXLUHG

A cricket ball, a tennis ball, etc.

Procedure:
7HOO \RXU IULHQG WR WDNH D FULFNHW EDOO DQG WKURZ LW WRZDUGV \RX

Catch the ball.
7HOO KLP WR UHSHDW WKH VDPH DFWLYLW\ ZLWK PRUH VSHHG RI WKH EDOO
3. Now, tell him to take a tennis ball and throw it towards you.

Catch the ball
7HOO KLP WR UHSHDW WKH DFWLYLW\ RI VWHS ·LLL· ZLWK PRUH VSHHG :KDW

do you observe?

2EVHUYDWLRQ
When the speed of the cricket ball is increased, it becomes more
GLIÀFXOW WR FDWFK ,W LV GXH WR PRUH NLQHWLF HQHUJ\ RI WKH EDOO ZKLFK
LV FDXVHG GXH WR LQFUHDVHG VSHHG 7R FDWFK WKH FULFNHW EDOO LV PRUH
GLIÀFXOW WKDQ WR FDWFK WKH WHQQLV EDOO HYHQ WKRXJK ERWK EDOOV KDYH
equal speed. It is due to more kinetic energy of the cricket ball due
to its more mass.

Conclusion:
7KH NLQHWLF HQHUJ\ SRVVHVVHG E\ D ERG\ GHSHQGV XSRQ PDVV DQG
VSHHG RI WKH ERG\ 7KH PRUH WKH PDVV DQG VSHHG RI WKH ERG\ WKH
more the kinetic energy.

Derivation of formula of Kinetic Energy

u=0 a v
F m m

s

Suppose a body of mass m is at rest state ( u = 0). When a force F is

69 Times' Crucial Science Book - 9

applied to it, it moves with an acceleration a. It travels a distance s
ZKHQ LWV ÀQDO YHORFLW\ EHFRPHV v.

7KH ZRUN GRQH RQ WKH ERG\ E\ WKH IRUFH ·)· LV FDOFXODWHG DV

Work done = Force × displacement

or, W = F × s

or, W = m × a × s …………… (i)

According to equation of motion,

v2=u2+2as

or, v2=0+2as

s= v2 …………… (ii)
2a
Substituting the value of 's' from equation (ii) in equation(i)

W=m×a× v2
?W= 1 mv2 2a

7KH NLQHWLF HQHUJ\ 2JDLQHG E\ WKH ERG\ LV GXH WR WKH IRUFH DSSOLHG
7KXV WKH NLQHWLF HQHUJ\ RI WKH ERG\ LV HTXDO WR WKH ZRUN GRQH E\
the applied force.

KE of the body = Work done

? KE = 1 mv2
2

Solved Numerical Problem 4.4

,I D ELF\FOH RI ZHLJKW NJ LV PRYLQJ ZLWK D VSHHG RI P V ZKDW LV LWV
kinetic energy?
Given,

Mass of bicycle(m)=20kg
6SHHG Y P V

Kinetic energy(KE)=?
Using the relationship,

KE = 1 mv2 = 1 × 20 × (10)2 = 1000J
7KXV WKH NLQH2WLF HQHU2J\ RI WKH ELF\FOH LV -RXOH

Times' Crucial Science Book - 9 70

b. Potential energy (PE)

(QHUJ\ SRVVHVVHG E\ D ERG\ E\ YLUWXH RI LWV SRVLWLRQ RU FRQÀJXUDWLRQ
is called potential energy. Stretched rubber, stretched bow,
compressed spring, leg lifted to kick a ball, water stored in a dam, a
body at a height, etc have potential energy. Stretched rubber,
compressed spring, HWF KDYH 3( GXH WR FKDQJH LQ FRQÀJXUDWLRQ
whereas water stored in a dam, a body at a height, etc have PE due
to height. Potential energy of a body at a certain height from the
ground is calculated by using a formula

PE = mgh m
Where

m = mass of the body h
J DFFHOHUDWLRQ GXH WR JUDYLW\ J P V2)

h = height from the surface m
Derivation of formula of potential energy

Suppose, a body having mass m is lying on the ground.
Let the body is lifted to a height h LQ YHUWLFDOO\ XSZDUG GLUHFWLRQ 7R
lift it, force mg is required.

Now, the work done is given by

Work done = Force × displacement

W = F × d or, W = mg × h ? W = mgh

7KLV ZRUN GRQH LV VWRUHG LQ WKH ERG\ LQ WKH IRUP RI SRWHQWLDO HQHUJ\
7KLV SRWHQWLDO HQHUJ\ LV FDOOHG JUDYLWDWLRQDO SRWHQWLDO HQHUJ\

Gravitational potential energy = mgh
? PE = m × g × h

Solved Numerical Problem 4.5

If a rice sack of weight 50kg is lifted above ground through a height of
P :KDW LV WKH SRWHQWLDO HQHUJ\ VWRUHG LQ WKH VDFN" *LYHQ J P V2)

Given, 15m
Mass of sack (m) = 50kg 50kg
Height(h) = 15m

$FFHOHUDWLRQ GXH WR JUDYLW\ J P V2
Potential energy(PE) = ?

We know that,
PE = m×g×h = 50×10×15 = 7500J.

Hence, the potential energy stored in the sack is 7500J

71 Times' Crucial Science Book - 9

iii. Electrical energy
7KH HQHUJ\ ZKLFK LV SURGXFHG GXH WR WKH ÁRZ RI HOHFWURQV WKURXJK D
conductor is called electrical energy.
Electrical energy is the most essential energy in modern days.
Devices like computer, television, radio, fridge, fax, telephone,
calculators, etc use electrical energy. Electrical energy can be easily
converted into other forms of energy. Dry cell, dynamo, hydro-power
station, solar cell, etc are the sources of electrical energy.
iv. Magnetic energy
7KH HQHUJ\ SRVVHVVHG E\ D PDJQHW LV FDOOHG PDJQHWLF HQHUJ\. A
conductor carrying electricity also has magnetic energy. Magnetic
energy is used in dynamo or generator to produce electricity. It is
used in electrical and electronic devices such as radio, television,
loudspeaker, microphone, motor, electric bell, etc.
v. Heat energy
Every substance is composed of tiny particles called molecules.
7KHVH PROHFXOHV DUH DOZD\V YLEUDWLQJ 7KXV HYHU\ PROHFXOH KDV
kinetic energy. 7KH VXP RI NLQHWLF HQHUJ\ RI DOO WKH PROHFXOHV RI D
substance is heat energy.
7KH PRUH WKH VSHHG RI YLEUDWLRQ RI PROHFXOHV WKH PRUH LV WKH KHDW
energy contained by the body. Heat energy is used to cook food, run
vehicles, dry clothes, etc.
vi. Sound energy
Sound is a form of energy which is produced due to vibration of
an object. Sound transmits in the form of longitudinal wave. An
electric bell, temple bell, a radio, a television, etc produce sound.
vii. Light energy
Light is a form of energy which produces sensation of vision. When
a body is extremely hot, it emits radiation. Light is also a radiation.
Light energy is used by plants during photosynthesis and stored
in food in the form of chemical energy. Light energy is also used to
SURGXFH HOHFWULFLW\ LQ SKRWR FHOOV 7KH VXQ HOHFWULF EXOEV EXUQLQJ
candle, etc are the sources of light.
viii. Nuclear energy (Atomic energy)
7KH HQHUJ\ FRQWDLQHG LQ WKH QXFOHXV RI DQ DWRP LV FDOOHG QXFOHDU
energy. Nuclear energy is released when a larger nucleus splits into

Times' Crucial Science Book - 9 72

VPDOOHU QXFOHL RU VPDOOHU QXFOHL IXVH WR IRUP D ODUJH QXFOHXV 7KH
process in which a larger nucleus splits into smaller nuclei along
with the release of large amount of energy is called QXFOHDU ÀVVLRQ
and the process in which smaller nuclei fuse to form a large nucleus
is called nuclear fusion.
Nuclear energy is used to make atom bomb, produce electricity, etc.
Energy is produced in the sun due to fusion of hydrogen nuclei into
a helium nucleus.

Transformation of energy
Energy exists in various forms. One form of energy can be converted
into other forms. 7KH SURFHVV RI FRQYHUVLRQ RI HQHUJ\ IURP RQH IRUP
to another form is called transformation of energy. 7KH SURFHVV RI
transformation of energy is very common in daily life. Some of the
FRPPRQ H[DPSOHV DUH DV IROORZV
i. Electric bulb operates with electrical energy. Electrical energy

is converted into light and heat energy by the electric bulb.
ii. Electric heater produces heat by converting electrical energy

into heat energy.
iii. Loudspeaker converts electrical energy into sound energy.
LY 7KH FKHPLFDO HQHUJ\ RI D EDWWHU\ LV FRQYHUWHG LQWR HOHFWULFDO

HQHUJ\ ZKHQ D UDGLR LV WXUQHG RQ 7KH HOHFWULFDO HQHUJ\ LV DJDLQ
converted into sound energy which we hear.
Y 7KH FKHPLFDO HQHUJ\ RI IRRG LV XVHG E\ KXPDQV IRU FDUU\LQJ
YDULRXV DFWLYLWLHV 7KLV FKHPLFDO HQHUJ\ LV FRQYHUWHG LQWR
PXVFXODU HQHUJ\ 7KH PXVFXODU HQHUJ\ FRQYHUWV LQWR NLQHWLF
energy while moving, sound energy while speaking, potential
energy while climbing trees, etc.

Chemical energy Muscular energy Kinetic energy
Food Human body Running

vi. Potential energy contained in water stored in a reservoir gets
converted into kinetic energy when it falls to rotate turbine in
K\GURSRZHU VWDWLRQV 7KH URWDWLQJ WXUELQH RSHUDWHV JHQHUDWRU
which converts kinetic energy into electrical energy.

Potential energy Kinetic energy Electrical energy
Water collected in dam Rotation of turbine Generator

vii. Electric fan converts electrical energy into mechanical energy.

73 Times' Crucial Science Book - 9

Fact Reason

:DWHU ÁRZV ZLWK JUHDWHU VSHHG IURP D GDP VWRUHG DW KLJKHU
altitude than from a dam at lower altitude ?
7KH ZDWHU VWRUHG LQ D GDP LQ KLJKHU DOWLWXGH KDV PRUH SRWHQWLDO
energy than that of lower altitude and that potential energy
LV WUDQVIRUPHG LQWR NLQHWLF HQHUJ\ LQ ÁRZLQJ ZDWHU 6R ZDWHU
ÁRZV ZLWK JUHDWHU VSHHG IURP D GDP VWRUHG DW KLJKHU DOWLWXGH
than from a dam at lower altitude.

Law of conservation of energy

When energy changes from one form to another, the energy of the
former form disappears and the energy in equal amount of later
IRUP UHDSSHDUV 7KXV HQHUJ\ GRHV QRW JHW ORVW %XW LW FKDQJHV LWV
IRUPV RQO\ 7KHUHIRUH WKH WRWDO VXP RI DOO IRUPV RI HQHUJ\ LQ WKH
XQLYHUVH UHPDLQV FRQVWDQW 7KLV LV FDOOHG FRQVHUYDWLRQ of energy.
7KH ODZ RI FRQVHUYDWLRQ RI HQHUJ\ VWDWHV WKDW HQHUJ\ FDQ QHLWKHU
be created nor be destroyed but can only be changed from one form
into another.

For example, when a kerosene lamp burns, the amount of kerosene
in the lamp decreases. Here, the chemical energy of the kerosene
LV FRQYHUWHG LQWR KHDW DQG OLJKW HQHUJ\ 7KH VXP RI KHDW DQG OLJKW
energy produced is equal to the amount of chemical energy used.

Relation between work and energy

Work and energy are very much related. A body can do h
work if it has energy. When work is done by a body, it
ORVHV HQHUJ\ 7KH DPRXQW RI ZRUN GRQH LV HTXDO WR WKH
amount of energy it lost. Similarly, when work is done
RQ D ERG\ LW JDLQV HQHUJ\ 7KH ZRUN GRQH LV HTXDO WR WKH
amount of energy gained. For example, when a person
lifts a body of mass m from the ground to a height h , he
GRHV ZRUN DJDLQVW JUDYLW\ 7KH ZRUN GRQH LV FDOFXODWHG m
as,

Work done = F × displacement

Work done = mg × h

∴ W = mgh

7KH ZRUN GRQH RQ WKH ERG\ LV mgh Joule. Here, mgh Joule energy
RI WKH SHUVRQ LV XVHG 7KXV mgh Joule of energy is decreased in

Times' Crucial Science Book - 9 74

the person’s body. But mgh Joule energy is increased to that lifted
body. When that body is on the ground, total gravitational potential
energy is 0 Joule. When it is lifted, the gravitation potential is mgh
Joule.

7KXV ZRUN GRQH (QHUJ\ JDLQHG RU ORVW
Fact Reason
Why are work and energy interrelated?
Energy is the capacity of doing work and work is done when
WKH HQHUJ\ JHWV WUDQVIRUPHG IURP RQH IRUP WR DQRWKHU 7KH\
even have the same unit, i.e. Joule. Hence, work and energy
are interrelated.

Power
Suppose, Ram carries a load of 200 N to a distance 30 m in 50
seconds and Shyam carries the same load to the distance of 30 m in
30 seconds. Here, both Ram and Shyam do equal amount of work.
Each of them do 6000 Joule work. But the time taken by them is
different. Ram takes more time than Shyam to do equal amount of
work. In other words, Shyam does more work in each second than
Ram. Ram does 120 Joule work in each second whereas Shyam does
200 J work in each second. Hence, the power of Shyam is more than
that of Ram.
7KH UDWH RI GRLQJ ZRUN LV FDOOHG SRZHU In other words, the rate of
change of energy is called power. Its SI unit is Joule per second or
Watt.

Mathematically,

Power(P) = Work done W
7LPH WDNHQ ?P= t
One watt power

W
P=

t

If W=1J, and t=1s,then P=1Watt 1J

7KXV a body is said to have One Wia.ett.Ppo=w1esr if it ?P=1Watt work
can do 1 Joule
in 1 second. Horse power (HP), Kilowatt (KW), Megawatt (MW), etc
are bigger units of power.

75 Times' Crucial Science Book - 9

746 Watt = 1 HP

1000 Watt = 1 Kilowatt

1000000 Watt = 1 Megawatt
If 100 Watt is written on an electric bulb, what does it mean?
100 Watt written on an electric bulb means that the bulb consumes
100 Joule electrical energy in 1 second and converts it into heat and
light energy.

Solved Numerical Problem 4.6

Deepta weighs 40 kg. He climbs a staircase of height 3 meter in 6 seconds.
Find his power.

Given,

Mass (m) = 40 kg

Force (F) = mg = 40 × 10 = 400 N

Displacement (d) = 3 m

7LPH W V

Power = ?

We have,

W F×d 400 × 3
P= t = t = 6 = 200Watt

7KHUHIRUH SRZHU RI 'HHSWD LV :DWW

Comparison of work, energy and power

Work Energy Power

1. It is the product of 1. It is the capacity of 1. It is the rate of doing
force applied and doing work. work.
displacement in the
direction of force.

2. Its SI unit is Joule. 2. Its SI unit is Joule. 2. Its SI unit is Watt.

3. Its value does not 3. Its value does not 3. Its value depends
depend upon time. depend upon time. upon time taken to
do work.

Times' Crucial Science Book - 9 76

4. Work is of two types. 4. Energy has several 4. It does not have any
7KH\ DUH ZRUN forms. types.
against friction and
work against gravity.

Learn and Write

1. A person standing for long time feels tired even though
he seems not doing work. Why?

When a person stands for long time, he is using his energy for
holding his body upright. Moreover, various internal works like
flow of blood, pumping of blood by heart, movement of lungs
and other organs, etc are taking place continuously. For all
these activities, energy is being used. Due to the use of energy,
the person feels tired even though he is standing only.

2. What will be the change in kinetic energy of a body
when its speed is increased 2 times?
Kinetic energy of a body having mass ’m’ and speed ’v’ is
calculated by using a formula

KE1 = 1 mv2..........................................(i)
2
When speed is increased two times, the kinetic energy will be

KDiEv2id=in21gme(q2uva)2tion = 1 ×4mv2 = 4× 1 mv2....................(ii)
by e2quation (i) 2
(ii)

4× 1 mv2
KE2 = 2
? KE2 = 4KE1
KE1 1 mv2

2

7KXV WKH NLQHWLF HQHUJ\ LQFUHDVHG E\ IRXU WLPHV ZKHQ VSHHG RI
a body is increases two times.

3. What conditions are necessary for work to be done?
7KH ZRUN LV GRQH LI WKH IROORZLQJ FRQGLWLRQV DUH IXO¿OOHG
a. Force must be applied to the body.
b. The body must move through some distance in the direction of
force.

77 Times' Crucial Science Book - 9

4. The power of an electric water pump is labeled as 5 HP.
What does it mean?

+RUVH 3RZHU +3 LV D XQLW RI SRZHU DQG LW LV UHODWHG WR ZDWW DV IROORZV

1 HP = 746 Watt

? 5 HP = 5 x 746 = 3730 Watt

Hence, it means that the water pump can convert 3730 Joules of electrical
energy into kinetic and other forms of energy in one second.

5. An electric bulb is marked with 60W. What does it mean?

The mark 60W shows the power of the bulb. It means that the bulb
converts 100 joules of electrical energy into heat and light energy in one
second.

Glossary

Overcome defeat,overpower, lift a load
Gravity the force with which the earth pulls every other
object toward its centre
Compresed pressed tightly together
Configuration form, shape
Possess have, have ownership
Reservoir a large collection or storage of water
Upright erect, vertical, straight up

Main points to remember

1. The product of force applied and displacement in the direction
of the force is called work.

:KHQ IRUFH LV DSSOLHG WR D ERG\ DQG WKH ERG\ PRYHV RSSRVLWH WR WKH
direction of friction, the work done is called work against friction.

:KHQ IRUFH LV DSSOLHG WR D ERG\ DJDLQVW WKH JUDYLW\ WKH ZRUN
GRQH LV ZRUN DJDLQVW JUDYLW\

4. Capacity of doing work is called energy.
5. The energy stored in a body which is released during chemical

change occurring in it is called chemical energy.
7KH HQHUJ\ SRVVHVVHG E\ D ERG\ GXH WR LWV PRWLRQ LV FDOOHG

kinetic energy.
7KH HQHUJ\ SRVVHVVHG E\ D ERG\ E\ YLUWXH RI LWV SRVLWLRQ RU

FRQÀJXUDWLRQ LV FDOOHG SRWHQWLDO HQHUJ\
7KH SURFHVV RI FRQYHUVLRQ RI HQHUJ\ IURP RQH IRUP WR DQRWKHU LV

called transformation of energy.

Times' Crucial Science Book - 9 78

7KH ODZ RI FRQVHUYDWLRQ RI HQHUJ\ VWDWHV WKDW WKH HQHUJ\ FDQ
neither be created not be destroyed but can only be changed
from one form to another.

10. The rate of doing work is called power.

Exercise
1. Choose the best alternative in each case.

a. In which case is work done?

i. Sitting of a watchman at the gate to safeguard a building.

ii. Reading a book by sitting in a chair.

iii. Giving lecture to a class by standing in front of the class.

iv. None of the above.

b. One joule energy is equivalent to the work of

i. 1 Ns ii. 1 Nm iii. 1 N/m iv. 1 Nm2

c. A burning candle possesses

i. Chemical energy ii. Heat energy

iii. Light energy iv. All of these

d. A bread has

i. Kinetic energy ii. Chemical energy

iii. Heat energy iv. None

e. Which sentence is incorrect about power?

i. It is the rate of doing work.

ii. Power is a vector quantity.

iii. Power is a scalar quantity.

iv. An electric device labeled 2 HP has more power than the
device labeled 500W.

2. Answer these questions
a. What are various forms of energy? Explain any two of them.
b. Which form of energy is contained in following objects.
Kerosene, water stored in a reservoir, red hot iron, flowing
water, dry cell, stretched rubber, leg lifted to kick a ball,
magnet, glowing bulb.
c. Define nuclear fission and nuclear fusion with examples.
d. What will be the change in kinetic energy of a body when its
velocity is increased by three times?

79 Times' Crucial Science Book - 9

e. What do you mean by the statement that the power of a
bulb is 60 watt?

'HILQH b. Energy c. Power
a. Work e. Potential energy
d. Kinetic energy

:ULWH IRUPXOD IRU b. Kinetic energy
a. Work d. Power
c. Potential energy

'HULYH b. PE = mgh
a. KE = 1 mv2
2

'LVWLQJXLVK EHWZHHQ
i. Potential energy and kinetic energy
ii. Work and energy
iii. Energy and power

1DPH WKH GHYLFHV ZKLFK FRQYHUW HQHUJ\ LQ WKH IROORZLQJ ZD\V
a. Chemical into electrical b. Electrical into mechanical
c. Electrical into heat d. Electrical into light
e. Chemical into mechanical f. Light into electrical

*LYH UHDVRQV
i. A person feels hungry even though he does not do any work.
ii. A flying bird has both potential and kinetic energy.
iii. A person cannot do more work than the energy he has.
LY 7ZR SHUVRQV ZKR GR WKH VDPH ZRUN PD\ KDYH GLIIHUHQW
powers.

9. Numerical problems
a. Calculate the work done when you lift a load of 50 kg
through a height of 6m.
b. How much work is done when a body of mass 80 kg is
pushed to distance of 2m?
F $ EXOOHW RI PDVV J LV PRYLQJ ZLWK D YHORFLW\ RI P V
Calculate its kinetic energy.
d. Calculate the energy stored in a body of mass 40 kg when it
is at a height of 30m.
e. How much energy is stored in 4m3 water collected in a reservoir
at a height of 50m. (Given, 1m3 water = 1000 kg)
f. Calculate the power of a crane if it can lift a load of 2400 N
to a height of 10 m in 15 seconds.

Times' Crucial Science Book - 9 80

g. An engine supplies 40,000 J of heat energy per minute.
Find the power of the engine in horse power.

h. A stone is thrown upward with a kinetic energy of 10 J. If it
goes up to a maximum height of 5 m, find the initial velocity
and mass of the stone.

i. If power of an engine is 600 W, how much time will it take
to lift a box of 20 kg by a distance of 10 m?

Answers 9. a. 3000 Joule b. 1600 Joule c. 40 Joule
e. 1960000 Joule f. 1600 Watt
d.11760 Joule K P V NJ L V
J +3

Project Work

1. Measure your weight.
2. Count the number of steps and measure height of each step of

staircase of your house.
3. Measure the time you take to run over all the steps of the staircase.

Calculate work done and your power by using the results of above
activity.

81 Times' Crucial Science Book - 9

Chapter /LJKW

5
Willebord Snell

+H LV NQRZQ IRU WKH 6QHOO
V /DZ

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ ϵ

Objectives

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗
• H[SODLQ WKH ODZV RI UHIUDFWLRQ ZLWK H[DPSOHV
• H[SODLQ WKH FDXVHV RI WRWDO LQWHUQDO UHÁHFWLRQ DQG WKHLU HIIHFWV
• GHÀQH GLVSHUVLRQ RI OLJKW ZLWK GLDJUDPV
• H[SODLQ WKH VWUXFWXUH RI VSHFWUXP
• H[SODLQ WKH XVHV RI OLJKW ZDYHV RI GLIIHUHQW IUHTXHQFLHV

Mind Openers

• How is it possible to see things?
• Why does refraction of light occur?
• :KDW DUH YDULRXV W\SHV RI OLJKW ZDYHV" 'LVFXVV

Introduction
Light is invisible itself but makes the other things visible on which
it falls. ,W LV GHÀQHG DV WKH IRUP RI HQHUJ\ ZKLFK SURGXFHV WKH
sensation of vision. In physics, there is a separate branch to study
OLJKW DQG UHODWHG SKHQRPHQD 7KH EUDQFK RI SK\VLFV ZKLFK GHDOV
with the study of light and its phenomena is called optics.
Refraction of light
Light travels with different velocities in different media. It travels
with maximum velocity in vacuum or air. But, it has less velocity
in water as compared to air. Again it has less velocity in glass as
FRPSDUHG WR ZDWHU 7KH YHORFLW\ RI OLJKW LQ DLU ZDWHU DQG JODVV LV
3×108P V, 2.2×108P V and 2×108P V respectively.
Light travels in a straight line as long as it is travelling in the same
medium. But, when light passes from one medium to another, it
EHQGV DW WKH MXQFWLRQ RI WZR PHGLD 7KLV EHQGLQJ RI OLJKW LV GXH
to the change in its velocity. 7KH SURFHVV RI EHQGLQJ RI OLJKW ZKLOH
passing from one optical medium to the another is called refraction
of light.

Times' Crucial Science Book - 9 82

,Q WKH ÀJXUH AM

AO = Incident ray P i Air Glass Q
OB = Refracted ray
MN = Normal at air-glass o B' R
interface r C

M'N' = Normal at gas-air N M'
interface
‘AOM = i = Angle of incidence
‘BON = r = Angle of refraction S Air B
BC =Emergent ray Refraction of Light e
‘CBN' = e = Angle of
emergence N'
PQRS = Glass slab

OB' = Original path of the incident ray.

7KH PHGLXP WKURXJK ZKLFK OLJKW WUDYHOV LV FDOOHG RSWLFDO PHGLXP
7KH RSWLFDO PHGLD FDQ EH FODVVLÀHG LQWR WZR W\SHV rarer medium
and GHQVHU PHGLXP 7KH PHGLXP LQ ZKLFK WKH YHORFLW\ RI OLJKW LV
more is called rarer medium. On the other hand, the medium in
which the velocity of light is less is called denser medium. 7KH WHUPV
rarer and denser are the relative terms and the same substance
may act as a rarer as well as a denser medium. For example, in
the pair of air and water, air is rarer and water is denser medium.
It is because the velocity of light in air is more than that in water.
But, for the pair of water and glass, water is a rarer and glass is a
denser medium.

Cause of refraction

As stated earlier, light travels with different velocities in different
media. Its velocity is more in optically rarer medium and less in
denser medium. When light passes from optically rarer to denser
medium, it travels with slower velocity as compared to that in
UDUHU PHGLXP 7KH OLJKW EHQGV GXH WR WKH FKDQJH LQ YHORFLW\ ZKLOH
entering from one optical medium to the another. Hence, the change
in velocity of light while passing from one optical medium to the
next is the cause of refraction of light.

Laws of refraction

7KH UHIUDFWLRQ RI OLJKW REH\V WKH IROORZLQJ ODZV
7KH LQFLGHQW UD\ UHIUDFWHG UD\ DQG QRUPDO DOO OLH DW WKH VDPH
plane.

83 Times' Crucial Science Book - 9

7KH UDWLR RI VLQH RI DQJOH RI LQFLGHQFH WR WKH VLQH RI DQJOH RI
refraction for a given pair of media is constant, i.e,
Sin i = μ (a constant, and is read as mu)

Sin r

7KH FRQVWDQW μ) is known as refractive index of the medium.
7KLV ODZ LV SRSXODUO\ NQRZQ DV 6QHOO·V ODZ

7KH 6QHOO·V ODZ FDQ DOVR EH VWDWHG LQ WKH IROORZLQJ ZD\V
a. When a ray of light travels from rarer to denser medium, it
bends towards the normal.
b. When a ray of light travels from denser to rarer medium, it
bends away from the normal.
F 7KH UD\ RI OLJKW GRHV QRW EHQG LI LW SDVVHV WKURXJK WKH
normal. It means that if a ray of light incidents on a surface
perpendicularly, it does not bend.

A M B

i Air Air o r
o
r Glass i Glass

A

NB (b) (c)
(a)

Refractive index

7KH UDWLR RI WKH YHORFLW\ RI OLJKW LQ YDFXXP RU DLU WR WKH YHORFLW\ RI
light in the given medium is called refractive index of the given
medium. Its value is constant for a given pair of media and is
denoted by μ.

Veloccity of light in vacuum or air
μ = Velocity of light in given medium

? μ = C
V

7KH UHIUDFWLYH LQGLFHV RI VRPH RI WKH FRPPRQ RSWLFDO PHGLD DUH
JLYHQ LQ WKH WDEOH EHORZ

Times' Crucial Science Book - 9 84

SN Optical Medium Refractive Index SN Optical Medium Refractive Index

1. Air 1.00 6. Alcohol 1.36

2. Water 1.33 7. Kerosene 1.39

3. Glass 1.5 8. 3DUDIÀQ 1.44

4. Diamond 2.24 9. Glycerine 1.47

5. Ice 1.31 10. Ruby 1.76

7KHVH YDOXHV RI UHIUDFWLYH LQGLFHV DUH FDOFXODWHG ZLWK UHVSHFW WR DLU
Solved Numerical Problem 5.1

Calculate the velocity of light in alcohol if the refractive index of alcohol is
1.36 (Given, velocity of light in air (c) = 3×108P V

Given,

Refractive index (w) = 1.36

Velocity of light in air (C) = 3×108P V

Velocity of light in alcohol (V) = ?

We know that,

μ=C C 3×108
or, V = μ = = 2.2×108P V
V
1.36
7KHUHIRUH WKH YHORFLW\ RI OLJKW LQ DOFRKRO LV 2.2×108P V

Real and apparent depth

If you don’t have proper Real depth
swimming skills, it is unwise to Apparent depth
jump into the swimming pool
seeing its apparent depth only. Image
It is because, we cannot see the
actual depth of a swimming pool
while observing from outside.

When the rays of light coming ĮƐŚ

from an object at the bottom
of the pool travel from water
(denser medium) to air (rarer medium), the rays of light bend away
from normal. If the refracted rays are produced backwards, they
PHHW WKH QRUPDO UD\ DERYH WKH DFWXDO SRVLWLRQ RI WKH REMHFW 7KXV
the bottom or any other object appears above its actual depth. Such
depth is called apparent depth.

85 Times' Crucial Science Book - 9

7KH DFWXDO GHSWK RI DQ REMHFW IURP WKH VXUIDFH RI ZDWHU LV FDOOHG UHDO
depth, while the virtual depth at which an object appears due to the
refraction of light is called apparent depth. 7KH UDWLR RI UHDO GHSWK
to the apparent depth for a given pair of media is constant and is
the measure of refractive index (μ).
7KXV WKH UHIUDFWLYH LQGH[ PD\ DOVR EH GHÀQHG DV WKH UDWLR RI UHDO
depth to the apparent depth.

Solved Numerical Problem 5.2

&DOFXODWH WKH DSSDUHQW GHSWK RI D ÀVK DW WKH ERWWRP RI D SRQG LI WKH UHDO
depth of pond is 20m and refractive index of water is 1.33.

Solution:

Given,

Real depth (d) = 20m

Refractive index (w) = 1.33

Apparent depth (d’) = ?

According to formula,

μ = Real depth 20
Apparent depth or, 1.33 = Apparent depth

20
or, Apparent depth = 1.33 = 15.037m

+HQFH WKH ÀVK DSSHDUV DW WKH GHSWK RI P IURP WKH ZDWHU VXUIDFH

Examples of refraction of light
We experience several events in our daily life due to the refraction
RI OLJKW 6RPH RI WKH H[DPSOHV DUH JLYHQ EHORZ

1. A straight stick appears bent if it is partially immersed
in water.
Eye

When a stick is partly
immersed in water, it appears
bent due to refraction of light. ^ƟĐŬ Image

7KH UD\V RI OLJKW FRPLQJ IURP
the immersed portion of the
stick pass from the water to tĂƚĞƌ

the air by bending away from

Times' Crucial Science Book - 9 86

QRUPDO 7KXV WKH LPPHUVHG SRUWLRQ RI WKH VWLFN VHHPV WR EH
raised up (as in the figure). But the light rays from the portion
of stick outside the water come to the observer’s eyes without
bending. Hence, the stick appears bent at the air water
interface.

Fact Reason

A stick partially immersed in water seems bent, why?
When the light travels from lower part of stick i.e. from denser
medium to rarer medium, it bends away from the normal due
to which the lower part appears somewhere above its original
position. So, a stick partially immersed in water seems bent.

2. Stars twinkle at night ƐƚĂƌ Apparent
If you look at the clear sky ƐƚĂƌ ƉŽƐŝƟŽŶ

at night, you will observe the ray parh

WZLQNOLQJ RI VWDUV 7KH UHDVRQ
behind this observation is the
refraction of light coming from
the stars. As you have studied ƌĞĨƌĂĐƟǀĞ ŝŶĚĞdž
in your previous grades that the ŝŶĐƌĞĂƐŝŶŐ

temperatures of different layers
of atmosphere are different.
7KH WHPSHUDWXUH FKDQJHV FRQWLQXRXVO\ 7KLV FKDQJHV WKH GHQVLW\
RI WKH OD\HUV FRQWLQXRXVO\ 7KH OLJKW FRPLQJ IURP WKH VWDUV SDVVHV
through the layers of varying density and changes its direction
of path due to refraction. Since the density of layers changes
continuously, the uneven refraction of light, in turn, occurs
FRQWLQXRXVO\ 7KHVH SURFHVVHV PDNH WKH VWDUV WZLQNOH DW QLJKW

Critical angle
When light passes from a denser medium to a rarer medium, it
bends away from the normal. In such case, the angle of incidence is
less than the angle RI UHIUDFWLRQ 7KH DQJOH IRUPHG E\ LQFLGHQW UD\
with normal is called angle of incidence whereas the angle formed
by refracted ray with the normal is known as angle of refraction.
If the angle of incidence is increased, the corresponding angle of
refraction also increases. At a particular value of angle of incidence
in the denser medium, the corresponding angle of refraction in the
UDUHU PHGLXP LV ƒ 7KLV YDOXH RI DQJOH RI LQFLGHQFH LV NQRZQ DV
critical angle.

87 Times' Crucial Science Book - 9

Air Or BN B2 Air Nʼn
Air O r
i Glass O r B3
i Glass
C Glass
A A2 A3
M M

(a) (b) (c)

7KXV the value of angle of incidence in the denser medium for
which the corresponding value of angle of refraction in the rarer
medium is 90° is called critical angle. 7KH FULWLFDO DQJOH LV IRUPHG LQ
the denser medium as an angle of incidence for which the angle of
refraction is 90° in the rarer medium.

7KH FULWLFDO DQJOH LV GHQRWHG E\ & ,W FDQ EH FDOFXODWHG E\ XVLQJ D
IRUPXOD
1 ( )or, C= sinï 1
μ
sinC = Refractive index

7KH FULWLFDO DQJOH RI D SDUWLFXODU PHGLXP LV FRQVWDQW 7KH FULWLFDO
DQJOHV RI VRPH RSWLFDO PHGLD DUH JLYHQ EHORZ

SN Optical Medium critical angle SN Optical Medium critical angle

1. Water 49° 6. 3DUDIÀQ 44°

2. Glass 42° 7. 7XUSHQWLQH 43°

3. Diamond 24° 8. Glycerine 43°

4. Ice 50° 9. Flint glass 38°

5. Alcohol 48° 10. Quartz 35°

Solved Numerical Problem 5.3

Calculate the critical angle of diamond if its refractive index is 2.24.

Given,

Refractive index (μ) = 2.24

Critical angle (C) = ?

We have,
( ) ( )C= sinï
1 or, = sinï 1 = 26.51°
μ 2.24

7KHUHIRUH FULWLFDO DQJOH RI GLDPRQG LV ƒ

Times' Crucial Science Book - 9 88

7RWDO LQWHUQDO UHÁHFWLRQ

When light passes from a denser to a rarer medium, it bends away
from the normal. In this situation, angle of refraction is more than
the angle of incidence. According to Snell’s law, when angle of
incidence is increased, angle of refraction also increases.

When the angle of incidence is made greater than the critical angle,
WKH ZKROH OLJKW LV UHÁHFWHG EDFN LQWR WKH VDPH PHGLXP ZLWKRXW DQ\
UHIUDFWLRQ REH\LQJ WKH ODZV RI UHÁHFWLRQ 7KLV SURFHVV LV NQRZQ DV
WRWDO LQWHUQDO UHÁHFWLRQ

N N

Air O r B1 Air O r B2

(a) i Glass (b) i Glass

A1 A2
M M

NN

(c) Air O r (d) Air O Glass

C Glass A4 B4
M
A3
M

7KXV the process of returning of light into the original denser
medium, when the light passes from a denser to rarer medium with
the angle of incidence greater than the critical angle is called total
LQWHUQDO UHÁHFWLRQ
&RQGLWLRQV IRU WRWDO LQWHUQDO UHÁHFWLRQ

1. Ray of light must pass from optically denser to rarer medium.
7KH DQJOH RI LQFLGHQFH PXVW EH JUHDWHU WKDQ WKH FULWLFDO DQJOH
([DPSOHV RI WRWDO LQWHUQDO UHÁHFWLRQ
1. Mirage
Mirage is an optical illusion in which a pool of water is seen in hot desert
ZKLFK LV SURGXFHG E\ WKH SKHQRPHQRQ RI WRWDO LQWHUQDO UHÁHFWLRQ
Mirage can be observed in hot desert region. In the desert, the sand
is heated faster and the layer of air in contact with sand becomes

89 Times' Crucial Science Book - 9

KRW 7KH KRW DLU H[SDQGV DQG LWV GHQVLW\ GHFUHDVHV %XW WKH XSSHU
layers are comparatively cooler and denser than those below them.
Due to this, the ray of
light coming from the
object is refracted for ŽŽů Ăŝƌ ; ĞŶƐĞƌͿ

many times. For
instance, a ray of light
coming from the top of
a plant passes from
denser to rarer layers ,Žƚ Ăŝƌ ;ZĂƌĞƌͿ dŽƚĂů ŝŶƚĞƌŶĂů ƌĞŇĞĐƟŽŶ

and is refracted away ƉƉĞĂƌƐ ĂƐ ƚŚŽƵŐŚ ŝƚ ŝƐ
from normal in every ƌĞůĨĞĐƟŶŐ ĨƌŽŵ Ă ƐƵƌĨĂĐĞ
VWHS 7KLV LQFUHDVHV
the angle of incidence ŽĨ ǁĂƚĞƌ

at every refraction and ultimately a stage is reached where the
angle of incidence becomes greater than the critical angle 7KLV
UHVXOWV LQ WKH WRWDO LQWHUQDO UHÁHFWLRQ RI WKH LQFLGHQW UD\ ZKLFK
then reaches the observer’s eyes. Due to this, the observer views the
LQYHUWHG LPDJH RI WKH SODQW DV LI UHÁHFWHG LQ SRQG RU ODNH %XW DV WKH
observer reaches nearby the plant, its image disappears.

2SWLFDO ÀEUH RU OLJKW SLSH

Light always travels in a straight line. But it can be made to travel
LQ D FXUYHG SDWK XVLQJ WKH SKHQRPHQRQ RI WRWDO LQWHUQDO UHÁHFWLRQ
A bent tube through which light travels in a curved path is called
light pipe. 2SWLFDO ÀEUH LV DOVR D IRUP RI OLJKW SLSH $Q RSWLFDO ÀEUH
LV D YHU\ WKLQ ÁH[LEOH JODVV RU TXDUW] URG ZKLFK FDQ FDUU\ LQIRUPDWLRQ
LQ WKH IRUP RI ZDYH E\ WKH SURFHVV RI WRWDO LQWHUQDO UHÁHFWLRQ

,Q DQ RSWLFDO ÀEUH D JODVV dŽƚĂů ŝŶƚĞƌŶĂů ƌĞŇĞĐƟŽŶ
of high refractive index ŝŶŶĞƌ ĐŽƌĞ͗ ǁŝƚŚ
is coated with thin layer ŚŝŐŚ ƌĞĨƌĂĐƟǀĞ ŝŶĚĞdž

of glass of low refractive
index. A ray of light
entering from one end of KƵƚĞƌ ĐůĂĚĚŝŶŐ͗ ǁŝƚŚ
WKH RSWLFDO ÀEUH VWULNHV ůŽǁ ƌĞĨƌĂĐƟǀĞ ŝŶĚĞdž
the interface between
two glass surfaces at an
angle greater than critical
DQJOH 7KXV WKH HQWHULQJ UD\ RI OLJKW VXIIHUV PXOWLSOH WRWDO LQWHUQDO
UHÁHFWLRQV DQG WUDYHOV DORQJ WKH ÀEUH

Times' Crucial Science Book - 9 90

Light pipes are used by doctors in endoscopy. 7KH HQGRVFRS\ LV
the practice of viewing internal parts of a patient such as stomach
using a light pipe.
7KH RSWLFDO ÀEUHV DUH EHFRPLQJ PRUH SRSXODU LQ PRGHUQ GD\V 7KH
XVHV RI RSWLFDO ÀEUHV FDQ EH VXPPDUL]HG DV

a. It is used in communication as high speed internet cable.
b. It is used to study about blood vessels, arteries and tissues.
c. It is used for sending video signals.

3. Sparkling of diamond
Since the refractive index of diamond is high, its critical angle is
VPDOO 7KH IDFHV RI GLDPRQG DUH FXW LQ VXFK D ZD\ WKDW D UD\ RI OLJKW
entering into the diamond always strikes it with an angle greater
WKDQ WKH FULWLFDO DQJOH 7KXV WKH UD\ RI OLJKW VXIIHUV total internal
UHÁHFWLRQ DW HDFK IDFH 7KH OLJKW UD\V FRPH RXW IURP D IHZ SRLQWV DV
EULJKW HPHUJHQW UD\V 7KHVH UD\V PDNH GLDPRQG VSDUNOH

7RWDOO\ UHÁHFWLQJ SULVP P

A prism is a wedge shaped block of glass 45° B
having three rectangular faces and two A 45°

triangular faces. A WRWDOO\ UHÁHFWLQJ
prism is an isosceles right-angled glass
SULVP ,Q WKH ÀJXUH 345 LV D WRWDOO\ 90° Q

UHÁHFWLQJ SULVP LQ ZKLFK ‘Q = 90°, ‘P = 45°
‘R = 45° and PQ = QR. In such prism, the D 45°
rays of light from an object are incident
C

45°

normally on the face PR of the prism. R

7KHVH UD\V SDVV VWUDLJKW DQG LQFLGHQW RQ IDFH 34 ZLWK ƒ DV WKH
angle of incidence. 7KLV DQJOH RI LQFLGHQFH L H ƒ LV JUHDWHU WKDQ
the critical angle (i.e. 42°) of glass. Hence, the light rays suffer total
LQWHUQDO UHÁHFWLRQ 7KH VDPH SURFHVV LV UHSHDWHG RQ IDFH 45 DQG
the rays emerge from the prism forming the image of the object.

$ WRWDOO\ UHÁHFWLQJ SULVP GHYLDWHV WKH OLJKW WKURXJK ƒ VLPLODU WR
D SODQH PLUURU 7KHVH SULVPV DUH XVHG LQVWHDG RI SODQH PLUURUV LQ
binoculars and telescopes. It is because a prism produces a single
image but a plane mirror produces multiple images.

Fact Reason

Diamond sparkles more than glass cut in the same size, why?
7KH FULWLFDO DQJOH RI WKH GLDPRQG LV YHU\ OHVV LQ FRPSDUH WR WKH JODVV
GXH WR ZKLFK WKHUH LV PRUH WRWDO LQWHUQDO UHÁHFWLRQ LQ GLDPRQG
Hence, diamond sparkles more than a glass cut in the same size.

91 Times' Crucial Science Book - 9

Prism a
A prism consists of two refracting faces and a bc
base. 7KH WZR IDFHV RI D SULVP WKURXJK ZKLFK
light passes are called refracting faces. 7KH hh
angle between these two refracting faces
LV FDOOHG DQJOH RI SULVP 7KH IDFH RI SULVP a
opposite to the angle is its base. Some prisms bc
DUH VKRZQ EHORZ

Dispersion of light

In rainy days, sometimes, we see a
rainbow (consisting of a beautiful
pattern of seven colours) in the direction
RSSRVLWH WR WKH VXQ 7KH VHYHQ FRORXUV RI
rainbow are due to splitting of white rays
of sun light by the tiny water droplets in
WKH DWPRVSKHUH 7KH FRORXU SDWWHUQ RI
rainbow can be obtained when a narrow
beam of sunlight is passed through a prism. It is due to the splitting
RI OLJKW UD\V E\ D SULVP 7KLV SURFHVV LV FDOOHG GLVSHUVLRQ

7KH SURFHVV RI VSOLWWLQJ RI D ZKLWH UD\ RI OLJKW LQWR LWV FRQVWLWXHQW
seven colours is called dispersion of light. 7KH EDQG RI VHYHQ FRORXUV
obtained from a white ray of light due to dispersion is called spectrum
of light. It contains violet, indigo, blue, green, yellow, orange and
red colour in the respective order.

Cause of dispersion of light

In vacuum or air, the rays of all colours of white light travel with
the same speed (i.e, with the speed of 3×108P V . When the optical
medium changes, the speed of rays of different colours of light
FKDQJHV 7KXV WKH GLIIHUHQW FRORXUV RI OLJKW PRYH ZLWK GLIIHUHQW
speeds through a denser medium.

/LJKW WUDYHOV LQ WKH IRUP RI ZDYH 7KH VSHHG RI D OLJKW ZDYH LV WKH
product of its frequency and wavelength, i.e.

V= f × O

Where, V = Speed of light, f = frequency and O = wavelength

When light enters from one medium to another, its frequency is
XQDIIHFWHG EXW LWV ZDYHOHQJWK FKDQJHV 7KH GLIIHUHQW FRORXUV RI OLJKW

Times' Crucial Science Book - 9 92

have different wavelengths. For example, the red colour has the
longest wavelength (8×10-4mm) and violet colour has the shortest
wavelength (4×10-4mm) 7KH ZDYHOHQJWKV RI LQGLJR EOXH JUHHQ
yellow and orange increase in order from violet to red. Similarly,
the different colours of light have different frequency. For example,
frequency of red light is 3.75 ×1014 Hz and that of violet light is 7.5
×1014 Hz.
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ZDYHOHQJWK LQFUHDVHV 7KXV WKH UHG FRORXU RI OLJKW KDV PD[LPXP
speed (due to long wave length) and hence it slows down less and
bends the least while entering into a denser medium. Similarly,
the violet colour has the least speed (due to short wave length)
DQG KHQFH VORZV GRZQ WKH PRVW DQG EHQGV WKH PRVW 7KH UHVW RI
the colours of light bend between the red and violet due to their
intermediate wavelengths.
7KXV ZH FDQ FRQFOXGH WKDW WKH GLVSHUVLRQ RI OLJKW LV GXH WR WKH
variation in the wavelength or speed when it enters from one optical
medium to another.

Fact Reason

Rainbow can be seen around waterfall, why?
A rainbow is a combined result of refraction, despersion
DQG UHÁHFWLRQ RI VXQOLJKW E\ ZDWHU GURSOHWV SUHVHQW LQ WKH
atmosphere. When sunlight passes through the water droplets,
it splits into its component colours due to dispersion and
rainbow is observed. Since waterfall releases enough droplets in
the nearby atmosphere a rainbow can be seen around waterfall.

Recombination of spectrum of light
We have studied that when light changes medium, its wavelength
changes and dispersion occurs. But when light passes from air to
glass slab, we observe only refraction but not dispersion of light.
Why is it so?
%HIRUH WKLQNLQJ DQVZHU WR WKLV TXHVWLRQ OHW·V GR DQ DFWLYLW\ 7DNH
two prisms of same shape, size, refracting angles and made up of
same materials, i.e. pure glass or plastic. Arrange these prisms
DV VKRZQ LQ WKH ÀJXUH 1RZ SDVV D ZKLWH UD\ RI OLJKW E\ PDNLQJ
DQ DQJOH LQ WKH ÀUVW SULVP 7KH SULVP FDXVHV WKH GLVSHUVLRQ RI
the light ray. As the dispersed rays pass through the next prism,

93 Times' Crucial Science Book - 9

WKH\ FRPELQH WR IRUP WKH ZKLWH UD\ DJDLQ 7KXV D VLQJOH ZKLWH UD\
emerges from the second prism.

A P Screen

RR R White light

White light V VV

P1 A

7KLV LV WKH UHDVRQ ZK\ D JODVV VODE FDQQRW VSOLW WKH UD\ RI OLJKW
into its constituent colours. A glass slab is made by combining two
prisms. Similarly, we can make two prisms by cutting glass slab
along its diagonal.

Electromagnetic waves

7KH ZDYHV ZKLFK GR QRW UHTXLUH DQ\ PDWHULDO PHGLXP IRU WKHLU
propagation and can travel even through vacuum are called
electromagnetic waves. 7KH OLJKW ZDYHV DUH HOHFWURPDJQHWLF ZDYHV
because they do not require material medium for their propagation.
Similarly, ultraviolet rays, infrared radiations, X-rays, -rays,
microwaves, radio waves, etc are electromagnetic waves.

7KH HOHFWURPDJQHWLF ZDYHV FDQ EH FDWHJRUL]HG LQWR GLIIHUHQW W\SHV
on the basis of their frequency or wavelength. 7KH FODVVLÀFDWLRQ RI
electromagnetic waves according to their frequency or wavelength
is known as electromagnetic spectrum. Gamma rays ( - rays) are
the electromagnetic waves of shortest wavelength while the radio
waves are the electromagnetic waves of longest wave length

Electromagnetic spectrum with wave length and frequency

Electromagnetic Wave length (O) Frequency (f) in
spectrum in meters hertz

Gamma rays 10-13ï -11 1019 (and above)
X-rays 10-11ï -9 1017ï 19
Ultraviolet radiation 10-9ï -7 1015ï 17
Visible light 10-7ï -6 1014ï 15
Infrared radiation 10-6ï -3 1011ï 14
Microwaves 10-3ï -1 109ï 11
Radio-wave 101ï 4 103ï 9

Times' Crucial Science Book - 9 94

Properties of electromagnetic waves
7KH HOHFWURPDJQHWLF ZDYHV SRVVHVV WKH IROORZLQJ FRPPRQ SURSHUWLHV

1. The electromagnetic waves are the transverse waves.
2. They do not require material medium for their propagation.
3. They travel with the speed of 3×108m/s in vacuum.
4. Most of the electromagnetic waves are blocked by metal plates except

X-rays and -rays. The x-rays and -rays can pass through the thin
plates of metals also.
5. These waves can show the property of reflection, refraction,
interference, diffraction and polarization.
6RPH SDUW RI HOHFWURPDJQHWLF VSHFWUXP LV YLVLEOH WR XV 7KH SDUW RI
electromagnetic spectrum which is visible to human eyes is called
visible spectrum.
Application of electromagnetic waves
7KH HOHFWURPDJQHWLF ZDYHV DUH YHU\ XVHIXO LQ RXU GDLO\ OLIH %XW
some of the waves are extremely harmful on long term exposure.
1. X - rays
; UD\V KDYH ZDYHOHQJWK RI WR PP 7KH [ UD\V FDQ HDVLO\
pass through skin and muscles of body. But they cannot pass
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DSSOLFDWLRQ 7KH\ DUH XVHG WR GHWHFW IUDFWXUHV RI ERQHV RU RWKHU
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However, the excessive exposure to x-rays can cause cancer.
2. Gamma rays
7KH JDPPD UD\V DOVR KDYH PHGLFLQDO DSSOLFDWLRQV 7KH\ DUH
used to sterilize the surgical instruments because they kill
harmful bacteria. Doctors can use gamma rays to kill cancer
cells inside a patient’s body.
3. Microwaves
0LFURZDYHV DUH XVHG LQ PLFURZDYH RYHQ 7KH PLFURZDYH RYHQV
produce the microwaves which are used to cook foods.
4. Radio waves
Radio waves are used to transmit radio and television programmes.

95 Times' Crucial Science Book - 9

5. Ultraviolet radiations
7KH XOWUDYLROHW UDGLDWLRQV KDYH WKH ZDYHOHQJWK RI QP

7KH\ DUH WKH KDUPIXO UDGLDWLRQV DQG FDXVH WKH FDQFHU RI VNLQ
and cataract of eyes. But their mild or lesser exposure is
VRPHKRZ XVHIXO 7KHVH UD\V DUH DEVRUEHG E\ RXU VNLQ WR PDNH
YLWDPLQ ' 7KH RWKHU XVHV RI 89 UDGLDWLRQV DUH
D 7KH\ DUH XVHG WR WHVW WKH SXULW\ RI JKHH HJJV RUQDPHQWV HWF
E 7KH\ DUH XVHG WR VWHULOL]H WKH PHGLFDO LQVWUXPHQWV
F 7KH\ DUH XVHG WR SURGXFH YLWDPLQ ' LQ WKH IRRG RI DQLPDOV DQG
plants.
6. Infrared radiations
7KHVH UDGLDWLRQV FDUU\ KHDW DQG DUH FDOOHG KHDW UD\V 7KHVH
rays can be used for heating and cooking purposes.
7KH ; UD\V DQG JDPPD UD\V DUH YHU\ KDUPIXO RQ WKH ORQJ WHUP
H[SRVXUH 7KH\ KDYH FDUFLQRJHQLF HIIHFW L H WKH\ FDQ FDXVH
cancer. A pregnant woman must avoid the exposure to such
rays because these rays can severely harm the foetus.
Learn and Write

1. An air bubble in a jar of water shines brightly. Why?
When light rays entering from water to the air bubbles make
incident angle more than the critical angle, the light rays
undergo total internal reflection. For the observers, the light
UD\V VHHP FRPLQJ IURP WKH EXEEOHV 7KXV WKH DLU EXEEOHV
shine.

2. A stick partially dipped in water seems bent. Why?
When a stick is partially dipped, the light rays coming to the
viewer’s eyes from the immersed part of the stick from water
WR WKH DLU EHQG DZD\ IURP WKH QRUPDO 7KXV WKH GLSSHG SDUW
of the stick seems raised up. But the light rays coming from
the undipped part to the viewer’s eyes do not bend. Hence, the
stick seems bent when partially immersed in water.

3. A rectangular glass slab does not disperse white light.
Why?
A rectangular glass slab is the combination of two prisms of same
size, shape and made up of same materials. When a white ray
of light enters the glass slab, the ray splits into its constituent

Times' Crucial Science Book - 9 96