Science 8

Revised and Enlarged

Approved by Curriculum Development
Centre, Government of Nepal as an
additional material for schools.

Times’ Crucial

SCIENCE
and ENVIRONMENT
8Grade

Authors/Editors:
Kamal Prasad Sapkota
Rajan Kumar Shrestha

Surendra Ojha Editors: Dipak Kumar Dangi
Sagar Dahal Hemanta Dhungana
Santosh Pokharel Kamal Kanta Dhungel
Govinda Paudel Mani Ram Rai

Times’ Crucial

SCIENCE
and ENVIRONMENT
8Grade

Published by:
Times International Publication Pvt. Ltd.

Authors/Editors:
Kamal Prasad Sapkota
Rajan Kumar Shrestha

Edition:
First: 2070 B.S.
Revised Edition: 2074 B.S.
Revised Edition: 2075 B.S.
Revised Edition: 2077 B.S.
Revised Edition: 2078 B.S.

Layout and Design:
Ramesh Maharjan

© Copyright: Publisher

Printed in Nepal

Preface

Human life is always in progress. Newer technologies and equipmen are
developed or discovered in every second. Lengthy and time consuming work of past
are now done within a few moments. People can travel far off distances within short
time. Several fatal diseases have been eradicated. The achievements, that we are
enjoying today are the results of advancement in science and technology. Hence,
science has become an integral part of our education system.

Times' Crucial Science and Environment is a series of text books for the
school level students of grades LKG to class Ten. The series has been prepared for
the young learners emphasizing on student-centred teaching techniques, Learning
Circle Based Activities RTCEVKECDNG CEVKXKVKGU UEKGPVKſE CRRTQCEJGU CPF KPPQXCVKXG
learning techniques. The text of each lesson is preceded by a warm up activity that
encourages students to take part actively in the learning process. The series includes
the teaching techniques and methods for the teachers under the title Note to the
Teacher. It is based on the latest syllabus prescribed by CDC Government of Nepal.
Hence, this series acts as the foundation of science for the curious and inquisitive
young minds.

'CEJ DQQM QH VJKU UGTKGU EQXGTU VJG U[NNCDWU QH 5EKGPEG 6JG ſTUV RCTV EQPVCKPU
chapters of Science and the second part, chapters of Environment Science. All the
chapters of Science and Environment are provided with a wide variety of exercises
which encourage the students in learning and sharpening their mind. A Project Work
is asked at the end of each lesson so that student can apply their knowledge to
solve the problems of their day-to-day life. In the beginning of each lesson, thought
provoking questions are given under the heading Mind Openers. This activity will
encourage students to use their brain.

We feel delighted to extend our sincere gratitude to all the editors for their
painstaking contribution in editing the book and making it more simpler and crucial.
We are equally thankful to our publishers Mr. Kamal Pokharel and Mr. Purushottam
Dahal without whom this series would not have been possible in the present form.
Moreover, We are thankful to Mr. Santosh Pokharel for his effort to make this book
error-free. We are also thankful to Mr. Ramesh Maharjan for the kind cooperation and
innovative skills in providing the outstanding form to the books.

Finally, it will be our pleasure to receive constructive suggestions and
recommendations from the teachers, students and well wishers for the further
improvement of the books.

- Authors

Contents Page No.

Chapter 1-127

Physics 1

1 Measurement 11
26
2 Force and Motion 40
3 Simple Machine 55
4 Pressure 68
5 Work, Energy and Power 79
6 Heat 98
7 Light 110
8 Sound 118
9 Magnetism 128-205
10 Electricity 128
Chemistry 146
11 Matter 162
12 Language of Chemistry 173
13 Separation of Mixture 185
14 Metals and Non-metals 198
15 Acid, Base and Salt 206-254
16 Some Useful Chemicals 206
Biology
17 Living Beings 226

18 Cell, Tissue and Organ 236
236
19 Life Processes 242
19.A Reproduction 249
19.B Blood Circulation 255-281
19.C Photosynthesis 255
266
Geology and Astronomy 273
20 Structure of the Earth 282-324
21 Weather and Climate 282
22 The Earth and Space
Environment Science 300
23 Environment and Its Balance
319
24 Environmental Degradation and Its Conservation
325
25 Environment and Sustainable Development
327
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Model Test Paper

1CHAPTER Measurement

Louis Essen
He is known for the precise measurement of time and

the determination of the speed of light.

Estimated Periods : 6
Objectives: At the end of the chapter, the students will be able to:

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PHQWLRQ WKH XQLWV RI PDVV ZHLJKW WLPH DQG VRPH RWKHU TXDQWLWLHV
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VROYH VRPH PDWKHPDWLFDO SUREOHPV UHODWHG WR FRQYHUVLRQ RI XQLWV

Can you say what fundamental and derived units are ?
In what units do you measure mass, length and time ?
Plastic jug floats on water but steel glass sinks. Why? Discuss.

Introduction

Measurement is one of the important activities of human life. We need
measurement many times in a day. When we have to cook food, proper amount
of rice, oil, salt, water, sugar, milk, etc are measured and used for cooking.
When we go to a shop, the shopkeeper measures the things and gives to us.
For measurement, a standard or known value is required. When we want to
measure mass of any substance, we keep it on one pan of the balance and a unit
of known mass is kept on another pan. By adding or taking out the substance,
we make the beam of the balance maintained. Similarly, a meter rod is used
to measure the length of cloth. Thus, measurement is the comparison of an
unknown quantity with a standard or known quantity.
The quantities which can be measured are called physical quantities. Mass,
volume, length, time, etc are physical quantities. The quantities which cannot
be measured are known as non-physical quantities. Sadness, happiness, love,
feelings, etc are non-physical quantities.

Think and Solve
We can express mass in terms of exact unit but not sadness. Why?

1 Times' Crucial Science & Environment Book - 8

Physical quantities are of two types:

a) Fundamental quantities and b) Derived quantities

Fundamental quantities

The quantities which are independent of other quantities are called
fundamental quantities. Length, mass, time, temperature, current, luminous
intensity and amount of substance are physical quantities.

Derived quantities

The quantities which are derived from fundamental quantities are called
derived quantities. Examples: velocity, area, speed, acceleration, density, etc.
Speed can be proved as derived quantity in the following ways:

Speed means the distance travelled by a body in unit time, i.e.

Speed = Distance
Time

Distance is measured as length. So,

Speed = Length
Time

Thus, speed is derived from length and time. Therefore, it is a derived quantity.
In the same way, area is derived from two lengths, i.e.

Area = (length)2
Thus, area is a derived quantity. There are several other derived quantities.

Units

Units are standard or known quantities in terms of which other physical
quantities are expressed or measured.

When we measure a piece of cloth, we compare it with a meter rod. We express
the measurement as 1 meter, 2 meter, etc. Here, meter is unit because the
length is expressed in ‘meter’ and value of one meter is known.
There are two types of units. They are fundamental units and derived units.

Fundamental units

The units which are used to measure fundamental quantities are called
fundamental units. In other words, fundamental units are the units which are
independent of other units.

S.N. Fundamental quantities Fundamental units
1. Length Meter (m)
2. Mass Kilogram (kg)
3. Time Second (s)

Times' Crucial Science & Environment Book - 8 2

4. Temperature Kelvin (K)
5. Current Ampere (A)
6. Luminous intensity Candela (cd)
7. Amount of substance Mole (mol)

Derived units

The units which are used to measure derived quantities are called derived units.

In other words, the units which are derived from fundamental units are called
derived units. For example, units of velocity, area, acceleration, force, volume,
density, etc are derived units because they are derived from two or more
IXQGDPHQWDO XQLWV 7KH XQLW RI YHORFLW\ DV D GHULYHG XQLW FDQ EH MXVWLÀHG DV

Velocity = Distance = Length = m
Time Time s

The unit of length is metre (m) and that of time is second (s). Thus, unit of
velocity is metre/second or m/s in short. It is derived from fundamental units
metre and second and is the derived unit.

Similarly, the unit of density is a derived unit. It is derived from fundamental
units kilogram (kg) and metre (m).

Density = Mass = kg
Volume m3

Since the unit of density involves the use of two fundamental units, it is a
derived unit. In this unit, the fundamental unit kg is used once and m is
repeated thrice. In the same way, unit of acceleration is a derived unit.

Acceleration = Change in velocity = m/s = m
Time s s2

It is derived from fundamental units metre (m) and second (s). The derived
units of some physical quantities are given below:

S.N. Derived quantities Derived units Fundamental units
involved
1. Velocity/ Speed m/s
2. Density kg/m3 metre and second
3. Acceleration m/s2
4. Volume m3 kilogram and metre
5. Area m2
6. Force Newton (kgms-2) metre and second
7. Work/ energy Joule (kgm2s-2)
8. Power Watt (kgm2s-3) metre for three times
9. Pressure Pascal(kgm–1s–2)
metre for twice

kilogram, metre and second

kilogram, metre and second

kilogram, metre and second

kilogram, metre and second

3 Times' Crucial Science & Environment Book - 8

Measurement of mass

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measures the total amount of matter contained in an object.

Mass depends upon the number and size of atoms present in the object. It is
measured by using a beam balance and standard masses. The object to be
measured is kept on one pan of the balance and the standard masses are kept on
another pan to maintain balance of the beam.

The SI unit of mass is kilogram. The units like gram, milligram, centigram,
decigram, etc are smaller than kilogram and are called sub-multiples of
kilogram. The units like quintal, tonne, etc are bigger than kilogram and are
called multiples of kilogram.

Sub-multiples of kilogram Multiples of kilogram
100kg = 1quintal
1kg = 10,00,000 milligram (mg) 1000kg = 1 tonne
1kg = 1,00,000 centigram (cg)
1kg = 1000 gram (g)
1kg = 100 decagram (dag)
1kg = 10 hectogram (hg)

How much mass is taken as 1 kilogram?

There is an international organization
that monitors the matters of weight and
measurements in the world. It is named as
International Bureau of Weights and Measures
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substance to be taken as 1 kilogram.
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International Bureau of Weights and Measures at Sevres near Paris.

Measurement of weight

Weight of an object is the force of gravity applied by the earth to attract that
object towards the centre of the earth.

Times' Crucial Science & Environment Book - 8 4

It depends upon the mass of the object and gravity of the earth or other
heavenly body. The object having more mass has more weight. An
object has more weight when it is pulled by the earth or heavenly
body with more gravity. It is measured by a spring balance. The
weight of a body is given by the product of mass of the body and
acceleraltion due to gravity,i.e.

Weight= Mass × Acceleraltion due to gravity
W =m×g

It is measured in unit ’Newton’.

ĐƟǀŝƚLJ ϭ͘ϭ Take a spring balance and measure weight of your

science book, instrument box, etc.
Mass and weight are used as synonyms but they are different from
each other in the following ways:

Mass Weight

1. Mass of an object is the total 1. Weight of an object is the measure
quantity of matter contained in it. of force of gravity applied by the
earth to pull that object towards the
centre of the earth.

2. It is measured in Kilogram. 2. It is measured in Newton.

3. It is measured by a beam balance. 3. It is measured by a spring balance.

4. Mass of an object remains constant. 4. Weight of an object differs from
place to place.

Measurement of length

The distance between any two points is called length.
It is measured by various devices such as measuring tape, scale, metre scale, etc.

The SI unit of length is metre (m) . The CGS unit of length is centimeter (cm).
The units like kilometer, decameter, hectometer, etc are bigger units than
metre and are called multiples of metre. The units like centimeter, millimeter,
decimeter, etc are smaller than metre and are called sub-multiples of metre.

5 Times' Crucial Science & Environment Book - 8

Sub-multiples of metre Multiples of metre
1 metre = 10 decimetre (dm) 10 metre = 1 decametre (dm)
1 metre = 100 centimetre (cm) 100 metre = 10 hectometre (hm)
1 metre = 1000 millimetre (mm) 1000 metre = 1 Kilometre (km)

Smaller units are used to measure smaller lengths and larger units are used
to measure larger lengths. Light year is a unit of length which is used for
measuring the distance of stars and other heavenly bodies.

ĐƟǀŝƚLJ ϭ͘Ϯ a) Take a scale and measure the length and breadth of your

science book. b) Measure your height with the help of a measuring tape or
metre-rod.

Measurement of time

The interval or duration between any two events is
called time. The units of time are based on position
of the earth about its own axis and its position
with respect to the sun. The time required for one
complete rotation of earth about its own axis is
called one day or one solar day. One day is divided
into 24 equal parts, each part is called an hour. The
'one hour' is divided into 60 equal parts, each part
is called a minute. One minute is again divided into
60 equal parts; each part gives one second time. Hence, one solar day can be
converted into seconds as follows:

1 solar day = 24 hours = 24×60 minutes
= 24 ×60×60 seconds = 86,400 seconds

Thus, there are 86,400 seconds in a solar day. 2QH VHFRQG LV GHÀQHG DV
1/ 86,400th the part of a solar day.
The time taken by the earth to revolve around the sun is called one year. The
one year is equal to 365¼ days. As it is impractical to add 1/4th of a day in a
year, one single day is added to a year in every four years. Hence, a year has
366 days in every four years. The year which has 366 days is called leap year.
The month February has 29 days in a leap year.
Time is measured by watches like quartz watch, pendulum clock, etc. The SI
unit of time is second. It is also measured in minutes, hours, days, months,
years, etc. These units are bigger than second. Therefore, they are called
multiples of second. The units like millisecond, microsecond, nanosecond,
picosecond, etc are smaller than second and are called sub-multiples of second.

Times' Crucial Science & Environment Book - 8 6

Multiples of second (s) Sub-multiples of second (s)
60 seconds = 1 minute 1 second = 1000 milliseconds
3600 seconds = 1hour 1 second = 10,00,000 microseconds
86400 seconds = 1 day 1 second = 1,00,00,00,000 nanoseconds
31536000 seconds = 1 year

Nowadays, many digital and atomic watches are also in use.

ĐƟǀŝƚLJ ϭ͘ϯ Radio Nepal broadcasts news at accurate Nepali times. Make

the watches or clocks of your home show exact time as the studio clock of Radio
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E\ ZDWFKHV DQG FORFNV DW \RXU KRPH" 'LVFXVV \RXU ÀQGLQJV LQ WKH FODVV

Measurement of volume

The total space occupied by an object is called its volume. The SI unit of
volume is cubic metre (m3) and CGS unit is cubic centimeter (cm3).

Volume of a regular solid object is determined by using formula. Volume of a
cuboid solid is calculated by formula (V) = l × b × h

Volume of a cube (V) = l3 height

9ROXPH RI D F\OLQGHU 9 ǑU2h

Volume of a sphere (V) = 4 ǑU3 breadth
3

Volume of an irregular object is determined by length
using a measuring cylinder.

ĐƟǀŝƚLJ ϭ͘ϰ To measure the volume of an irregular solid.

Materials required:

Measuring cylinder, a stone, string, etc.

Procedure:

(a) Take a measuring cylinder and
ÀOO LW ZLWK ZDWHU XSWR WKH KDOI
part.

(b) Note the level of water in the
cylinder. Let the volume be V1.

(c) Tie a stone with a string and
immerse it into the water
completely. This increases the
volume of water in the cylinder.

(d) Note the level of water now. Let
the volume be V2.

7 Times' Crucial Science & Environment Book - 8

The difference in the level of water is the volume of the stone.

Therefore, volume of stone = V2 ï 91.
)URP WKH ÀJXUH YROXPH RI WKH VWRQH ï FF

fundamental : basic

derived : something obtained from

1. Measurement is the comparison of an unknown quantity with a standard or known
quantity.

2. Units are the standard or known quantities in terms of which other physical
quantities are expressed or measured.

3. The units which are independent of other units are called fundamental units.

4. The units which are derived from fundamental units are called derived units.

5. Mass is the total quantity of matter contained in an object.

6. Weight of an object is the force of gravity applied by the earth to attract that
object towards the centre of the earth.

7. The distance between any two points is called length.

8. The interval or duration between any two events is called time.

9. The total space occupied by an object is called its volume.

Exercise

A. Choose the best alternative.

1. Which of the following is a derived quantity?

a. Current b. Luminous intensity

c. Amount of substance d. Density

2. What is the correct unit of energy?

a. Newton b. Watt c. Joule d. Pascal

3. How many decagrams are there in 1 kilogram?

a. 105dag b. 103dag c. 102dag d.10-2dag

4. The weight of a body is given by.

Times' Crucial Science & Environment Book - 8 8

a. W= m b. W= m×g c. W= m d. F=m×a
g v d. l×b×h
5. The volume of a sphere is given by.

a. 4 ǑU3 E ǑU2h c. l3
3

B. Copy the correct statements. And correct the false statements.

1. Derived units are the units which are independent of other units.

2. Units of work, power, acceleration and mass are derived units.

3. Mass of an object is measured by a spring balance.

4. The SI unit of density is g/cc.

5. The unit light year is uesd to measure the distance between stars.

6. The standard or known quantities in terms of which other quantities
are measured are called units.

& 'HÀQH ii. Length iii. Volume iv. Time
i. Mass

D. Answer these questions in short.
1. What is measurement?
2. What are physical quantities? Give some examples.
3. What are units? Give examples of derived and fundamental units.
4. What is weight? On what factors does the weight of an object depend?
5. Differentiate between mass and weight.
6. What is meant by 1kg mass?
7. What is volume? Mention the formulae for the volume of a sphere,
cylinder, and a cube.
:KDW LV VHFRQG" 'HÀQH PXOWLSOHV DQG VXE PXOWLSOHV RI D VHFRQG
with examples.
9. What is a leap year?

E. Answer these questions.

1. How do you measure the volume of irregular objects? Explain with
diagram.

2. Explain the importance of measurement in daily life.
3. What is mass? How is it measured?

9 Times' Crucial Science & Environment Book - 8

F. Solve the following numerical problems.

1. Convert the following:

i. 340cm into m ii. 65 kg into g

iii. 87570 seconds into hour iv. 7.5 km into m.

2. The length, breadth and height of an object are 20cm, 15cm and
25cm respectively. Its mass is 7kg. Find its volume and density.

Answers ii. 65000g iii. 24.325 hrs

F. 1. i.3.4m 2. 7500cm3, 0.933g/cc
iv. 7500m

Take a brick and measure its length, breadth and height. Calculate its volume
by using formula. Find its mass by using a beam balance. Calculate its density.
'RHV LW ÁRDW RU VLQN LQ WKH ZDWHU " :K\" ([SODLQ

Times' Crucial Science & Environment Book - 8 10

2CHAPTER Force
and Motion

Sir Isaac Newton

He discovered Newtonian mechanics, Universal gravitation, Calculus,
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Estimated Periods : 12
Objectives: At the end of the chapter, the students will be able to:

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ZULWH DQG XVH WKH HTXDWLRQV RI PRWLRQ
VROYH VLPSOH QXPHULFDO SUREOHPV UHODWHG WR WKH YHORFLW\ DFFHOHUDWLRQ HWF

Are you in rest or motion state at present?
What is velocity? Is it the same as speed?
What is acceleration? Tell its unit.
Are motions always uniform? Discuss.

Introduction

We see many objects around us. Some of them are at rest and some are in the
state of motion.
A body is said to be at rest if it does not change its position with respect to its
surrounding. For example, building, table, chair, blackboard, road, etc are in
the state of rest.
A body is said to be in motion if it changes its position with respect to its
surrounding. )RU H[DPSOH Á\LQJ ELUG UXQQLQJ YHKLFOHV ÁRZLQJ ZDWHU HWF
are in the state of motion.
For determining the rest or motion state of a body, we have to compare it with
a certain stationary object or point. This stationary object or point is called
reference point or reference frame. Thus, a stationary object with respect to
which rest or motion state of a body is determined is called reference frame.
When we are in a moving bus, we will be in motion state if we are compared
with outside tree or building. It is because we change our position with respect
to the tree or building. Here, tree or building is a reference point. When we are

11 Times' Crucial Science & Environment Book - 8

compared with other passengers of the same bus, we will be in the state of rest.
Here, we are not changing our position with respect to the other passengers.
In the above example, we appear in the state of rest as well as motion at the
same time. Hence, a body in motion with respect to one object can be at rest
with respect to another object. It depends upon the comparison. Therefore,
rest and motion are relative terms.

Vector and scalar quantity

There are some quantities such as force, velocity, acceleration, etc which have
both magnitude and direction. They are vector quantities. When anyone says,
I applied 50 Newton force to a book. There may be the question to which
direction? The answer may be toward the east, north, etc . Here, 50 Newton
is magnitude and east or north is direction.
The quantities that have both magnitude and direction are called vector
quantities.
The quantities like mass, time, volume, temperature, area, etc have only
magnitude but no direction. Such quantities are called scalar quantities.
When some body says my mass is 50 kg, we never ask the question to which
direction?. Thus, mass has only magnitude but no direction.
The quantities that have only magnitude but no direction are called scalar
quantities.

Differences between vector and scalar quantities

Vector quantities Scalar quantities

1. The quantities that have both 1. The quantities that have only
magnitude and direction are called magnitude but no direction are
vector quantities. called scalar quantities.

2. Their sum may be positive, negative 2. Their sum is always positive.
or zero.

3. They are represented by an 3. They are not represented by
arrowhead over the letter (s). arrowhead.

4. Displacement, force, velocity, 4. Distance, mass, volume, density, etc
acceleration, etc are vector quantities. are scalar quantities.

Distance and displacement 3

When a person has to move from 2

point A to B, he/she can travel along 1 B
the paths 1, 2, 3 or 4 as shown in A
WKH ÀJXUH
4

Times' Crucial Science & Environment Book - 8 12

When a person travels along paths 1, 2, 3 and 4, he has to travel 20m, 30m,
40m and 50 meters respectively. These are the actual lengths of the paths.

The actual length of a path travelled by a body is called distance. The SI unit
of distance is meter (m). It is a scalar quantity.

But, the shortest distance between the points A and B is the length of the path
1. It is displacement.

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called displacement. ,W FDQ DOVR EH GHÀQHG DV WKH VKRUWHVW GLVWDQFH EHWZHHQ
any two points measured in a particular direction.

The SI unit of displacement is meter (m). C

Suppose a person moves from point A to B then to
C. Let the distance between A and B is 4m and the
distance between B and C is 3m. Thus, the total 5m 3m

distance travelled by the person sums to be 7m. But,
the shortest distance between A and C is the length
of AC. It is 5m. Here, the actual distance travelled A 4m B
by the person is 7m but the displacement is only 5m.

Differences between distance and displacement

Distance Displacement

1. It is the actual length of a path 1. It is the shortest distance between
travelled by an object. any two points.

2. It is a scalar quantity. 2. It is a vector quantity.

3. It can be added or subtracted by 3. It can be added or subtracted by
simple mathematical rule. vector rules only.

4. The sum of distances is always 4. Its value may be positive, negative or
positive. zero.

Speed

Suppose a body moves from one place to another either in a straight or curved
path and covers 20 meter distance in 2 seconds. Here, we can say that it covers
10m distance in 1 second. It can be expressed as 10m/s. It is the speed of the
moving body. Hence, VSHHG LV GHÀQHG DV WKH GLVWDQFH WUDYHOOHG E\ DQ REMHFW LQ
unit time.

Speed = Distance covered (d)
Time taken (t)

Since the distance is measured in ‘meter’ and time in ‘second’, speed is
measured in meter per second (m/s). It is also measured in kilometer per hour
(km/hr). Speed is a scalar quantity.

13 Times' Crucial Science & Environment Book - 8

Velocity

When a body moves from one point to another point in a particular direction, its
speed is called velocity. 7KHUHIRUH YHORFLW\ FDQ EH GHÀQHG DV VSHHG RI D ERG\ LQ D
particular direction.
Since the distance in a particular direction is displacement, velocity can be
GHÀQHG DV WKH GLVSODFHPHQW RI D ERG\ SHU XQLW WLPH

Velocity = Displacement ? v = s
Time taken (t) t

Comparison of speed and velocity C
5m
Suppose a body moves from point A to B then to the 3m
point C. Let the distance of AB and BC is 4m and 3m
respectively. The total distance is 7m. Suppose, total
time taken by the body is 2 seconds.

The speed of the body is calculated by using formula,

Speed = Distance covered (d) A 4m B
Time taken (t)

= 4 + 3 = 3.5 m/s
2

If the body directly travels from A to C in the same time, the velocity of the
body is calculated by using formula,

Velocity = Displacement
Time taken

= 5m = 2.5 m/s
2s

Since the unit of displacement is meter and that of time is second, the unit of
velocity is meter per second (m/s).

Solved Numerical Problem 2.1
If a car travels a distance of 3000m in 2 minutes, what will be its speed?
Given,

Distance (d) = 3000m

Time (t) = 2 minute = 2 × 60 = 120s

Speed (v) = ?

We have,

Speed (v) = Distance travelled (d) = 3000 = 25m/s
Time taken (t) 120

Therefore, the speed of the car is 25m/s.

Times' Crucial Science & Environment Book - 8 14

Differences between speed and velocity

Speed Velocity

1. It is the distance travelled by 1. It is the displacement of an object in unit
an object in unit time. time.

2. It is a scalar quantity. 2. It is a vector quantity.

3. Its value is always positive. 3. Its value may be positive, negative or zero.

4. It can be added by simple 4. It can be added by using the rules of vector.
mathematical rule.

Uniform and variable motion

When a body travels equal distance in equal interval of time, its motion is
called uniform motion.

0 sec 1 sec 1 sec 1 sec 1 sec

a 5m b 5m c 5m d 5m e

,Q WKH DERYH ÀJXUH D FDU WUDYHOV P GLVWDQFH LQ WKH ÀUVW RQH VHFRQG ,W IXUWKHU
travels 5m distance in the next one second and so on. Here, the car is travelling
equal distance in equal interval of time. Thus, its motion is uniform motion.
Examples of uniform motion are: motion of hands of watch, motion of planets,
motion of the moon, etc.

When a body travels unequal distance in equal interval of time, its motion is
called variable motion or non-uniform motion.

0 sec 1 sec 1 sec 1 sec

a 5m b 10m c 4m d

,Q WKH DERYH ÀJXUH D FDU WUDYHOOHG P LQ WKH ÀUVW RQH VHFRQG P LQ WKH VHFRQG
one second and 4m in the third one second. Thus, it covers different distances
in equal interval of time. Therefore, its motion is variable motion. Examples:
motion of animals, motion of vehicles, motion of water in a river, etc.

Average velocity

When a body moves in a non-uniform motion, its velocity changes continuously.
In such situation, we use average velocity.

Average velocity of a body is the total displacement of the body divided by total
time taken.

Average velocity = Total displacement
Total time taken

15 Times' Crucial Science & Environment Book - 8

Its SI unit is m/s and CGS unit is cm/s.
If a body covers total displacement of 50 m in 10 seconds, its average velocity is
calculated as,

Average velocity = Total displacement = 50 = 5m/s
Total time taken 10

If velocity of a body changes with a constant rate, its average velocity is
calculated by using the formula,

Average velocity = Initial velocity + Final velocity =u + v
2 2

Initial velocity is the velocity of a body at the beginning of the time and is
denoted by u. Final velocity is the velocity at the end of time and is denoted
by v.

If a body is travelling with the velocity of 20m/s in the beginning and its velocity
increases to 30m/s at the end, its average velocity can be calculated as:

Average velocity = Initial velocity + Final velocity = 20 + 30 = 25 m/s
2 2

Acceleration

When a body moves in a non-uniform motion, its velocity keeps on changing.
The rate by which velocity of a body changes is its acceleration. Thus, the rate
of change of velocity of a moving body is called acceleration.

Suppose, a car is moving from point A to B. Its velocity is 5m/s at the beginning.
It increases continuously and becomes 20m/s after 5 seconds.

u=5m/s v=20m/s

0 sec 5 sec

t=5sec

Here, the velocity of the car increases from 5m/s to 20m/s in 5s. The increase
in the velocity of the car in 5s is 15m/s. It shows that its velocity has increased
by 3m/s in every one second. Thus, its acceleration is 3m/s2.

Suppose, a body is moving with initial velocity u and its velocity increases to v
in time t, then the acceleration a is calculated by using the formula,

Acceleration = Final Velocity - Initial Velocity ?a = v – u
Time taken t

Acceleration is a vector quantity.

Unit of acceleration

The change in velocity is measured in meter per second and time is measured
in second. Therefore, unit of acceleration is meter per second per second or m/s2.

Times' Crucial Science & Environment Book - 8 16

Acceleration = Change in Velocity = m/s = m = m
Time taken s s×s s2

If the velocity of a moving body decreases in every second, its acceleration will
be negative. It is called retardation. Hence, the rate of decrease of velocity of a
body is called retardation. It is also a vector quantity and its unit is the same
as that of acceleration, i.e. m/s2.

Solved Numerical Problem 2.2

A bus is moving with a velocity of 20m/s. It increases its velocity to
30m/s after 5 seconds. Find its acceleration.

Given,

Initial velocity (u) = 20m/s

Final velocity (v) = 30m/s

Time (t) = 5s

We have,

a = v–u or, a = 30–20 or, a = 10 = 2m/s2
t 5 5

Therefore, the acceleration of the bus is 2m/s2.

Solved Numerical Problem 2.3

A car starts from rest and gains a velocity of 30m/s after 6s. Find the
acceleration of the car.

Given,

Initial velocity (u) = 0m/s

Final velocity (v) = 30m/s

Time (t) = 6s

We have,

a = v–u or, a = 30–0 or, a = 30 = 5m/s2
t 6 6

Therefore, the acceleration of the car is 5m/s2.

Solved Numerical Problem 2.4

A car moving with the velocity of 40m/s stops at 5 seconds after
applying the brakes. Calculate the acceleration of the car.

Given,

Initial velocity (u) = 40m/s

Final velocity (v) = 0m/s

Time (t) = 5s

17 Times' Crucial Science & Environment Book - 8

We have,

a = v–u
t
0–40
Or, a = 5

Or, a = –40 – 8m/s2
5

Therefore, the acceleration of the car is –8m/s2.
Since, the negative acceleration is called retardation, the retardation of the
car is 8m/s2.

Equations of motion

If a body moves with a uniform acceleration, its initial velocity u ÀQDO YHORFLW\
v, distance travelled s, time taken t and uniform acceleration a are related.
The relations can be shown by equations. These equations are called equations
of motions. They are:

1. v = u + at

2. s = u+v ×t ua v
2 s
t
3. v2 = u2 + 2as

4. s = ut + 1 at2
2

To prove: v = u + at

$FFHOHUDWLRQ LV GHÀQHG DV WKH UDWH RI FKDQJH RI YHORFLW\

Acceleration = Final Velocity - Initial Velocity
Time taken
v – u
a = t Or, at = v – u

? v = u + at

To prove: s = u+v ×t
2
We have,

Average velocity = Initial velocity + Final velocity
2
u + v ....................(i)
=
2

Also,

Average velocity = Total displacement = s ............................(ii)
Total time taken
t

Times' Crucial Science & Environment Book - 8 18

From equation (i) and (ii), we have

s = u+v
t 2

? s= u+v ×t
2

To prove: v2 = u2 + 2as

We know that,

Acceleration = Final Velocity – Initial Velocity
Time taken
v–u v–u ...................(i)
a= t Or, t = a

Again,

Total distance = Average velocity × Time

s= Initial velocity + Final velocity × Time
2
u + v
s= 2 × t .....................(ii)

Substituting the value of ‘t’ from equation (i) in equation (ii), we have

s= u+v × v–u Or, s = v2 – u2 Or, 2as = v2 ï X2
2 a 2a

? v2 = u2 + 2as

To prove: s = ut + 1 at2
2
Final Velocity – Initial Velocity
Acceleration = Time taken

a = v – u Or, at = v – u ? v = u + at ............(i)
t

Again,

Total distance = Average velocity × Time

s = Final Velocity + Initial Velocity × Time
2

s= u+v × t............................(ii)
2
Substituting the value of ‘v’ from equation (i) in equation (ii), we have

s= u + (u + at) ×t = 2u + at ×t = 2ut + at2
2 2 2 2
1
? s = ut + 2 at2

Special conditions
When a body starts from rest, u = 0.
When a moving body comes to rest, v = 0.

19 Times' Crucial Science & Environment Book - 8

Solved Numerical Problem 2.5

,I D ERG\ FRYHUV P GLVSODFHPHQW LQ V ÀQG LWV DYHUDJH YHORFLW\

Given,

Total displacement (d) = 500m

Total time (t) = 10s

Average velocity = ?

We have,

Average velocity = Total displacement = 500
Total time taken 10

Therefore, average velocity of the body is 50m/s.

Solved Numerical Problem 2.6

A body changes its velocity from 45km/hr to 72km/hr in 2s. Find its
acceleration and distance travelled.

Given,

Initial velocity (u)= 45km/hr = 45km = 45 × 1000m = 12.5m/s
1hr 3600s

Final velocity (v) = 72km/hr = 72km = 72 × 1000m = 20m/s
1hr 3600s
Acceleration (a) = ?

Distance (s) = ?

We have,

v = u + at Or, 20 = 12.5 + a × 2 Or, 2a = 7.5 or, a = 3.75m/s2
Again,

s= u+v ×t or, s= 12.5+20 ×2 = 32.5m
2 2

Therefore, the acceleration is 3.75m/s2 and distance travelled is 32.5m.

Solved Numerical Problem 2.7
A bus increases its velocity from 30m/s to 45m/s and covers 300m
distance. Find: a) time required and b) acceleration.
Given,

Initial velocity (u) = 30m/s
Final velocity (v) = 45m/s
Distance travelled (s) = 300m
Time (t) = ?
Acceleration (a) = ?

Times' Crucial Science & Environment Book - 8 20

We have,

s= u+v ×t Or, 300 = 30 + 45 ×t Or, t = 600 Or, t = 8s
Again, 2 2 75

v = u + at or, 45 = 30 + a × 8 or, a = 45 – 30 Or, a = 15
8 8
? a = 1.875 m/s2

Hence, the required time is 8 seconds and the acceleration of the bus is 1.875
m/s2.

Relative motion

The motion of a body with respect to another body when both bodies are in
motion is called relative motion.

a) Relative motion when both bodies are moving in the same direction

6XSSRVH WZR FDUV ɇ$· DQG ɇ%· VWDUW IURP UHVW DQG PRYH WRZDUGV WKH VDPH
GLUHFWLRQ IURP SRLQW ɇ;· ,I WKH YHORFLW\ RI FDU ɇ$· LV P V DQG WKH YHORFLW\
RI FDU ɇ%· LV P V KRZ IDU ZLOO WKH\ EH IURP HDFK RWKHU DIWHU VHFRQG"

AA

B 10m B

15m

,Q VHFRQG WKH FDU ɇ$· PRYHV WKH GLVWDQFH RI P ZKHUHDV FDU ɇ%· PRYHV
the distance of 15m. Thus, they will be 5m apart from each other after 1
second.
Here, the relative displacement is 5m.
,Q VHFRQG WKH FDU ɇ%· WUDYHOV P PRUH WKDQ WKH FDU ɇ$· WKHUHIRUH UHODWLYH
YHORFLW\ RI WKH FDU ɇ%· ZLWK UHVSHFW WR WKH FDU $ LV P V
5HODWLYH YHORFLW\ 9HORFLW\ RI D ERG\ ï9HORFLW\ RI DQRWKHU ERG\
? VBA = VB ï VA
In the example,
Relative velocity of car 'A' with respect to car 'B'

9HORFLW\ RI FDU
$
ï 9HORFLW\ RI FDU
%

ï ï P V
Velocity of car 'B' with respect to car 'A'

21 Times' Crucial Science & Environment Book - 8

9HORFLW\ RI FDU
%
ï 9HORFLW\ RI FDU
$


ï P V

b) Relative motion when both bodies are moving in opposite direction

6XSSRVH FDU
$
DQG FDU
%
VWDUW PRYLQJ IURP D SRLQW
;
DQG PRYH LQ
opposite directions. Car 'A' moves with a velocity of 10m/s and car 'B' moves
with a velocity of 15m/s.

After 1 second, the car 'A' travels the distance of 10m and the car 'B'
travels the distance of 15m. After 1 second, the cars 'A' and 'B' will be
25m(=10m+15m) apart from each other. Thus, relative distance of the
car'A' with respect to the car 'B' is 25m and relative velocity of the car
'A'with respect to the car 'B' is 25m/s.

B BA A

15m x 10m

Relative velocity = Velocity of a body + Velocity of another body
? VAB = VA + VB
In the above example,
Relative velocity of the car 'A' with respect to the car 'B'
= Velocity of the car 'A' + Velocity of the car 'B'
= 10 + 15 = 25 m/s
The relative velocity of a body 'A' with respect to another body 'B' when
both are in motion, is the rate at which the body 'A' changes its position
with respect to the body 'B'.

Solved Numerical Problem 2.8

Two vehicles ‘A’ and ‘B’ are moving towards the same direction with
velocity of 20m/s and 30m/s respectively. Calculate (i) the relative
velocity of ‘A’ with respect to ‘B’. (ii) the relative velocity of ‘B’ with
respect to ‘A’.

Given,

Velocity of vehicle A (VA) = 20m/s
Velocity of vehicle B (VB) = 30m/s
Relative velocity of A with respect to ‘B’ (VAB) = ?
Relative velocity of B with respect to ‘A’ (VBA) = ?

Times' Crucial Science & Environment Book - 8 22

We have,

Since the vehicles are moving to the same direction,

the relative velocity of A with respect to B is given by:

VAB = VA — VB
³ ï P V

Again, the relative velocity of B with respect to A is given by:

VBA = VB — VA
= 30 —20 = 10m/s

Therefore, relative velocity of A with respect to B (VAB LV ï P V DQG UHODWLYH
velocity of B with respect to A (VBA) is 10m/s.

Solved Numerical Problem 2.9

Two vehicles ‘X’ and ‘Y’ are moving with velocity of 20m/s and 30m/s
respectively in opposite directions. If they start from the same place
and same time, calculate the relative distance between them after 1
minute.

Given,

9HORFLW\ RI YHKLFOH ; 9x) = 20m/s
Velocity of vehicle Y (Vy) = 30m/s
Time (t) = 1 minute = 60s

Relative distance after 1 minute = ?

'LVWDQFH WUDYHOOHG E\ WKH YHKLFOH ¶;· LQ PLQXWH 9x × t
Sx = 20 × 60 ? Sx = 1200m

Distance travelled by the vehicle ‘Y’ in 1 minute = Vy × t
Sy = 30 × 60 ? Sy = 1800m

'LVWDQFH EHWZHHQ WKH YHKLFOHV ; DQG < DIWHU PLQXWH

= Sx + Sy (Since, they are moving in opposite direction)
= 1200 + 1800m = 3000m

Hence, the vehicles are at a distance of 3000m after one minute.

reference : a standard with which other are compared
ƌĞůĂƟǀĞ ͗ ĐŽŵƉĂƌĂƟǀĞ
magnitude : value

23 Times' Crucial Science & Environment Book - 8

1. A body is said to be at rest if it does not change its position with respect to its
surrounding.

2. A body is said to be in motion if it changes its position with respect to its surrounding.

3. Rest and motion are relative terms.

4. The quantity which requires both magnitude and direction for its complete
specification is called vector quantity.

5. The quantity which has only magnitude but no direction is called scalar quantity.

6. The actual length of a path travelled by a body is called distance.

7. The shortest distance between the initial and final positions of a body is called
displacement.

8. The rate of change of distance is called speed.

9. The rate of change of displacement is called velocity.

10. The rate of change of velocity is called acceleration.

11. Average velocity of a body is the ratio of total displacement travelled by a body to
the total time taken.

12. The motion of a body with respect to another body when both bodies are in motion
is called relative motion.

Exercise

A. Answer these questions in short.
1. What is rest? Give some examples.
2. What is motion? Name some objects which are in motion state.
3. What is average velocity? How is it calculated?
4. What is relative motion?
5. What is relative velocity? How is relative velocity calculated at
different situations?
6. What is uniform motion? Give some examples.
7. What is acceleration? Mention its unit.
'HÀQH QRQ XQLIRUP PRWLRQ ZLWK H[DPSOHV

B. Differentiate between:
1. Vector and scalar quantities
2. Distance and displacement
3. Speed and velocity

Times' Crucial Science & Environment Book - 8 24

C. Give reasons:
1. Rest and motion are relative terms.
2. A body may have zero velocity even though its speed is 10m/s.
3. Speed is a scalar quantity but velocity is a vector quantity.

D. Prove:

i. v = u + at ii. s = u+v ×t
2
1
iii. s = ut + 2 at2 iv. v2 = u2 + 2as

E. Numerical Problems:
1. A bus increases its velocity from 20m/s to 30m/s in 2 seconds. Find
its acceleration.
2. A motorcycle starts from rest and gains a velocity of 40m/s in 8 seconds.
Find (i) acceleration and (ii) distance travelled by the motorcycle.
3. Two buses A and B are moving in opposite directions with velocities
36km/hr and 108 km/hr respectively. Find the relative velocity of A with
respect to B and the total displacement between them after 2 minutes.
4. Two buses 'A' and 'B' are moving in the same direction with the
velocities 30m/s and 40m/s respectively. Find the relative velocity
of the bus 'A' with respect to the bus 'B'. Calculate the relative
displacement between them after 4 minutes.
,I D ERG\ WUDYHOV NP LQ KRXU ÀQG WKH DYHUDJH YHORFLW\ RI WKH
body in m/s.

Answers 2. i. 5m/s2, ii. 160m 3. 144 km/hr, 4800m
5. 5m/s
E. 1. 5m/s2
ï P V P

Take two toy cars A and B and place them on a smooth and long table. Tie
both the cars with separate pieces of threads and place them facing opposite
directions such that both cars touch each other by their back ends. Pull the
cars in opposite directions with the same speed. Record the time with the help
of stop watch. Now, let the cars be undisturbed and measure the distance
between them. Then calculate the relative motion.

25 Times' Crucial Science & Environment Book - 8

3CHAPTER Simple
Machine

Archimedes
+H LV NQRZQ IRU $UFKLPHGHV· SULQFLSOH $UFKLPHGHV· VFUHZ
+\GURVWDWLFV /HYHUV ,QÀQLWHVLPDOV 1HXVHLV FRQVWUXFWLRQV

Estimated Periods : 6
Objectives: At the end of the chapter, the students will be able to:

LQWURGXFH VLPSOH PDFKLQH DQG H[SODLQ LWV W\SHV
H[SODLQ WKH SULQFLSOH RI OHYHU
FODVVLI\ OHYHU DQG LQWURGXFH 0$ 95 DQG HIILFLHQF\ LQYROYHG LQ LWV ZRUNLQJ
VROYH VLPSOH QXPHULFDO SUREOHPV LQYROYLQJ 0$ 95 DQG HIILFLHQF\

Why do we use different devices at our home?
Which one is easier either cutting nails with a knife or cutting them with a nail
cutter? Why?
What are simple machines? Discuss.

Introduction

We use a number of devices in our daily life. These devices are useful to us
in many ways. For instance, we use a nail-cutter to trim our nails, we use a
EURRP WR VZHHS GXVW DQG GLUW IURP WKH ÁRRU ZH XVH VFLVVRUV WR FXW FORWKHV DQG
paper, etc. These devices are simple in structure but they make our work easier
and faster. Due to their simple structure, they are called simple machines.
Thus, the devices which are simple in structure and make our work easier
and faster are called simple machines. 1DLO FXWWHU EURRP VFLVVRUV ÀUH WRQJV
beam balance, crowbar, wheelbarrow, etc. are some simple machines.

Broom Spanner Fire tongs Scissors

There are some other machines which are complex in structure. They are
formed by the combination of large number of simple machines. Such machines

Times' Crucial Science & Environment Book - 8 26

are called complex machines. For example, sewing machine, motor engine,
water mill, wind-mill, etc.

Advantages of simple machines
The simple machines are useful to us in the following ways:
1. Simple machines multiply force, i.e. more load can be lifted by applying

less 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 applied force.

Some terms used in simple machines

Mechanical Advantage (MA)

0HFKDQLFDO DGYDQWDJH RI D PDFKLQH LV GHÀQHG DV
the ratio of load overcome by the machine to the
effort applied.
Mathematically, E
L

Mechanical advantage = Load
Effort
L F
? MA = E

Mechanical advantage has no unit because it is the ratio of similar physical
quantities (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.

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.

Numerical example: ,Q WKH ÀJXUH D VWRQH RI E=300N

load 900N is lifted by the effort of 300N using a
lever. The load distance is 25 cm and the effort
distance is 100 cm. In such case, the lever can L=o9a02d05Nc(Lm) 100cm
magnify the effort and makes it easy to lift load.
We have to calculate the mechanical advantage of the lever to know by how
many times the effort has been multiplied.

Here, Mechanical advantage (MA) = Load = 900N = 3
Effort 300N
The mechanical advantage of the machine is 3. It shows that the effort can lift
three times heavier load.

27 Times' Crucial Science & Environment Book - 8

The mechanical advantage is affected by friction. As the friction increases, the
mechanical advantage decreases.

Velocity Ratio (VR)

7KH YHORFLW\ UDWLR RI D PDFKLQH LV GHÀQHG DV WKH UDWLR RI GLVWDQFH WUDYHOOHG E\
the effort applied in a machine to the distance travelled by the load.

Mathematically,

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

Velocity ratio has no unit as it is the ratio of similar physical quantities (two
distances).

The velocity ratio of a machine shows the distance up to which an effort is
applied to lift a load up to a particular height. If the velocity ratio of a machine
is 2, it means that the effort has to move twice more distance than the load
distance to lift a load up to a particular height.

Numerical example: ,Q WKH JLYHQ ÀJXUH WKH ORDG GLVWDQFH LV FP DQG WKH
effort distance is 100 cm.

The velocity ratio is calculated as:

Velocity ratio = Effort distance = 100 cm =4
Load distance 25 cm

The Velocity ratio of a simple machine is not 100cm
affected by friction. As the mechanical advantage
is affected by friction but the velocity ratio is not, 25cm

the mechanical advantage of a machine is always less than velocity ratio.

(IÀFLHQF\ Lj

When an effort is applied in a machine, some work is done. The work done in
the machine is called input work. On the other hand, the machine performs
some work due to the input work. The work performed by the machine is
called output work.

The ratio of output work to the input work in a machine expressed in percentage
LV FDOOHG HIÀFLHQF\ Mathematically,

(IÀFLHQF\ Output work × 100%
Input work

,I WKH HIÀFLHQF\ RI D VLPSOH PDFKLQH LV LW PHDQV WKDW RI RXU HQHUJ\
(input work) is converted into useful work (output work). Rest 25% of our
energy is wasted to overcome frictional 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

Times' Crucial Science & Environment Book - 8 28

than the input work because no machine can be made frictionless. Due to this
UHDVRQ D VLPSOH PDFKLQH FDQQRW KDYH RU PRUH HIÀFLHQF\

Numerical example: ,Q WKH DERYH H[DPSOH WKH HIÀFLHQF\ RI WKH PDFKLQH LV
calculated as follows:

(IÀFLHQF\ Mechanical advantage × 100%
Velocity ratio

= 3 x 100% = 75%.
4

5HODWLRQ EHWZHHQ 0$ 95 DQG Lj

)URP WKH GHÀQLWLRQ RI HIÀFLHQF\ ZH KDYH

Lj Output work × 100%
Input work

Here, Output work = Load × Load distance

Input work = Effort × Effort distance

1RZ Lj L × Ld × 100%
E × Ed

It can be rearranged as

Lj L /E × 100% MA = L ,
Ed/Ld E
Ed
? Lj MA × 100% VR = Ld
VR

Thus, HIÀFLHQF\ RI D PDFKLQH PD\ DOVR EH GHÀQHG DV WKH UDWLR RI PHFKDQLFDO
advantage to the velocity ratio expressed in percentage. Due to friction, the
value of mechanical advantage is always less than the value of velocity ratio.
+HQFH WKH HIÀFLHQF\ RI D PDFKLQH LV QHYHU RU PRUH

Principle of simple machine

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

The force applied in a machine is called effort or input force whereas the force
which is to be overcome by the effort is called load or output force.

All simple machines work on a basic principle which can be stated as:

In a balanced condition, 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

E × Ed = L × Ld

It is the basic principle of simple machines.

29 Times' Crucial Science & Environment Book - 8

Types of simple machines

7KH VLPSOH PDFKLQHV FDQ EH FODVVLÀHG LQWR VL[ W\SHV RQ WKH EDVLV RI WKHLU
structure and function. They are:
1. Lever 2. Pulley 3. Wheel and axle
4. Inclined plane 5. Screw 6. Wedge

$ OHYHU LV D ORQJ ULJLG EDU WKDW LV FDSDEOH RI URWDWLQJ DERXW D À[HG SRLQW The
À[HG SRLQW DERXW ZKLFK D OHYHU URWDWHV LV FDOOHG IXOFUXP $ OHYHU FRQVLVWV RI
three main parts - load, effort and fulcrum. The 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 (object).

1. Lever

The advantages of the lever depend upon the load distance and effort distance.
In a lever, the distance between the fulcrum and the load is called load distance
(Ld) or load arm (La) whereas the distance between the fulcrum and the effort
is called effort distance (Ed) or effort arm (Ea).

7KH OHYHU FDQ EH FODVVLÀHG LQWR WKUHH W\SHV RQ WKH EDVLV RI SRVLWLRQ RI ORDG
effort and fulcrum. They are:

a. First class lever

The lever in which the fulcrum lies at any point in the middle of load and
HIIRUW LV FDOOHG ÀUVW FODVV OHYHU Crowbar, scissors, see-saw, beam balance,
GKLNL SOLHUV FXWWLQJ VKHDUV HWF DUH VRPH H[DPSOHV RI ÀUVW FODVV OHYHU

E

LE L E L
F Crowbar F F
See-saw
Scissors

7KH ÀUVW FODVV OHYHU VKRZV DOO WKH DGYDQWDJHV RI OHYHU L H LW FKDQJHV WKH
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

The lever in which the load lies at any point in the middle of effort and
fulcrum is called second class lever. In the second class lever, the effort

Times' Crucial Science & Environment Book - 8 30

distance is always greater than the load distance. Hence, the second
class lever multiplies effort more than any other class of lever. But it
cannot change the direction of applied force. It cannot accelerate the
work because the load distance is always less than the effort distance.

LE FL
EE

FL

F

tŚĞĞůͲďĂƌƌŽǁ KŶŝŽŶ ĐƵƩĞƌ ŽƩůĞ ŽƉĞŶĞƌ

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

c. Third class lever
The lever in which effort lies in the middle of load and 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.

FE L

EE

FL

LF

Broom Fishing rod Fire tongs

)LUH WRQJV VWDSOHU EURRP VKRYHO ÀVKLQJ URG VSRRQ HWF DUH WKH H[DPSOHV
of third class lever.

In a lever,

MA = L , VR = Ed Lj MA ð RU Lj Output work × 100%
E Ld VR Input work

In balanced condition: E × Ed = L × Ld

2. Pulley

A pulley is a hard metallic or wooden disc with a grooved rim. A pulley moves
DURXQG D À[HG KRUL]RQWDO EDU FDOOHG D[OH The axle is supported by a frame. A
rope moves around the groove of the disc. The load is tied to one end of the
rope and it is pulled from another end of the rope by applying effort.

3XOOH\ FDQ EH FODVVLÀHG LQWR WZR W\SHV VLQJOH À[HG SXOOH\ DQG VLQJOH PRYDEOH
pulley.

31 Times' Crucial Science & Environment Book - 8

D 6LQJOH À[HG SXOOH\ Frame
$ SXOOH\ LQ ZKLFK WKH IUDPH LV À[HG WR D Wooden disc
rigid support and the disc rotates along
ZLWK WKH URSH LV FDOOHG VLQJOH À[HG SXOOH\ Rope
The mechanical advantage of single
À[HG SXOOH\ LV L H WKHUH LV QR JDLQ LQ Load
mechanical advantage. But we use such īŽƌƚ
pulley because it helps to change the
direction of applied force and makes work ^ŝŶŐůĞ ĮdžĞĚ ƉƵůůĞLJ
convenient. Such pulley is used in well,
ÁDJ VWDQG HWF

Think and Solve

$OWKRXJK D VLQJOH À[HG SXOO\ GRHV QRW PXOWLSO\ HIIRUW LW LV LQ FRPPRQ XVH
Why?

b. Single movable pulley īŽƌƚ

A pulley which moves up and down along Rope
with the load is called single movable
pulley. In such pulley, one end of the Wooden disc
rope is tied to a rigid support and the Frame
load is pulled from the other end of the
rope. The circular disc moves along with
load.

The single movable pulley does not Load
change the diretion of applied force but ^ŝŶŐůĞ ŵŽǀĂďůĞ ƉƵůůĞLJ
it helps to multiply the force. In such
pulley, the effort distance moves twice
more than the load distance.

c. Combined or compound pulley &ŝdžĞĚ ƉƵůůĞLJ
Rope
A pulley which consists of Frame
combination of two or more DŽǀĂďůĞ ƉƵůůĞLJ
pulleys is called combined pulley. īŽƌƚ

A combined pulley is also called
compound pulley or block and
tackle system.

Load
ŽŵďŝŶĞĚ ƉƵůůĞLJ

Times' Crucial Science & Environment Book - 8 32

In a pulley,

MA = Load
Effort
VR = No. of pulleys used (Except in single movable pulley)

VR = No. of sections of rope that support load (in a single movable pulley)

Lj MA × 100%
VR

:KHHO DQG $[OH

A wheel and axle is a simple machine which consists of two wheels (cylinders)
of different diameters arranged co-axialy. 7KH ODUJHU F\OLQGHU ZKHHO LV À[HG
rigidly with the small wheel (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.

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

In a wheel and axle, MA = L
E
&LUFXPIHUHQFH RI ZKHHO ›5
VR = &LUFXPIHUHQFH RI D[OH ›U

? VR = R , (Where R is the radius of wheel and r is the radius of axle.)
r
MA
Lj VR × 100%

Wheel

Axle

Wheel

Axle

String roller Screw driver Steering of car

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

In an inclined plane, MA = L , VR = Length of inclined plane (l) Lj MA × 100%
E Height of the plane (h) VR

33 Times' Crucial Science & Environment Book - 8

5. Screw

A screw is a simple machine which seems to have an inclined
plane wrapped around a cylindrical surface. The spiral ridge
of the screw is called thread and the distance between two
threads of a screw is called pitch. A screw having less pitch
multiplies the applied force to a greater extent. Jack screw,
screw nail, driller, etc are the examples of screw. A jack
screw is used to lift vehicles while changing their wheels.

In a screw, MA = L , VR = ›5 Lj MA × 100%, p = Pitch of the screw
E p VR

6. Wedge īŽƌƚ load
t
A wedge is a simple machine which has a sharp
SDUW DW RQH HQG DQG ÁDW SDUW DW WKH RWKHU The l
HIIRUW LV JHQHUDOO\ DSSOLHG XSRQ WKH ÁDW SDUW DQG load
the sharp part performs the work. A wedge is used
to split the wooden logs. Axe, knife, khukuri, nail,
chisel, needle, etc are the examples of wedge.

In an wedge, MA = L , VR = Length (l) Lj MA × 100%
E Thickness (t) VR

Solved Numerical Problem 3.1

A load of 3600N is lifted by a crowbar by placing the load at the
distance of 20cm from the fulcrum. Calculate the effort required to be
applied at a distance of 90cm from the fulcrum.

Given, E=?
Load (L) = 3600N

Load distance (Ld) = 20cm

Effort distance (Ed) = 90cm

Effort (E) = ? L=o3a6d0(0LN) 90cm
Now, we have
20cm
E × Ed = L×
Ld

or, E = L × Ld Or, E = 3600N × 20cm ? E = 800 N
Ed 90cm

Therefore, effort required is 800N.

Times' Crucial Science & Environment Book - 8 34

Solved Numerical Problem 3.2

,Q D ÀUVW FODVV OHYHU D ORDG RI 1 LV OLIWHG E\ DQ HIIRUW RI 1 ,I
the load distance and effort distance are 10cm and 40cm respectively,
FDOFXODWH 0$ 95 DQG Lj

Given, E=500N
Load (L) = 1500N

Load distance (Ld) = 10cm

Effort (E) = 500N

Effort distance (Ed) = 40cm L=o1a5d0(0LN) 40cm
10cm
Now, we have:

MA = Load = 1500N = 3
Again, Effort 500N

VR = Effort distance = 40cm = 4
Similarly, Load distance 10cm

Lj MA × 100% = 3 × 100% = 75%
VR 4

7KXV 0$ 95 DQG Lj RI WKH OHYHU DUH DQG UHVSHFWLYHO\

Solved Numerical Problem 3.3
&DOFXODWH WKH HIIRUW UHTXLUHG WR OLIW D ORDG RI 1 E\ D VLQJOH À[HG
SXOOH\ $OVR FDOFXODWH LWV HIÀFLHQF\
Given,

Load (L) = 750N.
,Q D VLQJOH À[HG SXOOH\ 0$ L H LW GRHV QRW PXOWLSO\ IRUFH
Now, we have

MA = Load or, 1 = 750N ? E = 750N
Effort E

Hence, the required effort is 750N.
Again,
95 RI D VLQJOH À[HG SXOOH\ LV EHFDXVH HIIRUW GLVWDQFH DQG ORDG GLVWDQFH DUH
equal in it. Hence,

Lj MA × 100% = 1 × 100% = 100%
VR 1

Therefore, if the frictional force can be neglected, the pulley will have 100%
HIÀFLHQF\

35 Times' Crucial Science & Environment Book - 8

Solved Numerical Problem 3.4

A load of 1200N is lifted by 4 pulley system using an effort of 400N.
&DOFXODWH 0$ 95 DQG Lj RI WKH SXOOH\ V\VWHP

Given,

Load (L) =1200N

Effort (E) = 400N

No. of pulleys = 4

Then,

0$ " 95 " Lj "

Now, we have

MA = L = 1200N = 3
E 400N

Again, VR = No. of Pulleys used = 4

Similarly,

Lj MA × 100% = 3 × 100% = 75%
VR 4

+HQFH 0$ RI WKH SXOOH\ V\VWHP LV 95 LV DQG WKH HIÀFLHQF\ LV

Solved Numerical Problem 3.5

&DOFXODWH D LQSXW ZRUN E RXWSXW ZRUN F 0$ G 95 DQG H Lj IURP
the given diagram.

Given, 250N
Load (L) = 1500N

Effort (E) = 250N length=30m height=3m
Length of slope (l) = 30m
Height of slope (h) = 3m. 1500N
E.d = l = 30m
L.d = h = 3m

Now,

a. According to the formula,

Input work = Effort × Effort distance

= 250N × 30m = 7500 Joule

The input work, i.e. work done by effort is 7500J.

b. Output work = Load × Load distance
= 1500N × 3m = 4500 Joule

Times' Crucial Science & Environment Book - 8 36

The output work, i.e. work done by load is 4500J.

c. M. A = L = 1500N =6
E 250N

d. V. R = l = 30m = 10
h 3m

H Lj MA × 100% = 6 × 100% = 60%
VR 10

Thus, the input work is 7500J, output work is 4500J, MA is 6, VR is 10 and the
HIÀFLHQF\ LV LQ WKH JLYHQ PDFKLQH

wheel : a circular disc or ring
dhiki ͗ Ă ƚƌĂĚŝƟŽŶĂů ƌŝĐĞ͕ ŵŝůůĞƚ ďĞĂƚĞƌ ƵƐĞĚ ŝŶ ǀŝůůĂŐĞƐ
chisel ͗ Ă ƚŽŽů ǁŝƚŚ Ă ƐŚĂƌƉ ĐƵƫŶŐ ĞĚŐĞ

1. The devices which are simple in structure and make our work easier and faster are
called simple machines.

2. The ratio of load overcome by the machine to the effort applied is called mechanical
advantage.

3. The velocity ratio of a machine is defined as the ratio of distance travelled by
effort applied in a machine to the distance travelled by load.

5. There are six types of simple machines. They are: lever, pulley, wheel and axle,
inclined plane, screw and wedge.

6. A lever is a long rigid bar which is capable of moving about a fixed point, called fulcrum.
7. A pulley is a round metallic or wooden disc having a grooved rim along which a rope

passes.
8. Basically, there are two types of pulleys: single fixed pulley and single movable pulley.
9. Wheel and axle is a simple machine which is made of two co-axial cylinders of

different diameters.
10. Inclined plane is a simple machine because it makes our work easier and faster.

Exercise

A. Answer these questions in very short.
1. What is a crowbar ?
2. What is a ramp ?
:KDW LV WKH PHFKDQLFDO DGYDQWDJH RI D VLQJOH À[HG SXOOH\"

37 Times' Crucial Science & Environment Book - 8

4. What is the velocity ratio of a single movable pulley ?
5. Give the formula to calculate VR in a wheel and axle.
6. Give any two examples of third class lever.
7. Name a simple machine that changes the direction of applied force

but does not multiply it.

% 'HÀQH LL 95 LLL Lj
L 0$ v. Wedge vi. Fulcrum
iv. Screw

C. Give reasons:
1. A tight nut can be opened easily by using a long spanner.
2. Mechanical advantage does not have unit.
3. Velocity ratio does not have unit.
4. Velocity ratio is not affected by friction.
5. Mechanical advantage is always less than velocity ratio.
6. The roads on hills are made winding.
7. Handles of metal-cutting shears are made long.
(IÀFLHQF\ RI D VLPSOH PDFKLQH LV QHYHU RU PRUH
9. It is easier to overcome load when the load is shifted towards the
wheel in a wheelbarrow.
10. A third class lever cannot multiply force.

D. Classify the following levers:

1. Broom 2. Beam balance 3. Fire tongs
6. Crowbar
4. Nut-cracker 5. Wheelbarrow
9. Nail-cutter
7. Bottle opener 8. Scissors

E. Classify the following simple machines:

1. Screw driver 2. Jack screw 3. Winding road on hill

4. Axe 5. See-saw

F. Answer these questions.
1. What are simple machines ? What are their advantages?
7KH HIÀFLHQF\ RI D VLPSOH PDFKLQH LV :KDW GRHV LW PHDQ"
'HULYH WKH UHODWLRQ EHWZHHQ 0$ 95 DQG Lj LQ D VLPSOH PDFKLQH
4. What is a pulley? Explain the structure of a single movable pulley.
5. What is wheel and axle ? Explain its structure with the help of a

Times' Crucial Science & Environment Book - 8 38

diagram.
:ULWH DQ\ ÀYH VLPSOH PDFKLQHV XVHG LQ \RXU GDLO\ OLIH DQG FODVVLI\

them.
7. What is screw? Give examples.

G. Solve the following numerical problems.
1. A load of 800N is lifted by an effort of 200N. If the load is placed at a
distance of 10cm from the fulcrum, what will be the effort distance?
2. Deepta and Abhinav having weights 450N and 350N are playing
see-saw. If Deepta sits at a distance of 2m from the fulcrum, how
far should Abhinav be seated in order to balance Deepta?
3. Calculate the effort, MA and VR from the given diagram:

E=?

L=o3a6d0(0LN) 90cm
20cm

4. In a three pulley system, a load of 900N is lifted by an effort of
1 &DOFXODWH 0$ 95 DQG HIÀFLHQF\ RI WKH SXOOH\ V\VWHP

5. In a nut-cracker, a nut is placed at a distance of 3cm and effort is
applied at a distance of 10cm from the fulcrum. If the effort is 300N,
calculate the force experienced by the nut.

Answers 2. 2.57m 3. 800N MA = 4.5, VR = 4.5
5. 1000N
G. 1. 40cm
4. MA = 1.5, VR = 3 and
Lj

Use a crowbar to lift a heavy rock (having weight 1000N or greater). Change
the load arm by changing the position of fulcrum in each case and expperience
the magnitude of effort needed to lift the rock. Explain your observations and
ÀQGLQJV ZLWK UHDVRQV

39 Times' Crucial Science & Environment Book - 8

4CHAPTER Pressure

Evangelista Torricelli

(YDQJHOLVWD 7RUULFHOOL 2FWREHU ² 2FWREHU
ZDV DQ ,WDOLDQ SK\VLFLVW DQG PDWKHPDWLFLDQ EHVW

NQRZQ IRU KLV LQYHQWLRQ RI WKH EDURPHWHU

Estimated Periods : 6
Objectives: At the end of the chapter, the students will be able to:

GHILQH SUHVVXUH
GHILQH DWPRVSKHULF SUHVVXUH
H[SODLQ WKH HIIHFWV DQG LPSRUWDQFH RI DWPRVSKHULF SUHVVXUH
GHILQH OLTXLG SUHVVXUH DQG H[SODLQ WKH IDFWRUV DIIHFWLQJ OLTXLG SUHVVXUH
GHULYH WKH IRUPXOD IRU OLTXLG SUHVVXUH
VROYH WKH QXPHULFDOV UHODWHG WR WKH OLTXLG SUHVVXUH

Why is it easier to cut vegetables with a sharp knife than with a blunt one?
What is atmospheric pressure?
Why is a bucket filled faster at the lower stair than at the upper stair when the
tap is opened? Discuss.

It is easier to cut vegetables with a sharp knife than with a blunt one. Nails
are made pointed. Why?

Here, in both cases, force is applied. The effectiveness of the applied force
depends upon the amount of force and the area upon which the force is applied.
This relation can be expressed by the term 'pressure'.

3UHVVXUH LV GHÀQHG DV WKH IRUFH DFWLQJ SHU XQLW DUHD

Pressure (P) = Force (F) ? P = F
Area (A) A
Since force is measured in Newton and area in m2, the unit of pressure is N/m2
or Pascal.

One Pascal Pressure: When one Newton force is exerted on the area of one
m2, the resultant pressure is one pascal pressure.

Pascal is surname of a scientist named Blaise Pascal. He has contributed a lot
LQ WKH ÀHOG RI SUHVVXUH 6R 3DVFDO LV XVHG DV XQLW RI SUHVVXUH WR JLYH KRQRXU WR
him. In short Pascal is written as Pa.

Other units of pressure are mm of Hg, atmospheric pressure, bar, millibar, etc.

Times' Crucial Science & Environment Book - 8 40

Factors affecting pressure

Pressure depends upon two factors. They are:

(i) The amount of force applied and

(ii) The area upon which the force is applied.

Pressure increases when the amount of applied force increases. It decreases
when the amount of force applied decreases. On the other hand, pressure
increases when the area upon which the force applied decreases. It decreases
when the area increases.

Atmospheric pressure Increase in
atm. pressure
The earth is surrounded by the air. The
layer of the air that surrounds the earth is
called atmosphere. The atmosphere consists
of mixture of different gases. It contains 78%
Nitrogen, 21% Oxygen, and 1% other gases.
The air has weight. Due to its weight, it
exerts pressure. The pressure exerted by the
atmosphere is called atmospheric pressure.

The air is not equally dense in all parts of the
atmosphere. At the lower altitude, the denser
air is accumulated due to gravity of the
earth. As we move upwards, the air becomes sĂƌŝĂƟŽŶ ŝŶ ĂƚŵŽƐƉŚĞƌŝĐ ƉƌĞƐƐƵƌĞ Ăƚ
thinner. Thus, the atmospheric pressure is ĚŝīĞƌĞŶƚ ĂůƟƚƵĚĞƐ ĨƌŽŵ ƚŚĞ ĞĂƌƚŚ͛Ɛ ƐƵƌĨĂĐĞ͘
more at the lower altitude than at higher altitude.

At the lower altitude, the whole air remaining above this altitude exerts force
downward which can create more pressure. But at the higher altitude, less
air remaining above this layer exerts force downwards which can create less
SUHVVXUH ,W LV FOHDU IURP WKH ÀJXUH

The atmospheric pressure at sea level is 105 N/m2. It decreases gradually as
we go upwards. The atmospere is very thin at the top of Mt. Everest.

Why do we not feel atmospheric pressure ?
The atmosphere exerts pressure not only on the surface of the earth but also
on any object on the earth. The atmosphere exerts pressure of 105 Pascal at
the sea level. It means atmosphere exerts force of 105 N on 1m2 area.

Thus, there is an enormous pressure on the surface of human body. But we
don’t feel it because there is blood pressure inside our body. The blood pressure
inside our body is nearly equal to atmospheric pressure. Due to the balance of
blood pressure and atmospheric pressure, we don’t feel huge force applied by

41 Times' Crucial Science & Environment Book - 8

the atmosphere. Furthermore, our body weight also exerts pressure against the
atmospheric pressure and we don’t feel the downward pressure of the atmosphere.
But, we feel uneasy at very low or high pressure.

When we go to higher altitude, nose bleeding occurs, why?

At higher altitude, there is less atmospheric pressure. But, our blood pressure
remains constant. Thus, our blood pressure is much more than atmospheric
pressure at higher altitude. Due to this, the blood vessels feel higher pressure
of the blood. This results in rupturing of the blood vessels and nose bleeding.

ĐƟǀŝƚLJ ϰ͘ϭ To show that air exerts pressure

Materials required:
Tin can, burner, tripod stand, etc
Procedure:
1. Take a tin can with little water.
2. Put the tin can on a tripod stand and heat

the water by putting a burner under the
tin can.
3. Let the water be boiled, then close the lid
of the tin.
4. Now put out the burner.
5. Pour cold water on the can. What happens ?
Observations:
The can gets crushed.
Explanation:
At the beginning, there was equal atmospheric pressure inside and outside the
tin and it was balanced. When the water is boiled, the gas and vapour formed
in the tin comes out from the lid. When the lid is suddenly closed and the tin
is cooled by pouring cold water, the vapour condenses and a partial vacuum is
created inside the can. The atmospheric pressure outside the tin can exceeds
the pressure inside it. Thus, the can gets collapsed inwards.
Conclusion:
The atmosphere exerts pressure on an object from all sides.

ĐƟǀŝƚLJ ϰ͘Ϯ To show that air has pressure.

Materials required:
A glass, a cardboard, water, etc
Procedure :
7DNH D JODVV DQG ÀOO ZLWK ZDWHU

Times' Crucial Science & Environment Book - 8 42

2. Cover the mouth of the glass with a cardboard so that no space is left for
the air.

3. Invert the glass by one hand pressing the cardboard against the glass
with another palm.

4. Remove the palm slowly from the cardboard. What do you observe?

Observation:
The cardboard does not fall.
Explanation:
The atmospheric pressure exerts upward pressure on the cardboard from
down. It can support the weight of the water which is acting downward. Thus,
the water does not fall.
Conclusion:
The atmosphere has pressure.

Think and Solve

If you try to immerse an inverted empty glass in water, it does not get
immersed Why?

Differences between atmospheric pressure and air pressure

Air pressure Atmospheric pressure

1. It is the pressure of air enclosed in a 1. It is the pressure exerted by
container. atmosphere.

2. It is measured by a pressure gauge. 2. It is measured by a barometer.

Pressure measuring devices
Manometer, barometer and pressure gauge are some of the pressure measuring
devices.

Manometer
Manometer is a device that measures air pressure. It consists of a U-shaped
tube containing liquid. Its both arms are open. The level of liquid in both arms
is equal due to the equal atmospheric pressure on the liquids of both arms.

43 Times' Crucial Science & Environment Book - 8

Generally, it is used to measure pressure exerted by lungs. To measure the
pressure exerted by lungs, we have to put one end
of the tube in our mouth. The level of the liquid in
this arm decreases due to the pressure exerted Out

by lungs. But, the level in another arm increases.
The difference in the level of liquid in two arms is
the measurement of pressure exerted by lungs.

Barometer

It is an instrument used for measuring
atmospheric pressure. Barometers are of two
types:

i. Mercury barometer
ii. Aneroid barometer

Mercury barometer

Mercury is used in a mercury barometer. A simple mercury barometer is
constructed in the following ways:

About one meter long glass tube closed at one Torricellian
HQG VKRXOG EH WDNHQ DQG ÀOOHG ZLWK PHUFXU\ vacuum
The air bubbles should be removed if present
inside the tube. By closing the open end of Pressure due to
the tube with thumb, it should be inverted Atmosphere
and kept in a trough containing mercury.

The thumb should be removed carefully from Pressure due to the
the open end when the open end is inside DĞƌĐƵƌLJ ĐŽůƵŵŶ
the mercury, so that no air bubbles enter the
tube. Trough

Then the level of mercury slowly falls and DĞƌĐƵƌLJ
UHPDLQV À[HG DIWHU VRPH WLPH 7KH PHUFXU\
in the glass tube is supported by the ĂƌŽŵĞƚĞƌ
atmospheric pressure exerted on the mercury
of the trough.

The level of the mercury in the glass tube is the measurement of atmospheric
pressure. When the atmospheric pressure is more, the level of mercury is also
more. The level decreases when the atmospheric pressure decreases. At sea
level, the height of mercury column in the tube is 760mm or 76cm. Thus, the
atmospheric pressure at sea level is 760mm of Hg (Mercury). As we move
towards higher altitude, the level of mercury in the tube decreases. This means
the atmospheric pressure decreases when altitude increases.

Times' Crucial Science & Environment Book - 8 44

A vacuum is created above the mercury level of the tube. This vacuum is called
Torricellian Vacuum.
Water cannot be used instead of mercury in a barometer because density of
water is very less. The atmospheric pressure can hold a liquid column of height
10m. Hence, the height of the barometer tube should be about 11 metres, which
is very inconvenient to handle.
In aeroplanes, barometer without mercury or any liquid is used. It is called
aneroid barometer.

Pressure gauge

7KH WXEHV RI W\UHV VKRXOG EH ÀOOHG ZLWK DLU XSWR RSWLPXP
OHYHO ,I PRUH DLU LV ÀOOHG WKH\ PD\ JHW EXUVW ,I OHVV DLU
LV ÀOOHG WKH W\UH LV QRW SURSHUO\ LQÁDWHG DQG WKH YHKLFOH
does not move smoothly.

Hence, it is necessary to measure the air pressure in WƌĞƐƐƵƌĞ ŐĂƵŐĞ
tubes of wheels of balls, etc.

Pressure gauge is the instrument used for measuring the pressure of air
contained in the tube of vehicles, ball, etc.

Uses of atmospheric pressure

a. To suck water or cold drinks with a pipe.

7DNH D JODVV DQG ÀOO LW ZLWK ZDWHU 6XFN WKH
water with the help of a straw. When you suck
the straw, the air is moved towards the mouth
and vacuum is created in the straw. Then
the water which is pressed by atmospheric
SUHVVXUH PRYHV WRZDUGV WKH PRXWK WR ÀOO WKH
vacuum.

6DPH SULQFLSOH LV XVHG WR ÀOO LQN LQ WKH SHQ
and medicine in syringe.

b. We measure altitude with the help of atmospheric pressure.
Altitude and atmospheric pressure are related. When altitude increases,
the atmospheric pressure decreases. By knowing the atmospheric
pressure of a particular place, its altitude can be calculated.

c. We use atmospheric pressure for the weather forecasting.
Meteorologists use atmospheric pressure for weather forecasting.
Gradual fall in the atmospheric pressure indicates the rainfall. Gradual
rise in the atmospheric pressure indicates a dry weather.

45 Times' Crucial Science & Environment Book - 8

d. Living things use atmospheric pressure to balance blood pressure.

Atmospheric pressure balances blood pressure in our body. In the absence
RI VXIÀFLHQW DWPRVSKHULF SUHVVXUH QRVH EOHHGLQJ RU DOWLWXGH VLFNQHVV
occurs at higher altitude.

e. Water is pumped up from lift pump with the help of atmospheric
pressure.

Liquid pressure

Liquid has weight. Due to its weight, it exerts pressure. It is called liquid pressure.

Pressure exerted by a liquid due to its weight is called liquid pressure.

Calculation of liquid pressure

Take a cylindrical vessel of base area A. Fill it with a liquid having density d
up to level h. The pressure given by the liquid at the bottom is given by:

Pressure = Force = Weight of the liquid
Area Area

= m× g ( Weight = mass × acceleration due to gravity) 500
A 400
300
= d× V× g ( d= m/V or m = d × V) 200
A
h
d× A× h × g
= A ( V = A × h)

? P = dgh 100

A

From the formula, it is clear that liquid pressure depends upon three factors:
a. Depth of the liquid from its free surface (h)
b. Density of the liquid (d)
c. Acceleration due to gravity (g)

Solved Numerical Problem 4.1

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(Density of water = 1000kg/m3)

Given,

Density of water (d) = 1000kg/m3.

Depth of water (h) = 3m.

Acceleration due to gravity (g) = 9.8m/s2

Liquid pressure (P) = ? 3m

We have,

P = dgh Or, P = 1000 × 9.8 × 3 Or, P = 29400 Pascal

Therefore, the pressure exerted is 29400 Pascal.

Times' Crucial Science & Environment Book - 8 46