Science 7

Based on New Curriculum

Approved by Curriculum Development Centre (CDC) Sanothimi, Bhaktapur,
Nepal as a reference material for schools.

Times’ Crucial

and ENVIRONMENT

7Grade

Authors/Editors:
Kamal Prasad Sapkota
Rajan Kumar Shrestha

Editors:

Surandra Ojha Santosh Pokhrel Dipak Kumar Dangi Sagar Dahal
Mani Ram Rai
Govinda Paudel Hemanta Dhungana Kamal Kanta Dhungel

Times’ Crucial

and ENVIRONMENT

7Grade

Published by: Times International Publication Pvt. Ltd.

Authors/Editors: Kamal Prasad Sapkota
Rajan Kumar Shrestha

Edition: First: 2070 B.S.
Layout: Revised: 2074 B.S.
Revised: 2078 B.S.

Ramesh Maharjan

© Copyright: Publisher

Printed in Nepal

Preface

Human life is always in progress. Newer technologies and equipments have
been developed or discovered in every second. Lengthy and time consuming works
of past are now done within few moments. People can travel far off distances within
short time. Several fatal diseases have been eradicated. These achievements, what
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’ Science, Environment, Health and Physical Education is a series of text
books for the school level students of grades LKG to ten. The series has been prepared
for the young learners emphasizing on student-centred teaching techniques, learning
EKTENG DCUGF CEVKXKVKGU RTCEVKECDNG CEVKXKVKGU UEKGPVKſE CRRTQCEJGU CPF KPPQXCVKXG
learning techniques. The text of each lesson is preceded by a warming up activity
that encourages students to take part actively in the learning process. The series
includes the teaching techniques and methods for the teachers in the title ‘Note to the
Teacher’. It is based on the latest syllabus prescribed by the Government of Nepal,
Ministry of Education, Curriculum Development Center. Hence, this series is believed
to act as the foundation of science for the curious and inquisitive young minds.

Each book of this series covers the syllabus of Science, Environment, Health
CPF 2J[UKECN 'FWECVKQP 6JG ſTUV RCTV EQPVCKPU EJCRVGTU QH 5EKGPEG CPF 'PXKTQPOGPV
the second part contains the chapters of Health Education and the last part contains
chapters of Physical Education. Chapters of science and health section are provided
with wide variety of exercises which help to encourage the learning and sharpen the
memory of the students. 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. But
physical education has not been provided with exercise because it is not necessary
to include this section in examinations.

I feel delighted to extend our sincere gratitude to Mr. Kamal Prasad Sapkota for
his painstaking contribution in editing the book and making it more simpler and crucial.
I am 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, I am thankful to Mr. Santosh Pokharel for his effort to make this book
error-free. I am 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 my pleasure to receive constructive suggestions and
recommendations from the teacher, students and well wishers for the further
improvement of the books.

- Authors

Contents Page No.

SN Contents 1
Physics 14
31
1 Measurement 41
2 &ŽƌĐĞ ĂŶĚ DŽƟŽŶ 48
3 Simple Machine 58
4 Pressure 72
5 Work, Energy and Power 83
6 Heat 92
7 Light 100
8 Sound
9 Magnet 109
10 Electricity 131
143
Chemistry 150
11 DĂƩĞƌ 160
12 Mixture
13 ^ŽůƵƟŽŶ 166
14 Metals and Non-metals 183
15 Chemicals in Daily Life 194
207
Biology 220
16 ůĂƐƐŝĮĐĂƟŽŶ ŽĨ ŶŝŵĂůƐ ĂŶĚ WůĂŶƚƐ
17 Parts of Plants 223
18 Cell and Tissue 231
19 Life Process 242
20 Life Cycle of Frog
253
Geology and Astronomy 266
21 Structure of The Earth 284
22 Weather and Climate 291
23 The Earth and Space 292
293
Environment Science 296
24 Environment and its Balance
25 ŶǀŝƌŽŶŵĞŶƚĂů ĞŐƌĂĚĂƟŽŶ ĂŶĚ ŝƚƐ ŽŶƐĞƌǀĂƟŽŶ
26 Environment and Sustainable Development

^ƉĞĐŝĮĐĂƟŽŶ 'ƌŝĚ
Terminal Break-down of the Courses
Model Test Papers
WƌĂĐƟĐĂů džĂŵŝŶĂƟŽŶ

Chapter

1 0HDVXUHPHQW

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϳ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ŵĞĂƐƵƌĞŵĞŶƚ ĂŶĚ ƵŶŝƚ͘
 ĚĞĮŶĞ ^/ ƵŶŝƚ ĂŶĚ ƚĞůů ƉŚLJƐŝĐĂů ƋƵĂŶƟƟĞƐ ĂŶĚ ƚŚĞŝƌ ^/ ƵŶŝƚƐ͘
 ĮŶĚ ĂƌĞĂ ŽĨ ƌĞŐƵůĂƌ ĂŶĚ ŝƌƌĞŐƵůĂƌ ŽďũĞĐƚƐ͘
 ĮŶĚ ǀŽůƵŵĞ ŽĨ ƌĞŐƵůĂƌ ĂŶĚ ŝƌƌĞŐƵůĂƌ ƐŽůŝĚ ŽďũĞĐƚƐ͘
 ĮŶĚ ƚŚĞ ǀŽůƵŵĞ ŽĨ ůŝƋƵŝĚ͘
 solve numerical problems related to area and volume.

MIND OPENERS
 What is measurement and unit?
 ƌĞ ƚŚĞ ƵŶŝƚƐ ŽĨ ǀĂƌŝŽƵƐ ƋƵĂŶƟƟĞƐ ƐĂŵĞ ŝŶ Ăůů ƉůĂĐĞƐ ŽĨ ƚŚĞ ǁŽƌůĚ͍
 ,Žǁ ĚŽ LJŽƵ ĮŶĚ ƚŚĞ ǀŽůƵŵĞ ŽĨ Ă ĐƵďŝĐĂů ďŽĚLJ ĂŶĚ ŝƌƌĞŐƵůĂƌ ďŽĚLJ͍
 ,Žǁ ĐĂŶ LJŽƵ ŵĞĂƐƵƌĞ ǀŽůƵŵĞ ŽĨ Ă ůŝƋƵŝĚ͍
 Can you say formula for volume of a cuboid object and sphere?

Introduction

Measurement is one of the important activities of our daily life. It is
necessary in every step of life. We measure amount of water, milk and
sugar to prepare tea in the morning. We measure quantity of rice to be
cooked. When we go to the shop, the shopkeeper measures the amount of
the things to be sold. When we go to the tailor for sewing clothes, he/she
takes measurement of different parts of our body. We measure time by
using a watch. Thus, measurement is
an important part of our daily life.
We need a standard quantity
for measurement. When we ask
shopkeeper for 1 kg of sugar, he keeps
one kilogram standard quantity on
one pan and sugar on another pan. He
adds or removes sugar from the pan

Times' Crucial Science and Environment 1 Book 7

until the balance is maintained. When length of cloth is to be measured,
meter scale is used. Here, standard one kilogram and meter are units.
Thus, PHDVXUHPHQW FDQ EH GHÀQHG DV D SURFHVV RI ÀQGLQJ YDOXH RI XQNQRZQ
TXDQWLW\ E\ FRPSDULQJ LW ZLWK D VWDQGDUG TXDQWLW\ RU XQLW. When we
perform measurement, we compare unknown quantity with standard or
known quantity. When we say the length of a piece of cloth is 5 meter,
it means the length of the piece of cloth is 5 times of one meter. If you
don’t know the value of one meter, you don’t understand the meaning of 5
meter. Therefore, value of one meter should be known. The quantity whose
value is known is called standard quantity or unit. Unit is the basis for
measurement.
8QLW FDQ EH GHÀQHG DV D VWDQGDUG RU NQRZQ TXDQWLW\ LQ WKH IRUP RI ZKLFK
RWKHU SK\VLFDO TXDQWLWLHV DUH PHDVXUHG
The units of length are meter or foot or centimeter, etc. The units of mass
are NLORJUDP or GKDUQL, or VKHU, etc.

Physical quantities

The quantities which can be measured are called physical quantities.
Length, mass, time, volume, speed, temperature, etc are physical quantities.
There are two types of physical quantities. They are:
a) Fundamental quantities
7KRVH TXDQWLWLHV ZKLFK DUH LQGHSHQGHQW RI RWKHU TXDQWLWLHV DUH FDOOHG
IXQGDPHQWDO TXDQWLWLHV 7KH\ GR QRW GHSHQG XSRQ RWKHU TXDQWLWLHV 7KH\
DUH DOVR NQRZQ DV EDVLF TXDQWLWLHV
Generally, length, mass and time are considered as fundamental quantities.
b) Derived quantities
Those quantities which are derived or obtained from fundamental quantities
are called derived quantities. For example, speed, volume, acceleration,
force, density, pressure, etc. Speed is a derived quantity because speed is
obtained from formula.

6SHHG '7LVLWPDQH FH

Distance means length. Thus, speed is derived from two fundamental
quantities- length and time.
In the same way, volume is obtained from formula:

Volume = length × breadth × height

Times' Crucial Science and Environment 2 Book 7

Length, breadth or height is measured as length.

? 9ROXPH OHQJWK 3

It means volume is obtained from the repetition of ’length’, the fundamental
quantity. Hence, volume is a derived quantity.

Systems of Units

There are various systems of units in different parts of the world. We use
one type of system of unit in our country, whereas people of other countries
use other system of units. There are mainly four systems of units. They are:

a) MKS system b) CGS system

c) FPS system d) SI system

MKS system
The system of units in which length is measured in meter, mass in kilogram
and time in second is called MKS system.

CGS system
The system of units in which length is measured in centimeter, mass in
gram and time in second is called CGS system.

FPS system
The system of units in which length is measured in foot, mass in pound
and time in second is called FPS system.

SI system
It is the short form of 6\VWHP ,QWHUQDWLRQDO GH 8QLWV. It is the extended
form of MKS System. Thus, WKH V\VWHP RI XQLWV ZKLFK ZDV DGRSWHG E\ WKH
,QWHUQDWLRQDO &RQIHUHQFH RI VFLHQWLVWV UHJDUGLQJ :HLJKWV DQG 0HDVXUHV LQ
LV NQRZQ DV 6, 6\VWHP. In SI system, there are seven fundamental
quantities. The SI units of the seven fundamental quantities are as follows:

&ƵŶĚĂŵĞŶƚĂů ƋƵĂŶƟƟĞƐ SI units Symbol
Length Meter m
Mass Kilogram Kg
Time Second s
Kelvin K
Temperature ŵƉĞƌĞ
Current Candela cd
Mole mol
Luminuous intensity
ŵŽƵŶƚ ŽĨ ƐƵďƐƚĂŶĐĞ

Times' Crucial Science and Environment 3 Book 7

Length

The shortest distance between any two points is called length. The SI
unit of length is metre (m). Different instruments can be used to measure
length. We use different units to measure length of different objects. Some
of the units such as millimeter (mm), centimetre (cm) and decimetre (dm)
are smaller than the unit metre (m). Such units are called sub-multiples
of metre (m). Similarly, the units namely decametre (dam), hectometre
(hm) and kilometer (km) are larger than metre (m). Such units are called
multiples of metre (m).

Measuring tape Measuring tape Measuring scale

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)

Mass

0DVV RI D ERG\ LV GHÀQHG DV WKH WRWDO TXDQWLW\
of matter contained in it. It 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
Beambalance

kilogram. The units like quintal, tonne, etc. are bigger than kilogram and
are called multiples of kilogram.

Times' Crucial Science and Environment 4 Book 7

Sub-multiples of kilogram Multiples of kilogram

1kg = 10,00,000 milligram (mg) 100kg = 1quintal
1kg = 1,00,000 centigram (cg) 1000kg = 1 tonne
1kg = 1000 gram (g)
1kg = 100 decagram (dag)
1kg = 10 hectogram (hg)

Time Clock

The interval between any two events is called
time. The time is measured with the help of
watches and clocks. The basic unit of time is
second and the other units are minute, hour,
day, week, etc.
60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
7 days = 1 week
52 weeks = 1 year
365 days = 1 year

Regular and Irregular Objects

The objects which have equal length, width and height in all parts are
called regular objects. Examples of regular objects are: instrument box,
duster, book, cube, football, volleyball, cylinder, etc. We can use certain
IRUPXOD IRU ÀQGLQJ DUHD DQG YROXPH RI UHJXODU REMHFWV
The objects that have no equal length, width or height in all parts are called
irregular objects. Stones, broken pieces of bricks, broken glass pieces, ink
pot, mango fruit, etc are some examples of irregular objects. There is no
SDUWLFXODU IRUPXOD IRU ÀQGLQJ DUHD RU YROXPH RI LUUHJXODU REMHFWV

Area of a Regular Object

7KH VSDFH RFFXSLHG E\ WKH VXUIDFH RI DQ REMHFW LV FDOOHG WKH DUHD RI WKH REMHFW
,WV 6 , XQLW LV VTXDUH PHWHU P2
We can use the formula, $UHD OHQJWK ð EUHDGWK for rectangular surface.

Times' Crucial Science and Environment 5 Book 7

Duster, book, blackboard, instrument box, eraser, etc have rectangular breadth
surface.
If a rectangular surface has a length of 4m
and the breadth 3m, its area can be
calculated as:

$UHD OHQJWK ð EUHDGWK

= 4m × 3m length
U FP
= 12m2
We can use

A = Sr2

IRUPXOD IRU ÀQGLQJ WKH DUHD RI FLUFXODU VXUIDFH

Where S = 22 and r = radius of the circle.
7
Area of the circle given in the picture is
calculated as:
A = Sr2

= 22 × 72
7

= 154 cm2.

Thus, area of the given circle is 154 cm2.

Area of Irregular Object

:H QHHG JUDSK SDSHU WR ÀQG WKH DUHD
of irregular object. For measuring the
area, the irregular object is placed
on the graph paper. Then the outline
of the object is drawn on the graph
paper with the help of a pencil. The
object is removed and the number of
squares occupied by the object in the
graph are counted.

If the square is incomplete, the sum
of the incomplete squares is divided
by 2 and added to the total complete
squares.
Sum of incomplete squares
Area =^No. of complete squares+ 2 } x area of unit square in graph paper

Times' Crucial Science and Environment 6 Book 7

Activity 1.1 7R ¿QG DUHD RI D UHFWDQJXODU REMHFW

Materials required

A blackboard and a measuring tape
Procedure

1. Measure length and breadth of the blackboard of your class
room.

8VH WKH IRUPXOD $ OHQJWK ð EUHDGWK WR ÀQG WKH DUHD RI WKH
blackboard.

Activity 1.2 7R ¿QG DUHD RI DQ LUUHJXODU REMHFW

Materials required
A peepal leaf and a graph paper

Procedure
1. Take a peepal leaf and keep it on a graph paper.
2. Draw an outline of the leaf.
3. Count the number of complete squares and incomplete
squares separately and calculate the area of the leaf by using
formula,

Area = {No. of complete squares + Sum of incomplete squares }× area of unit square in graph paper
2

Volume

7RWDO VSDFH RFFXSLHG E\ DQ REMHFW LV FDOOHG LWV JGKIJV
YROXPH. The SI unit of volume is cubic meter (m3).
It is measured in cubic centimeter FP3) in CGS NGPIVJ DTGCFVJ
system. Volume of cuboid object is calculated by
using formula. T

8 OHQJWK î EUHDGWK î KHLJKW

:H FDOFXODWH YROXPH RI D VSKHUH E\ XVLQJ IRUPXOD

9 QU YJGTG ŎUŏ KU TCFKWU QH VJG URJGTG

([DPSOH ,I WKH UDGLXV RI D VSKHUH LV FP WKH YROXPH RI
WKH VSKHUH LV FDOFXODWHG E\ XVLQJ WKH IRUPXOD DV IROORZV

Times' Crucial Science and Environment 7 Book 7

9 QU
ª ª
ª ª ª ª

FP

Thus, volume of the given sphere is 1437.3 cm3.

Volume of Liquid

We can measure the volume of a liquid by using measuring cylinder,
burette, pipette, etc.

Activity 1.3 7R ¿QG WKH YROXPH RI D OLTXLG

Materials required

Measuring cylinder and water ϱϬϬ

Procedure ϰϬϬ
Take a measuring cylinder and pour some water into it. ϯϬϬ
Observation ϮϬϬ
ϭϬϬ

The water level is not seen uniform on the surface. The surface of the
water in the middle is depressed whereas the surface of the water
at the side is raised upward. Such curved surface is called FRQFDYH
PHQLVFXV.

7KH UHDGLQJ LV WDNHQ E\ À[LQJ WKH H\HV VWUDLJKW WR WKH ORZHU PHQLVFXV
of water.

Precautions

You have to keep
your eyes at the
ϱϬϬ ϱϬϬ

level of water. If you ϰϬϬ ϰϬϬ

keep your eyes lower ϯϬϬ ϯϬϬ
than the water level,
9CVGT ϮϬϬ /GTEWT[ ϮϬϬ

the volume of water ϭϬϬ ϭϬϬ

seems less. If you
keep your eyes above the water level, the volume of the water seems to be
more.

Times' Crucial Science and Environment 8 Book 7

Liquids like mercury do not wet the vessel. They form FRQYH[ PHQLVFXV in
the vessel. Their middle surface is raised upward whereas the surface at
the side is depressed.
To measure the volume of such liquids, the level of eyes should be straight
to the upper surface of meniscus.

Activity 1.4 To measure the volume of a cuboid object

Materials required

A brick and measuring tape

Procedure
1. Take a brick and measure its length, breadth and height.
8VH WKH IRUPXOD YROXPH OHQJWK ð EUHDGWK ð KHLJKW WR ÀQG
the volume of the brick.

If length = 4cm, breadth = 3cm and height = 2cm, then the
9ROXPH OHQJWK ð EUHDGWK ð KHLJKW

= 4cm × 3cm × 2cm
= 24cm3

Volume of irregular object

:H FDQ ÀQG WKH YROXPH RI LUUHJXODU REMHFWV E\ XVLQJ PHDVXULQJ F\OLQGHU
For this, we have to put water in a measuring cylinder up to a certain level.
Then we should note the volume of water. Again, we should immerse an
irregular object and note down the water level after immersing the object.
The difference in the volume of water before and after immersing the
irregular object is the volume of the irregular object.

Activity 1.5 7R ¿QG WKH YROXPH RI DQ LUUHJXODU REMHFW

Material required

A stone, a measuring cylinder and a thread
Procedure

1. Take a stone and a measuring cylinder.
2. Put water in the measuring cylinder up to a certain level and

note the volume of water.
3. Tie the stone with a thread and dip it slowly inside the water.

Times' Crucial Science and Environment 9 Book 7

4. Note down the volume of water after immersing the stone
inside the water.

5. Calculate the volume of the stone
by using formula:
Volume of stone = volume of water
after immersing the
stone —volume of the water before
immersing the stone

Volume of a gas

9ROXPH RI D JDV LV QRW À[HG ,W FDQ EH FRPSUHVVHG E\ DSSO\LQJ IRUFH 0RUH
amount of air can be compressed inside a small tyre. Therefore, volume of
gas depends upon the amount of pressure exerted.

Activity 1.6 7R ¿QG WKH YROXPH RI JDV

Materials required

A measuring cylinder, water trough,
bee-hive shelf and a balloon
Procedure

7DNH D ZDWHU WURXJK DQG ÀOO LW ZLWK
water.

2. Keep a beehive shelf inside the water
of the trough.

3. Fill a measuring cylinder completely with water and close its
mouth with a lid and invert it over the beehive shelf. The
mouth should be completely inside the water.

4. Then, remove the lid carefully.

7DNH DQ LQÁDWHG EDOORRQ DQG RSHQ LWV PRXWK LQ WKH EHHKLYH
shelf.

Observation

The air moves slowly towards the measuring cylinder and remains
at the top of the cylinder.

After sometime, the whole air of the balloon goes inside the measuring
cylinder and occupies some place of the water at the upper level.
Keep your eyes at the level of water and note the volume of the
cylinder occupied by the air. It is the volume of the air.

Times' Crucial Science and Environment 10 Book 7

Dharni ͗ ƵŶŝƚ ŽĨ ŵĂƐƐ ƵƐĞĚ ŝŶ EĞƉĂů͘ /ƚ ŝƐ ĞƋƵĂů ƚŽ Ϯ͘ϱ ŬŐ͘
Sher ͗ ƵŶŝƚ ŽĨ ǀŽůƵŵĞ ƵƐĞĚ ƚƌĂĚŝƟŽŶĂůůLJ ŝŶ ǀŝůůĂŐĞ ŽĨ EĞƉĂů͘ /ƚ ŝƐ ĂůŵŽƐƚ

Meniscus ĞƋƵĂů ƚŽ ŽŶĞ ůŝƚƌĞ͘
͗ ƐŚĂƉĞ ŽĨ ůŝƋƵŝĚ ƐƵƌĨĂĐĞ

Main Points to Remember

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

2. Unit is a standard or known quantity in the form of which other
physical quantities are measured.

3. Those quantities which can be measured are called physical
quantities.

4. Those quantities which are independent of other quantities are
called fundamental quantities.

5. Those quantities which are derived from fundamental quantities
are called derived quantities.

6. SI system is the extended form of MKS system.
:H FDQ XVH FHUWDLQ IRUPXODH IRU ÀQGLQJ DUHD DQG YROXPH RI UHJXODU

objects.
:H XVH JUDSK SDSHU IRU ÀQGLQJ DUHD RI LUUHJXODU REMHFWV
:H XVH PHDVXULQJ F\OLQGHU IRU ÀQGLQJ YROXPH RI OLTXLGV
10. We can measure volume of irregular objects and air by using a

measuring cylinder.

Exercise

1. Choose the best alternative in each case.

a. What instrument is used to measure the volume of liquid in a science lab?

i. Beaker ii. Balance iii. Scale iv. Measuring cylinder

b. What is the SI unit to measure the quantity of matter?

i. Candela ii. Ampere iii. Mole iv. Kelvin

c. How many litres are there in a cubic metre (m3) of a liquid?

i. 1000 L ii. 10,000 L iii. 10 L iv. 100 L

Times' Crucial Science and Environment 11 Book 7

d. Which liquid forms convex meniscus?

i. Mercury ii. Water iii. Alcohol iv. All of these

e. What formula is used to calculate the area of a circle?

L $ O [ E LL $ ʌU2 iii. V = l x b x h iv. A = V/h

2. Copy the correct statements and correct the false statements if any.
D 0HDVXUHPHQW LV WKH SURFHVV RI ÀQGLQJ YDOXH RI DQ XQNQRZQ
quantity by comparing it with a standard or known quantity.
b. The SI unit of length is meter.
c. Total space occupied by an object is its area.
G :H FDQ ÀQG WKH YROXPH RI D FXERLG REMHFW E\ XVLQJ IRUPXOD
Volume = length × breadth × height.
H :H QHHG JUDSK SDSHU IRU ÀQGLQJ WKH DUHD RI LUUHJXODU REMHFWV
f. We measure volume of liquids by using measuring cylinder.

3. Match the following:

Mass Volume of liquid

Current Standard quantity

Measuring cylinder Kilogram

Volume Ampere

Unit m3

Fundamental quantity

4. Answer these questions in short:

a. What is measurement?

b. What is unit? Give examples.

c. What is physical quantity? Give some examples.

d. What are various systems of unit?

e. What is fundamental quantity? Give some examples.

I :KDW LV DUHD" :KDW IRUPXOD GR \RX XVH WR ÀQG DUHD RI D
rectangular surface?

J :KDW LV YROXPH" +RZ GR \RX ÀQG YROXPH RI D FXERLG REMHFW"

h. What is S.I. unit? Mention S.I. units of area, volume, mass,
length and current.

L 'HÀQH FRQFDYH DQG FRQYH[ PHQLVFXV

Times' Crucial Science and Environment 12 Book 7

j. What precautions should we take while measuring the volume
of liquid having concave meniscus?

k. What is time? What is S.I. unit of time?

l. What is length? What is S.I. unit of length?

m. What is mass? What is S.I. unit of mass?

5. Answer in detail.

a. Why is measurement important?

b. Why are area and speed derived quantities?

c. How do you measure the area of a peepal leaf? Explain.

G +RZ GR \RX PHDVXUH WKH YROXPH RI D VWRQH" ([SODLQ ZLWK ÀJXUH

e. How do you measure the volume of milk?

6. Numerical problems:

a. A brick has length of 8 cm and breadth of 6 m. Find its area.

$QV FP2

b. What is the area of a rectangle whose length is 14 cm and
breadth is 8cm? $QV FP2

c. A circle has a radius of 8 cm. Find its area. $QV FP2

d. A cuboid body has length of 8 cm, breadth of 6 cm and height of
4 cm. Find its volume. $QV FP3

e. A sphere has a radius of 8 cm. Find its colume. $QV FP3

Project Work

a. Take a stone. Find its volume with the help of a measuring
cylinder.

b. Measure the length and breadth of a wall of your classroom and
ÀQG LWV DUHD

Times' Crucial Science and Environment 13 Book 7

Chapter

2 )RUFH DQG 0RWLRQ

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ĨŽƌĐĞ͘
 tell the types of force.
 ĚĞĮŶĞ ƐĐĂůĂƌ ĂŶĚ ǀĞĐƚŽƌ ƋƵĂŶƟƚLJ ǁŝƚŚ ĞdžĂŵƉůĞƐ͘
 ĚĞĮŶĞ ĚŝƐƚĂŶĐĞ ĂŶĚ ĚŝƐƉůĂĐĞŵĞŶƚ͘
 ĚĞĮŶĞ ƐƉĞĞĚ ĂŶĚ ǀĞůŽĐŝƚLJ ĂŶĚ ĚŝīĞƌĞŶƟĂƚĞ ďĞƚǁĞĞŶ ƚŚĞŵ͘
 ĚĞĮŶĞ ĂĐĐĞůĞƌĂƟŽŶ ǁŝƚŚ ŝƚƐ ƵŶŝƚ͘
 solve simple numerical problems.

MIND OPENERS
 What happens when you kick a ball?
 tŚĂƚ ƚLJƉĞ ŽĨ ĨŽƌĐĞ ĐŽŵĞƐ ŝŶƚŽ ƵƐĞ ǁŚĞŶ Ă ŵĂŐŶĞƚ ĂƩƌĂĐƚƐ

ŵĂŐŶĞƟĐ ƐƵďƐƚĂŶĐĞ͍
 ƌĞ ĚŝƐƚĂŶĐĞ ĂŶĚ ĚŝƐƉůĂĐĞŵĞŶƚ ƐĂŵĞ ƋƵĂŶƟƟĞƐ͍ džƉůĂŝŶ͘

Force

We do various types of works in our daily life. We push carts, kick balls,
lift bags, lift water from well, etc. We open door by pushing its handle;
similarly we close door by pulling its handle. In all the above cases, we
apply force. Thus, force is applied by pulling or pushing. When we apply
force to an object, it changes its state of rest or motion. When we apply
force to an object at rest, it comes to motion. When we apply force to a
moving object in opposite direction to the motion, it comes to rest.
7KXV IRUFH FDQ EH GHÀQHG DV DQ H[WHUQDO DJHQW ZKLFK FKDQJHV RU WHQGV WR
FKDQJH VWDWH RI UHVW RU PRWLRQ RI DQ REMHFW )RUFH LV HLWKHU D SXOO RU D SXVK
,WV 6, XQLW LV 1HZWRQ 1 DQG LWV &*6 XQLW LV '\QH

Types of Force

There are eight types of forces. They are:
i) Pushing force

Times' Crucial Science and Environment 14 Book 7

ii) Pulling force
iii) Muscular force
iv) Magnetic force
v) Gravitational force
vi) Centripetal and centrifugal force
vii) Frictional force
viii) Electrostatic force

Pushing Force

We push a door to open it. We push a cart to carry a load on it. We kick a
football while playing football game. We push load to shift it. In all of the
above cases, we apply force by pushing.

3XVKLQJ IRUFH FDQ EH GHÀQHG DV D IRUFH WKDW SXVKHV RU WULHV WR SXVK DQ REMHFW

Pulling Force

When we draw water from well, we pull a bucket of water. To close a door,
we pull the door. Bullocks and buffaloes pull carts. In these cases, forces
are applied by pulling. Therefore, they are pulling forces.

3XOOLQJ IRUFH FDQ EH GHÀQHG DV D IRUFH WKDW SXOOV RU WULHV WR SXOO DQ REMHFW

Times' Crucial Science and Environment 15 Book 7

Muscular Force

When we lift a stone from the ground, we apply force. Buffaloes, Oxen,
etc apply force to pull carts. Elephants apply force to uproot the plants. A
weight lifter applies force to lift a heavy load.
Here, force applied by human being and other animals is due to the force
exerted by their muscles. Such force is called PXVFXODU IRUFH.
0XVFXODU IRUFH FDQ EH GHÀQHG DV WKH IRUFH H[HUWHG E\ WKH PXVFOH RI KXPDQ
being and other animals.

+XPDQ EHLQJ DQG RWKHU DQLPDOV XVH PXVFXODU IRUFH WR GR YDULRXV DFWLYLWLHV

Magnetic Force

When you bring a magnet near some iron pins, they
are attracted to the magnet. This is due to the force
applied by the magnet. 7KH IRUFH ZKLFK LV H[HUWHG E\
D PDJQHW LV FDOOHG PDJQHWLF IRUFH.
Magnet can attract the metals like iron, cobalt,
nickel, etc. They are called magnetic substances.

Activity 2.1 7R ¿QG DUHD RI DQ LUUHJXODU REMHFW

Materials required

A magnet and iron clips or pins

Procedure 6 1
1. Take a magnet and some iron clips
or pins and then spread the clips
or the pins on a table.

2. Bring the magnet near the clips. What happens?

Times' Crucial Science and Environment 16 Book 7

Observation
The clips or pins get attracted to the poles of the magnet due to
LQÁXHQFH RI WKH PDJQHWLF IRUFH

Gravitational Force

When a stone is thrown upwards, it falls to the ground. The earth moves
DURXQG WKH VXQ LQ D À[HG RUELW ,Q WKH VDPH ZD\ WKH PRRQ UHYROYHV
DURXQG WKH HDUWK LQ D À[HG RUELW 7KHVH DOO DUH GXH WR JUDYLWDWLRQDO IRUFH
Gravitational force exists between any two objects due to their masses.

*UDYLWDWLRQDO IRUFH LV WKH IRUFH RI DWWUDFWLRQ EHWZHHQ DQ\ WZR REMHFWV GXH WR
WKHLU PDVVHV

Centripetal and Centrifugal Force

When an object moves in a circular motion, it Centrifugal Force
experiences two types of forces. One force pulls the
object towards the centre whereas the another force
SXOOV WKH REMHFW DZD\ IURP WKH FHQWUH 7KH ÀUVW IRUFH
is centripetal force and the second force is centrifugal
force. These two forces make the object move in a
circular path. When centrifugal force is more, the
object moves away. When centripetal force is more
then the object is pulled to the centre.

When you turn your cycle in a circular path, you bend your body towards
centre. It is due to centripetal force. When a bus turns in a curved path, the
passengers of the bus are pushed away due to centrifugal force.

When an object moves in a circular path, the force that acts to the object
towards the centre is called centripetal force and the force that acts away
from the centre is called centrifugal force.

Times' Crucial Science and Environment 17 Book 7

Activity 2.2 To show centripetal and centrifugal force

Materials required

A rubber ball and one metre long rope
Procedure

1. Take a rubber ball and tie it with
a rope of about one meter length.

2. Hold the rope by the palm tightly
and rotate it in a circular motion
DV VKRZQ LQ WKH ÀJXUH :KDW GR
you experience?

Observation
You can experience that the ball rotates in a circular path due to
the force applied by you. It is centripetal force. You can experience
another force too, which tries to pull the ball away from the centre.
This force is called centrifugal force.

Frictional Force

When a ball rolls over a ground, it stops after sometime. Why? It is due
to frictional force. When one object slides or moves over another object,
frictional force is produced between the surface of these objects. The
frictional force opposes the motion of the objects.
7KH IRUFH ZKLFK RSSRVHV WKH PRWLRQ RI WKH REMHFW ZKHQ LW VOLGHV RU PRYHV RYHU
DQRWKHU REMHFW LV FDOOHG IULFWLRQDO IRUFH
The magnitude of friction depends upon the roughness and smoothness of
the surface of objects. The magnitude of friction is less when the surface is
smooth. The magnitude of the friction is more when the surface is rough.

How is friction produced?
The surface of an object is not completely
smooth. There are some projections and
depressions on the surface. When one object
slides over another object, projections of one
VXUIDFH ÀW LQWR WKH GHSUHVVLRQV RI DQRWKHU
surface and they interlock. Interlocking resists
the motion. In this way friction is created.
Rough surface can create more friction because the projections and
depressions are bigger than in smooth surface.

Times' Crucial Science and Environment 18 Book 7

Activity 2.3 To show production of frictional force depends
upon roughness of the surface

Materials required: a ball

Procedure:

7DNH D EDOO DQG UROO RQ D FHPHQWHG ÁRRU
2) Roll the same ball on the rough ground.
3) What difference do you observe in these two cases? Why?

Observation: 7KH EDOO UROOV IDVWHU RQ WKH FHPHQWHG ÁRRU WKDQ RQ

WKH URXJK JURXQG 0RUHRYHU LW VWRSV VORZHU RQ WKH FHPHQWHG ÁRRU
than on the rough ground.

Conclusion: More friction is produced on the rough surface than

on the smooth surface.

Effects of friction
Frictional force has following effects:
1) Friction opposes motion

A rolling ball stops after some time. It is due to friction. Friction
opposes motion of a moving object and causes the object to come to
rest state.

2) Friction produces heat
:KHQ \RX UXE \RXU SDOPV IRU VRPH WLPH \RX ZLOO ÀQG WKH SDOPV
warmer. Why does it happen? In this activity, heat is produced
due to friction between palms. In ancient period, ancestors used to
SURGXFH ÀUH E\ UXEELQJ WZR VWRQHV )URP DERYH VWDWHPHQWV ZH FDQ
say that friction produces heat.

3) Friction causes wears and tears.
When we walk on the road, sole of our shoes becomes thinner and
get torn after some time. This is due to friction between sole of our
shoes and road. Similarly, tyres of vehicles tear after some time
when vehicles run on the road. This is also due to friction between
tyres and road. From above facts, we can say that heat causes wears
and tears.

Times' Crucial Science and Environment 19 Book 7

Electrostatic Force

When you bring a plastic comb near small pieces of paper, it can not attract
them. But when you rub the comb in your hair for several times and bring
it near to the pieces of paper, it can attract them. This is due to electrostatic
charge produced in the comb by the rubbing of the comb in the dry hair.

(OHFWURVWDWLF IRUFH LV WKH IRUFH GHYHORSHG GXH WR SURGXFWLRQ RI FKDUJH
(OHFWULFDOO\ FKDUJHG ERG\ FDQ H[HUW HOHFWURVWDWLF IRUFH

Activity 2.4 To produce charge in a plastic comb

Materials required
A piece of a paper and a plastic comb

Procedure
1. Take a paper and
tear it into small
pieces.
2. Bring a plastic
comb near the
paper pieces.

:KDW GR \RX ÀQG"

Rub the plastic comb in your dry hair for several times. Now bring
the comb near the paper pieces. What happens?
Observation
,Q WKH ÀUVW FDVH WKH SDSHU SLHFHV GR QRW JHW DWWUDFWHG WR WKH FRPE
because the plastic comb is not charged. In the second case, the
paper pieces get attracted to the comb. This is due to production of
charge in the comb.

Vector and Scalar Quantities

There are some quantities such as force, displacement, velocity, acceleration,
etc which have both magnitude and direction. Such quantities are called
vectors. When somebody says, “I applied 50 Newton force to a book”, there
may be the question “to which direction?” The answer may be “towards
the east or north, etc”. Here 50 Newton is magnitude and east or north is
direction. Thus, WKH TXDQWLWLHV WKDW KDYH ERWK PDJQLWXGH DQG GLUHFWLRQ DUH
FDOOHG YHFWRU TXDQWLWLHV

Times' Crucial Science and Environment 20 Book 7

The quantities like mass, time, distance, volume, temperature, area, etc
have only magnitude but no direction. They are scalar quantities. When
somebody says “My mass is 50 kg”. We never ask the question “to which
direction?”. Thus, mass has only magnitude but no direction.

7KH TXDQWLWLHV WKDW KDYH RQO\ PDJQLWXGH EXW QR GLUHFWLRQ DUH FDOOHG VFDODU
TXDQWLWLHV

'L൵HUHQFHV EHWZHHQ 9HFWRU DQG 6FDODU TXDQWLWLHV

Vector Scalar

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

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

3. They are represented by an 3. They are not represented by an
arrowhead. arrowhead.

4. Examples: Displacement, force, 4. Examples: Distance, mass,

velocity, acceleration, etc. volume, density, etc.

Distance and Displacement

When a person has to move from point A to B, he/she can go along the
SDWKV RU DV VKRZQ LQ WKH ÀJXUH
When he travels through paths 1, 2, 3 and 4, he has to travel 20m, 30m,
40m and 50 meter respectively.




# $


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

Times' Crucial Science and Environment 21 Book 7

Here, the actual length of paths 1, 2, 3 and 4 are 20m, 30m, 40m and 50m
respectively. Therefore, the distance of path 1, 2, 3 and 4 are 20m, 30m,
40m and 50m respectively.

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

LQLWLDO DQG ÀQDO SRVLWLRQV RI DQ
object is called displacement. It
FDQ DOVR EH GHÀQHG DV WKH VKRUWHVW
distance between any two points 5m 3m

in a particular direction. Hence,
it is a vector quantity.

The SI unit of displacement is A 4m B
meter (m).

Suppose a person moves from point A to B then to C. The distance between
A and B is 4m and that between B and C is 3m. Thus, the total distance he
traveled is 7m. But the shortest distance between A and C is the length of
AC. It is 5m. Thus, the actual distance traveled by the person is 7m but the
displacement is only 5m.

'L൵HUHQFHV EHWZHHQ 'LVWDQFH DQG 'LVSODFHPHQW

Distance Displacement

ϭ͘ /ƚ ŝƐ ƚŚĞ ĂĐƚƵĂů ůĞŶŐƚŚ ŽĨ ƉĂƚŚ ƚƌĂǀĞůĞĚ ϭ͘ /ƚ ŝƐ ƚŚĞ ƐŚŽƌƚĞƐƚ ĚŝƐƚĂŶĐĞ ďĞƚǁĞĞŶ

by an object. any two points.

Ϯ͘ /ƚ ŝƐ ƐĐĂůĂƌ ƋƵĂŶƟƚLJ͘ Ϯ͘ /ƚ ŝƐ ǀĞĐƚŽƌ ƋƵĂŶƟƚLJ͘

ϯ͘ /ƚ ĐĂŶ ďĞ ĂĚĚĞĚ Žƌ ƐƵďƚƌĂĐƚĞĚ ďLJ ϯ͘ /ƚ ĐĂŶ ďĞ ĂĚĚĞĚ Žƌ ƐƵďƚƌĂĐƚĞĚ ďLJ

ƐŝŵƉůĞ ŵĂƚŚĞŵĂƟĐĂů ƌƵůĞ͘ vector methods.

ϰ͘ /ƚƐ ǀĂůƵĞ ŝƐ ĂůǁĂLJƐ ƉŽƐŝƟǀĞ͘ ϰ͘ /ƚƐ ǀĂůƵĞ ŵĂLJ ďĞ ƉŽƐŝƟǀĞ͕ ŶĞŐĂƟǀĞ Žƌ
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, we can say that it
covers 10m distance in 1 second and hence its speed is 10m/s.
6SHHG LV GHÀQHG DV WKH GLVWDQFH WUDYHOOHG E\ DQ REMHFW LQ XQLW WLPH

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

Times' Crucial Science and Environment 22 Book 7

Since, 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.

Velocity

When a body moves from one point to another in a straight path in a
particular direction, its speed is called velocity.
7KHUHIRUH YHORFLW\ FDQ EH GHÀQHG DV VSHHG RI DQ REMHFW LQ D SDUWLFXODU
direction.
Since distance in a particular direction is displacement,
9HORFLW\ FDQ EH GHÀQHG DV GLVSODFHPHQW RI D ERG\ LQ XQLW WLPH

Velocity = Displacement
Time taken

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

Comparison between Speed and Velocity

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

The speed of the body is calculated by using formula

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

= 4+3
2

= 3.5 m/s

If the body moves directly form A to C in time two seconds, its displacement is
5m. Therefore, the velocity of the body is calculated by using a formula,

Velocity = Displacement (d)
Time taken (t)
5
= 2

= 2.5 m/s

Times' Crucial Science and Environment 23 Book 7

Worked out example (1) "

A car travels 300 m distance in 1 minute. Calculate its speed.
Solution: Given,

Distance (d) = 300m

Time (t) = 1 minute = 60s

Speed (v) = ?

We have, Distance travelled (d)
Time taken (t)
Speed =

= 300
60

= 5 m/s

Therefore, the speed of the man is 5m/s.

'L൵HUHQFHV EHWZHHQ 6SHHG DQG 9HORFLW\

Speed Velocity

1. It is the distance covered by an 1. It is the displacement of an object

object in unit time. in unit 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 or subtracted by 4. It cannot be added or subtracted

simple mathematical rule. by simple mathematical rule.

Uniform and Non-uniform Motion

When a body covers equal distance in equal interval of time, its motion is
called XQLIRUP PRWLRQ.

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 LV FRYHULQJ P GLVWDQFH LQ WKH ÀUVW RQH VHFRQG ,W
further covers 5m distance in the next one second and so on. Its motion is
uniform motion.
Motion of hands of watch, motion of planets, moon, etc are examples of
uniform motion.

Times' Crucial Science and Environment 24 Book 7

:KHQ D ERG\ FRYHUV XQHTXDO GLVWDQFH LQ HTXDO LQWHUYDO RI WLPH LWV PRWLRQ LV
FDOOHG QRQ XQLIRUP PRWLRQ ,W LV DOVR NQRZQ DV YDULDEOH PRWLRQ

0 sec 1 sec 1 sec 1 sec

A 5m B 10 m C 5m D

,Q WKH DERYH ÀJXUH D FDU WUDYHOOHG P LQ WKH ÀUVW RQH VHFRQG P LQ WKH
next one second and 4m in the third one second. Thus, it covers different
distance in equal interval of time. Therefore, its motion is non-uniform
motion.

Examples: Motion of animals, motion of vehicles, motion of water in river, etc.

Acceleration

When a body has variable motion in a straight line, its velocity keeps on
changing. The rate by which velocity of a body changes is its acceleration.
Thus, WKH UDWH RI FKDQJH RI YHORFLW\ RI D PRYLQJ ERG\ LV DFFHOHUDWLRQ

Suppose, a car is moving from a point. Its velocity is 5m/s in the beginning.
It slowly increases and becomes 20m/s after 5 seconds.

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

X P V Y P V

0 sec W V 5 sec

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 the formula,

Acceleration =Final Velocity - Initial Velocity
Time

?a = v - u
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 will be meter per
second per second or P V2. It can be shown as:

Times' Crucial Science and Environment 25 Book 7

#EEGNGTCVKQP %JCPIG KP 8GNQEKV[
6KOG

P V
V

P
VªV

P OU
V

If the velocity of a moving body decreases in every second, its acceleration
will be negative. It is called UHWDUGDWLRQ. Hence, WKH UDWH RI GHFUHDVH RI
YHORFLW\ RI D ERG\ LV FDOOHG UHWDUGDWLRQ ,W LV D YHFWRU TXDQWLW\. Its unit is m/s2.

Worked out example (2) "

A bus is moving with a velocity of 20P V. If it increases its velocity
to 30P V DIWHU VHFRQGV ÀQG LWV DFFHOHUDWLRQ

Solution: +GPiKvVKeCnN ,8GNQEKV[
X P V
(KPCN 8GNQEKV[
Y P V
6KOG
W V
#EEGNGTCVKQP
D !

9G JCXG Y X P V
D W

7KHUHIRUH WKH DFFHOHUDWLRQ RI WKH EXV LV O U

Worked out example (3) "

A car moving with 40P V stops after 5 seconds when brakes are
applied. Find the acceleration of the car.

Solution: Given,

+PKVKCN 8GNQEKV[
X P V
P V
(KPCN 8GNQEKV[
Y V 6LQFH WKH FDU VWRSV
!
6KOG
W

#EEGNGTCVKQP
D

9G JCXG Y X
W
D

Times' Crucial Science and Environment 26 Book 7

Ō


P V

7KHUHIRUH WKH DFFHOHUDWLRQ RI WKH FDU LV O U

6LQFH WKH QHJDWLYH DFFHOHUDWLRQ LV UHWDUGDWLRQ WKH UHWDUGDWLRQ RI WKH FDU
LV O U

ŐĞŶƚ : any person or thing which does certain work
'ƌĂǀŝƚLJ : pulling force of the earth or planet
Cobalt ͗ ĂŶ ĞůĞŵĞŶƚ ǁŚŝĐŚ ĐĂŶ ďĞ ĂƩƌĂĐƚĞĚ ďLJ Ă ŵĂŐŶĞƚ
Nickel ͗ ĂŶ ĞůĞŵĞŶƚ ǁŚŝĐŚ ĐĂŶ ďĞ ĂƩƌĂĐƚĞĚ ďLJ Ă ŵĂŐŶĞƚ

Main Points to Remember

1. Force is an external agency which changes or tends to change the
state of rest or motion of an object.

2. Living things use muscular force stored in their muscle.
3. The force which is exerted by a magnet is called magnetic force.
4. When an object moves in a circular path, the force that acts to the

object towards the centre is called centripetal force and the force
that acts away from the centre is called centrifugal force.
5. The force that opposes motion of an object when it slides or moves
over another object is called frictional force.
6. Electrostatic force is the force developed due to the production of
charge.
7. The quantities that have both magnitude and direction are called
vector quantities.
8. The quantities that have only magnitude but no direction are
called scalar quantities.
9. The actual length of a path travelled by a body is called distance.

Times' Crucial Science and Environment 27 Book 7

7KH VKRUWHVW GLVWDQFH EHWZHHQ WKH LQLWLDO DQG ÀQDO SRVLWLRQV RI DQ
object is called displacement.

11. The distance covered by an object in unit time is called speed.
12. The displacement of an object in unit time is called velocity.
13. The rate of change of velocity of an object is called acceleration.

Exercise

1. Choose the best alternative in each case.

a. What is the CGS unit of force?

i. Dyne ii. Erg

iii. Newton iv. Watt

b. What is the force that pulls an object towards the centre of a
circular path called?

i. Pulling force ii. Centrifugal force

iii. Centripetal force iv. Frictional force

c. Which of the following is a vector quantity?

i. Mass ii. Energy

iii. Speed iv. None of these

d. The change in the displacement of a body per unit time

i. Velocity ii. Speed

iii. Acceleration iv. Uniform motion

e. The actual length of path travelled by an object is called

i. Speed ii. Displacement

iii. Distance iv. All of these

Times' Crucial Science and Environment 28 Book 7

2. Answer these questions in short.
a. What is force?
b. What are various types of force? Mention them.
c. What is pushing force? Give some examples of pushing force.
d. What is muscular force?

e. What is magnetic force?

f. What is gravitational force? Write down its effects.

g. What are centripetal and centrifugal forces?

h. What is frictional force? How is friction created?

i. What is vector quantity? Give some examples.

j. What is speed? Mention its unit.

3. Differentiate between

i. Vector and scalar quantity

ii. Distance and displacement

iii. Velocity and speed

iv. Acceleration and retardation

v. Centripetal and centrifugal force

4. Answer these questions:
a. Explain an activity which shows that centripetal and centrifugal
force are produced when an object moves in circular path.
b. How does friction depend upon surface? Explain.
c. How are distance and displacement related?
d. What is acceleration? Show that unit of acceleration is m/s2.
e. Speed is a scalar quantity but velocity is a vector quantity. Why?
f. Why is area a scalar quantity?
g. What is uniform motion? Give examples of uniform motion.
h. Explain non-uniform motion with a suitable example.
i. Mention effects of friction and explain them.

Times' Crucial Science and Environment 29 Book 7

5. Numerical problems

a. A car covers 400m distance in 50 seconds. Find its speed.

$QV P V

b. If the speed of a car is 20m/s. How long does it take to cover a

distance of 1 km? $QV V

c. A bus increases its velocity from 20m/s to 30m/s in 4s. Find its

acceleration. $QV P V2

d. A motorcycle starts from rest and gains a velocity of 20m/s after

4 seconds. Find its acceleration. $QV P V2

e. A vehicle moving with a velocity 25m/s stops after 5 seconds,
when brakes are applied. Find its retardation. $QV P V2

Project Work

1. Walk slowly from one end to another end of your playground in a
straight line. Note the time taken to your journey. Then walk little
faster and travel the same distance. Note the time. Now travel
the same distance by running and note the time taken. Finally,
travel the same distance by running as fast as you can and note
the time required. Calculate your velocity in different cases using
the formula:

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

2. Take two bar magnets ( A and B). Bring one end of the bar magnet
‘A’ close to one end of the bar magnet ‘B’. What happens? Now
reverse ends of the bar magnet ‘A’ without changing the ends of
the bar magnet ‘B’. What happens? Why does it happen? Discuss
with friends.

+LQWV WZR HQGV DWWUDFW LQ RQH VLWXDWLRQ ZKLOH WKH\ UHSHO LQ DQRWKHU
VLWXDWLRQ 7KLV LV GXH WR DWWUDFWLRQ IRUFH RI RSSRVLWH SROHV DQG UHSXOVLRQ
IRUFH RI VLPLODU SROHV

Times' Crucial Science and Environment 30 Book 7

Chapter

3 6LPSOH 0DFKLQHV

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 tell the meaning of simple machine and explain its advantages.
 ĐůĂƐƐŝĨLJ ĂŶĚ ĞdžƉůĂŝŶ ĚŝīĞƌĞŶƚ ƐŝŵƉůĞ ŵĂĐŚŝŶĞƐ͘
 ĞdžƉůĂŝŶ ƚŚĞ ƵƟůŝƟĞƐ ŽĨ ƐŝŵƉůĞ ŵĂĐŚŝŶĞƐ͘

MIND OPENERS
 How can you make your work easier?
 How can you make your work faster?
 Can you give names of some machines used in your daily life? How are they

useful? Discuss.

Introduction

We perform several work in our daily life. Some of the works are easy to
GR ZLWK RXU KDQGV EXW VRPH DUH GLIÀFXOW DQG WLPH FRQVXPLQJ LI ZH GR ZLWK
RXU KDQGV RQO\ +HQFH ZH XVH WKH LQVWUXPHQWV OLNH NQLIH VSDQQHU ÀUH
tongs, nail-cutter, crow-bar, etc to make our works easier and faster. These
instruments are called simple machines.
Hence, WKH VLPSOH GHYLFHV RU LQVWUXPHQWV WKDW PDNH RXU ZRUN HDVLHU DQG
IDVWHU DUH FDOOHG VLPSOH PDFKLQHV
We use some other machines which are constructed by using several simple
machines. Such machines are complicated in structure and are known as
complex machines. Thus, WKH PDFKLQHV ZKLFK DUH FRPSOLFDWHG LQ VWUXFWXUH
DQG FRQVLVW RI VHYHUDO VLPSOH PDFKLQHV DUH NQRZQ DV FRPSOH[ PDFKLQHV.
Sewing machine, engines of vehicles, water mill, wind mill, etc are some
examples of complex machines.

Advantages of Simple Machines

Simple machines make our work easier and faster in the following ways:
a) A simple machine can change the direction of force applied.
b) It can magnify the force applied.
c) It can transfer effort from one point of the machine to another point.
d) It can increase the speed of doing work.

Times' Crucial Science and Environment 31 Book 7

Mechanical Advantage (MA)

The number of times by which a simple machine multiplies the applied
force is called mechanical advantage. Mathematically, LW LV WKH UDWLR RI ORDG
WR HIIRUW LQ D VLPSOH PDFKLQH. It has no unit.

MA = Load
(൵RUW

Worked out example (1) "

If a load of 1000N is lifted by the effort of 200N in a simple machine,
what will be the mechanical advantage?

Solution: = 1000 N
Given,
Load (L)

(൵RUW ( 1

MA = ?

0QY YG JCXG . 0
/# ' 0

,W PHDQV WKH VLPSOH PDFKLQH PXOWLSOLHV WKH DSSOLHG IRUFH E\ ÀYH WLPHV

Types of Simple Machines

7KH VLPSOH PDFKLQHV FDQ EH FODVVLÀHG LQWR VL[ W\SHV RQ WKH EDVLV RI VWUXFWXUH
and function. They are:

1. Lever 2. Pulley 3. Wheel and axle

4. Inclined plane 5. Screw 6. Wedge

1. Lever

$ 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. A lever consists of
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.

(IIRUW /RDG

HIIRUW GLVWDQFH ORDG
)XOFUXPGLVWDQFH

Times' Crucial Science and Environment 32 Book 7

The mechanical advantage of the lever depends upon the load distance and
effort distance. In a lever, WKH GLVWDQFH EHWZHHQ WKH IXOFUXP DQG WKH ORDG
LV FDOOHG ORDG GLVWDQFH /G RU ORDG DUP /D whereas the distance between
the fulcrum and the effort is called effort distance(Ed) or effort arm(Ea).

When a lever is in a balanced condition, Effort × Effort arm = Load × load arm.
It is called principle of lever.

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,
'KLNL &XWWLQJ VKHDUV HWF DUH VRPH H[DPSOHV RI ÀUVW FODVV OHYHU

īŽƌƚ Load

ĞīŽƌƚ ĚŝƐƚĂŶĐĞ load distance
Fulcrum
L
F

F L F
E LE
E
Scissors

Beam balance See-saw

b. Second class lever

The lever in which the load lies at any point in the middle of effort and
fulcurm is called second class lever. In the second class lever, the effort
distance is always greater than the load distance. Hence, the second class
lever has mechanical advantage more than any other class of lever.

Load īŽƌƚ

load distance

ĞīŽƌƚ ĚŝƐƚĂŶĐĞ

Fulcrum

Times' Crucial Science and Environment 33 Book 7

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

LE E FL
F E
Wheel barrow L
F Bottle opener

Onion cutter

c. Third class lever
The lever in which effort lies at any point in the middle of load and fulcrum is
called third class lever. Such lever makes the work safe and easy. But it cannot
multiply effort because the load distance is always greater than the effort
distance. The mechanical advantage of third class lever is always less than one.

īŽƌƚ Load

ĞīŽƌƚ ĚŝƐƚĂŶĐĞ

Fulcrum load distance

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

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

2. Pulley

$ SXOOH\ LV D KDUG PHWDOOLF RU ZRRGHQ GLVF ZLWK D JURRYHG ULP 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.

Times' Crucial Science and Environment 34 Book 7

$ SXOOH\ FDQ EH FODVVLÀHG LQWR WZR W\SHV VLQJOH À[HG SXOOH\ DQG VLQJOH
movable pulley.

D 6LQJOH À[HG SXOOH\ Frame F
$ SXOOH\ LQ ZKLFK WKH IUDPH LV À[HG WR D Wooden disk
rigid support and the disc rotates along
ZLWK WKH URSH LV FDOOHG VLQJOH À[HG SXOOH\ Rope
7KH PHFKDQLFDO DGYDQWDJH RI VLQJOH À[HG
pulley is 1, i.e., there is no gain in mechanical L
advantage. But we use such pulley because
it helps to change the direction of applied E
force.
6LQJOH ¿[HG SXOOH\

b. Single movable pulley E Rope
A pulley which moves up and down along
with the load is called single movable F Wooden disk
pulley. In such pulley, one end of the rope is L Frame
tied to a rigid support and the load is pulled
from other end of the rope. The circular
disc moves along with load. In such pulley,
mechanical advantage is 2.

Single movable pulley

Combined pulley Wooden disk Fixed pulley
$ SXOOH\ ZKLFK FRQVLVWV RI Rope
FRPELQDWLRQ RI WZR RU PRUH SXOOH\V LV E
FDOOHG FRPELQHG SXOOH\ $ FRPELQHG Movable pulley
SXOOH\ LV DOVR FDOOHG FRPSRXQG
SXOOH\ RU EORFN DQG WDFNOH V\VWHP In
such pulley, mechanical advantage
is more than 1.

L

3. Wheel and Axle Combined pulley

$ ZKHHO DQG D[OH LV D VLPSOH PDFKLQH ZKLFK FRQVLVWV RI WZR ZKHHOV F\OLQGHUV
RI GLIIHUHQW GLDPHWHUV. 7KH ODUJHU F\OLQGHU RU ZKHHO LV À[HG ULJLGO\ ZLWK WKH

Times' Crucial Science and Environment 35 Book 7

small wheel 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 Steering of car Wheel
Wheel and axle
Axle
Kite string reel

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

4. Inclined Plane Inclined plane Height

$ VODQWHG VXUIDFH RYHU ZKLFK ORDG FDQ
EH SXOOHG RU SXVKHG LV FDOOHG LQFOLQHG
SODQH In general, it is a sloping
surface. Winding roads on hills, spiral
staircase, wooden plank used for
loading goods in a truck, etc are some
examples of inclined plane.

5. Screw
$ VFUHZ LV D VLPSOH PDFKLQH ZKLFK VHHPV WR
KDYH DQ LQFOLQHG SODQH ZUDSSHG DURXQG D
F\OLQGULFDO VXUIDFH. Jack screw, screw nail,
driller, etc are the examples of screw. A jack
screw is used to lift vehicles while changing
their wheels.
Screw nail Jack screw

6. Wedge (IIRUW
$ ZHGJH LV D VLPSOH PDFKLQH ZKLFK KDV D VKDUS SDUW DW /RDG
RQH HQG DQG ÁDW SDUW DW WKH RWKHU. The effort is generally
DSSOLHG XSRQ WKH ÁDW SDUW DQG WKH VKDUS SDUW SHUIRUPV
the work. A wedge is used to split the wooden logs. Axe,
knife, khukuri, nail, chisel, needle, etc are also regarded
as the examples of wedge. /RDG

Times' Crucial Science and Environment 36 Book 7

Worked out example (2) "

&DOFXODWH WKH HIIRUW ( IURP WKH JLYHQ ÀJXUH
UH
Solution: Given, 'HHQTV
'
!
.QCF
. 0
FP
.QCF &KUVCPEG
.F FP .QCF
.
'HHQTV &KUVCPEG
'F FP 0

FP

#EEQTFKPI VQ VJG RTKPEKRNG QH NGXGT

' ª 'F . ª .F

QT ' ª FP 0 ª FP

QT ' 0 ª FP
FP

6JGTGHQTG ' 0

Worked out example (3) "

$ ORDG RI 1 LV OLIWHG E\ DQ HIIRUW RI 1 LQ D ÀUVW FODVV OHYHU ,I
the load is at 20 cm away from the fulcrum, what will be the effort
distance?

Solution

Given, = 600N . Q C F 0
F.P 'HHQTV
'
Load (L) = 20cm 0
Load Distance (Ld)
(൵RUW ( 1 !

(൵RUW 'LVWDQFH (G "

#EEQTFKPI VQ VJG RTKPEKRNG QH NGXGT

' ª 'F . ª .F

QT 'F . ª .F QT 'F 0 ª FP
' 0

6JGTGHQTG 'F FP

Uses of simple machines

Uses of simple machine can be explained in following points:
1) It multiplies the applied force. Thus, bigger load can be lifted by

applying smaller effort.
2) It transfers force from one point to another. Due to this, a load kept at

one point of the machine can be lifted by applying force at another point.
3) It makes works faster.
4) It changes the direction of the applied force. Due to this, a load can

be lifted upward by applying effort in downward direction.

Times' Crucial Science and Environment 37 Book 7

Water mill : a mill which works with the energy of water

Wind mill : a mill which works with the energy of wind

Steering wheel ͗ Ă ǁŚĞĞů ƌŽƚĂƚĞĚ ďLJ ĚƌŝǀĞƌ ƚŽ ĐŚĂŶŐĞ ĚŝƌĞĐƟŽŶ ŽĨ ƌƵŶŶŝŶŐ ǀĞŚŝĐůĞ͘

Jack screw : ƐĐƌĞǁ ǁŚŝĐŚ ŝƐ ƵƐĞĚ ƚŽ ůŝŌ ǀĞŚŝĐůĞ ĨŽƌ ĐŚĂŶŐŝŶŐ ƚLJƌĞƐ͕ ŵĂŝŶƚĞŶĂŶĐĞ͕
etc.

Main Points to Remember

1. Simple machine is a device or instrument that makes our work
easier and faster.

2. The number of times by which a simple machine multiplies the
applied force is called mechanical advantage.

3. Lever, pulley, wheel and axle, inclined plane, screw, and wedge are
various types of simple machines.

4. $ OHYHU LV D ORQJ ULJLG EDU WKDW LV FDSDEOH RI URWDWLQJ DERXW D À[HG SRLQW
5. A pulley is a hard metallic or wooden disc with grooved rim.
6. Wheel and axle is a simple machine which consists of two wheels

of different diameters.
7. A slanted surface over which load can be pulled or pushed is called

inclined plane.
8. A screw is a simple machine which seems to have an inclined plane

wrapped around a cylindrical surface.
9. A wedge is a simple machine which has a sharp part at one end

DQG ÁDW SDUW DW DQRWKHU

Exercise

1. Choose the best alternative in each case.

a. The ratio of load to effort in a simple machine is called

i. Load arm ii. Effort arm

iii. Mechanical advantage iv. None

b. If a screw driver is used to open the lid of a can, what kind of
simple machine is it?

i. Screw ii. Wheel and axle

iii. Lever iv. Wedge

Times' Crucial Science and Environment 38 Book 7

c. The string roller of a kite is

i. Lever ii. Wheel and axle

iii. Pulley iv. Screw

d. A ramp used in a hospital is an example of

i. Lever ii. Pulley iii. Inclined plane iv. Wedge

e. Which of the following is not a Screw?

i. Screw nail ii. Driller

iii. Jack screw iv. Screw driver

2. Answer these questions in short.
a. What is simple machine?
b. What are advantages of a simple machine?
c. What is complex machine ? Give some examples.
d. What is mechanical advantage? What is its unit?
e. Mention various types of simple machines.
f. What is pulley?
g. What is wheel and axle? Give some examples.
K 'HÀQH L LQFOLQHG SODQH LL VFUHZ LLL ZHGJH
L *LYH DQ\ WZR H[DPSOHV RI ÀUVW FODVV OHYHU VHFRQG FODVV OHYHU DQG
third class lever each.

3. Differentiate between
i. First class lever and second class lever

ii. Inclined plane and screw

4. Identify the classes of lever of the given diagrams.

i. īŽƌƚ Load

ii. īŽƌƚ Load

iii. Load īŽƌƚ

5. Give reasons.
a. MA of second class lever is always more than 1.
b. Road in hills are made winding.
c. Even though third class lever does not multiply the force, it is in
practice.

Times' Crucial Science and Environment 39 Book 7

6. Draw diagrams of: ii. A single moveable pulley
i. Wheel and axle iv. A third class lever
iii. Wedge

7. Numerical problems

a. A pulley lifts an weight of 50N by applying an effort of 20N.
Calculate its MA. $QV

b. In a lever, effort arm and load arm are 50 cm and 20 cm
respectively. What effort is required to lift a load of 500N?
$QV 1

c. Calculate

i. MA and load distance.

.QCF 'HHQTV 0

0

EO
$QV 0$ /D FP

ii. Calculate the effort from the following information.

'

.QCF

0 EO

EO (Ans: 400N)

d. If the load and effort in a simple machine are in the ratio of 5:2,
what is the mechanical advantage? $QV

e. What load can be lifted by an effort of 800N if the effort arm is
80cm and the load arm is 20cm? $QV 1

Project Work

Take a wooden disk, some pieces of wood, iron nails and rope. Make a
pulley of your own using these materials and try to lift loads with the
help of this pulley.

Times' Crucial Science and Environment 40 Book 7

Chapter

4 3UHVVXUH

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ƉƌĞƐƐƵƌĞ͘
 tell the examples and uses of pressure in daily life.
 ĚĞĮŶĞ ĚĞŶƐŝƚLJ͘
 ƚĞůů ĐŽŶĚŝƟŽŶƐ ŽĨ ƐŝŶŬŝŶŐ ĂŶĚ ŇŽĂƟŶŐ ŽĨ ĂŶ ŽďũĞĐƚ͘
 solve simple numerical problems.

MIND OPENERS
 tŚĞŶ LJŽƵƌ ůĞŐ ŝƐ ƐƚĞƉƉĞĚ ďLJ Ă Őŝƌů ǁŝƚŚ ŇĂƚ ƐůĞĞƉĞƌƐ͕ LJŽƵ ĨĞĞů ůĞƐƐ

pain, whereas you feel more pain when stepped by a girl with
pointed heel. Why?
 Why are nails pointed?
 ƉůĂƐƟĐ ũƵŐ ŇŽĂƚƐ ŽŶ ǁĂƚĞƌ ďƵƚ ƐƚĞĞů ƐƉŽŽŶ ƐŝŶŬƐ ŝŶ ǁĂƚĞƌ͘ tŚLJ͍
Discuss.

Introduction

:H XVH NQLIH WR FXW YHJHWDEOHV 6LPLODUO\ ZH XVH SRLQWHG QDLO WR À[ RQ WKH
wall. Here, in both cases, we apply force. 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 in the term ‘pressure’.
3UHVVXUH LV GHÀQHG DV WKH IRUFH DFWLQJ RQ XQLW DUHD

3UHVVXUH 3 )RUFH ) = 2 (
$UHD $ #

Force is measured in Newton and area in m2. Therefore, the unit of pressure
is N/m2 or Pascal.
Pascal is surname of a scientist named Blaise Pascal. For the honour of his
FRQWULEXWLRQ LQ VFLHQWLÀF ÀHOG 3DVFDO LV XVHG DV XQLW RI SUHVVXUH 3DVFDO LV ZULWWHQ
as ‘Pa’ in short. Other units of pressure are mm of Hg, atmospheric pressure, etc.

Times' Crucial Science and Environment 41 Book 7

)DFWRUV D൵HFWLQJ SUHVVXUH

Pressure depends upon two factors:
i) Amount of force applied
ii) The area upon which the force is applied.
Pressure increases when amount of the force applied increases. It decreases
when amount of force applied decreases.
Similarly, pressures increases when the area upon which the force applied
decreases and it decreases when the area increases.

Activity 4.1 Show that pressure depends upon area

Materials required
A piece of foam and a brick

Procedure
1. Take a piece of foam.
2. Put a brick on the foam in different positions as shown in the
ÀJXUH

In position ’a’, the brick exerts force on large area. Therefore, the
pressure exerted is less and the foam is depressed less.
In position ’b’ the brick exerts force on small area. Therefore, the
pressure exerted is more and the foam is depressed more.

Activity 4.2 Show that pressure depends upon force

Materials required
A piece of foam and three bricks

Procedure
1. Take a piece of foam.

Times' Crucial Science and Environment 42 Book 7

3XW D EULFN RQ WKH IRDP :LWK ÁDW VXUIDFH RYHU WKH IRDP
Observe the depression.

Put 2 more bricks one over another on the previous brick and observe
the depression. In the second case the foam deeps or depresses more
than the previous case. Here, the area upon which the force acts is the
same. But, the force exerted is more due to the weight of three bricks.

Worked out example (1) "

A wooden box has mass of 50 kg. It occupies area of 2m2. Find the
pressure exerted by the box on the ground.
Solution:

)KXGP #EEQTFKPI VQ VJG HQTOWNC
9GKIJV QH VJG DQZ
( MI
ª 0 2 (
0 #
9G JCXG
#TGC
# P
2TGUUWTG
2 !
2CUECN
Worked out example (2)
6JGTGHQTG VJG RTGUUWTG GZGTVGF D[ VJG
DQZ QP VJG ITQWPF 2C

"

A brick has mass of 1.5 kg. Its length is 0.2m and breadth is 0.1m. What
LV WKH SUHVVXUH H[HUWHG RQ WKH ERWWRP ZKHQ LW LV NHSW ÁDW RQ WKH JURXQG"

Solution:

)KXGP

9GKIJV QH VJG DTKEM
( MI

ª 0 0

9G JCXG
#TGC
#
ª P P

2TGUUWTG
2 ! 2 ( 2CUECN
#EEQTFKPI VQ VJG HQTOWNC #

6JGTGHQTG RTGUUWTG GZGTVGF D[ VJG DTKEM QP VJG DQVVQO KU 2C

Times' Crucial Science and Environment 43 Book 7

Uses of Pressure in Daily life

1. It is easier to cut things with sharp knife than with a
blunt one: When we use sharp knife to cut the things, the force
applied acts on small area of the things. Due to this, more pressure
will be exerted. Thus, it is easier to cut things with a sharp knife.

2. The foundation of building is made wider than walls:
Foundation has to support the load of whole building. When the
foundation is made wider, it occupies more area. The weight of the
building exerts on large area causing the decrease in pressure. Thus,
the building is supported.

3. Nails are made pointed at one end: When nails are made
pointed, the force applied acts on small area. As a result, the nail can
easily penetrate inside the woods.

4. Camels can easily walk on the sand of desert: Camels have
ÁDW IHHW 7KHLU IHHW RFFXS\ ODUJH DUHD (YHQ WKRXJK WKHLU ZHLJKW LV
more, their feet exert less pressure on the sand due to more area. Due
to less pressure, they can easily walk on the sand.

5. The wheels of a tractor are wider but the wheels of a car
are not so: Tractors are used to carry heavier loads but a car is
used to carry less number of persons. When the wheels of tractors are
wider, the load exerts force on large area causing less pressure on the
road or soil. But the car does not give more force on the wheels.

'L൵HUHQFHV EHWZHHQ )RUFH DQG 3UHVVXUH

Force Pressure

ϭ͘ /ƚ ŝƐ ĂŶ ĞdžƚĞƌŶĂů ĂŐĞŶĐLJ ǁŚŝĐŚ ĐŚĂŶŐĞƐ ϭ͘ /ƚ ŝƐ ƚŚĞ ĨŽƌĐĞ ĂĐƟŶŐ ŽŶ ƵŶŝƚ ĂƌĞĂ͘
or tends to change the state of rest or
ŵŽƟŽŶ ŽĨ ĂŶ ŽďũĞĐƚ͘

Ϯ͘ /ƚƐ ƵŶŝƚ ŝƐ EĞǁƚŽŶ͘ Ϯ͘ /ƚƐ ƵŶŝƚ ŝƐ WĂƐĐĂů͘

ϯ͘ /ƚ ŝƐ Ă ǀĞĐƚŽƌ ƋƵĂŶƟƚLJ͘ ϯ͘ /ƚ ŝƐ Ă ƐĐĂůĂƌ ƋƵĂŶƟƚLJ͘

ϰ͘ /ƚ ŝƐ ŵĞĂƐƵƌĞĚ ďLJ ƚŚĞ ƌĞůĂƟŽŶƐŚŝƉ͗ ϰ͘ /ƚ ŝƐ ŵĞĂƐƵƌĞĚ ďLJ ƚŚĞ ƌĞůĂƟŽŶƐŚŝƉ͗

&ŽƌĐĞ с ŵĂƐƐ п ĂĐĐĞůĞƌĂƟŽŶ Pressure = Force
ƌĞĂ

Density

Some substances are heavier whereas some are lighter. The substances
like iron, copper, stones, soils, etc are heavier whereas paper, cork, plastic,

Times' Crucial Science and Environment 44 Book 7

etc are lighter. The heaviness or lightness of a substance is expressed in
the term ’density’.
'HQVLW\ RI D VXEVWDQFH LV GHÀQHG DV WKH PDVV SHU XQLW YROXPH

/CUU
/
&GPUKV[
& 8QNWOG
8

& /
8

Unit of mass is kilogram and unit of volume is cubic metre m3. Therefore,
unit of density is kilogram per cubic meter which is written as NJ P3.

Density of a substance shows its compactness or heaviness. The substance
having more density is heavier than the substance having less density.
Thus, density of iron is more than the density of wood.

Density of some common substances is given below.

SN. Substance Density (kg/m3)

1. ůĐŽŚŽů 800

2. Water 1000

3. /ĐĞ 920

4. /ƌŽŶ 7800

5. Copper 8900

6XEVWDQFHV KDYLQJ OHVV GHQVLW\ WKDQ WKDW RI ZDWHU ÁRDW RQ ZDWHU ,FH FRUN
SODVWLF HWF KDYH OHVV GHQVLW\ WKDQ WKDW RI ZDWHU 7KHUHIRUH WKH\ ÁRDW RQ
water. Substances like iron, stone, copper, etc have more density than that
of water. Therefore, they sink in water.

Worked out example (3) "

An iron piece has mass of 78 kg and its volume is 0.01 m3. Find its
density.
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Times' Crucial Science and Environment 45 Book 7

Pointed : having a tapering end
&ŽƵŶĚĂƟŽŶ : base
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Main Points to Remember

1. Pressure is the force acting on unit area.
2. The amount of force applied and the area upon which the force acts

are the factors which affect the pressure.
3. Pressure increases when the amount of force applied increases.
4. Pressure decreases when the area increases and vice versa.
5. Camels can walk on the sand easily due to less pressure exerted by

the feet of the camel.
6. Density of a substance is the mass per unit volume of the substance.

Exercise

1. Choose the best alternative in each case.

a. What is the SI unit of pressure?

i. N/m2 ii. Pascal

iii. Both i and ii iv. None of these

b. If a box of 500N is placed over the land of area of 2 m2, what
pressure is exerted by the box on the land?

i. 25 N/m2 ii. 250 N/m2

iii. 500 N/m2 iv. 1000 N/m2

c. Which of the following sentences is incorrect about pressure?
i. Pressure depends upon the amount of force applied.
ii. Pressure depends on the area upon which the force is applied.
iii. Pressure is a vector quantity.
iv. The force acting per unit area is called pressure.

d. What is the CGS unit of density?

i. kg/m3 ii. g/ cc iii. g/m3 iv. Pa

Times' Crucial Science and Environment 46 Book 7