Oasis Science and Technology 7

Approved by Government of Nepal, Ministry of
Education, Curriculum Development Centre, Sano

Thimi, Bhaktapur as an additional material

Oasis

Grade

7

Editor Author
Dr. Rameshwar Adhikari Jayananda Kapadi

Reader M.Sc. (Zoology), B.Ed. (Health Education)
Tribhuvan University, Kirtipur, Kathmandu
Central Department of Chemistry

Tribhuvan University, Kirtipur, Kathmandu

Oasis

7

Publisher and Distributor:

Oasis Publication Pvt. Ltd.

Tel: 01-4313205

Author:
Jayananda Kapadi

Language Editors:
Sedunath Dhakal
Bhan Dev Kapadi

Edition:
First 2066
Second 2067
Third 2068
Fourth 2069
Fifth 2070
Sixth 2071
Seventh 2072 (Completely Revised)
Eighth 2073 (Revised)
Ninth 2074
Tenth 2075 (Revised)
Reprint 2076

Copyright 
Publisher

Computer layout:
Ramesh Bhattarai

Printed in Nepal

Preface

Oasis School Science and Environment for Grade 7 is an attempt to make learning process
a joyful experience. This textbook has been written in strict conformity with the latest
syllabus prescribed by the Curriculum Development Centre, Sano Thimi, Bhaktapur,
Nepal. This book has been designed to help students develop their conceptual thinking
and scientific skills. I think this book is an excellent introduction to experimentation and
practical application of Science. I hope it will greatly facilitate teaching learning process
in an easy and enjoyable manner.

The beauty of this textbook lies in having high resolution pictures, attractive layout, and
clear illustrations with lucid language. It emphasizes concept building rather than merely
focusing on providing and collecting information without critical thinking. I expect
this book will assist students to make them eager and quizzical learners that reinforces
their conceptual learning in the classroom. Besides the learning process of the students,
this book will help in teaching process of the teachers. Each unit of this book presents
subject matter in an interesting, understandable and enjoyable manner. The exercise at
the end of each unit includes a variety of questions to facilitate the integration of various
concepts taught. Above all, I sincerely believe that this book will be helpful in overall
understanding of Science in an interesting manner.

It is not a hidden fact that modern era is the era of science and technology. Science is
a part of the world we live in and an avenue to the technology. A good textbook in
science should assist the learners to realize different activities and events around us that
encourages them for further discovery and innovation interestingly. I strongly believe
that students should enjoy science and this book will be a means of enjoying and learning
science in the modern era of science and technology.

I wish to express my sincere gratitude to Mr. Harish Chandra Bista, Managing Director
of Oasis Publication Pvt. Ltd. for publishing this book. Similarly, my hearty thanks go to
Focus Computer for layout. Thanks are due to Mr. Surendra Mishra, Mr. Navneesh Prasad
Yadav, Mr. Ram Maharjan, Mrs. Bimala Shah, Mrs. Jamuna Maharjan, Mr. R.C. Neupane,
Mr. Ujjwol Bhomi, Mr. Shivendra Karki, Mr. Binod Kumar Yadav, Mr. Prakash Bhatta
and Mr. Rabindra Agrawal for their valuable help during the preparation of the book.
Likewise, thanks are due to Mr. Ramesh Lamsal and Mr. Bhan Dev Kapadi for their
praiseworthy language editing. I gratefully acknowledge teachers across the country as
well as my well-wishers for their inspiration and support during the preparation and
publication of the book.

In my opinion, the real judges of a book are the teachers concerned and the students for
whom it is meant. Despite all my efforts, there might be textual as well as technical errors.
Therefore, constructive suggestions for rectification and improvement of the book would
be gratefully acknowledged and incorporated in further editions.

February 2016 Author
Kathmandu, Nepal

Contents

Physics

Unit 1 Measurement ................................................................ 1
Unit 2 Force and Motion.......................................................... 17
Unit 3 Simple Machine............................................................. 35
Unit 4 Pressure.......................................................................... 49
Unit 5 Energy, Work and Power............................................ 55
Unit 6 Heat................................................................................. 65
Unit 7 Light................................................................................ 76
Unit 8 Sound.............................................................................. 86
Unit 9 Magnet............................................................................ 97
Unit 10 Electricity........................................................................ 104

Chemistry

Unit 11 Matter.............................................................................. 114
Unit 12 Mixture........................................................................... 125
Unit 13 Metal and Non-Metal................................................... 138
Unit 14 Some Useful Chemicals................................................ 145

Biology

Unit 15 Living Beings: Animal life........................................... 150

Unit 16 Living Beings: Plant Life.............................................. 161

Unit 17 Cell and Tissue.............................................................. 179
Unit 18 Life Processes................................................................ 193

Geology and Astronomy

Unit 19 Structure of the Earth.................................................... 208
Unit 20 Weather and Climate.................................................... 218
Unit 21 The Earth and Space..................................................... 228

Environment Science

Unit 22 Environment and Its Balance...................................... 238
Unit 23 Environmental Degradation and Its Conservation.. 259
Unit 24 Environment and Sustainable Development............ 274

Teaching Strategies

Science and Environment deals with the systematic knowledge of different activities and events that
occur in our surroundings. Therefore, various teaching learning activities can be adopted to Science and
Environment according to nature of the subject matter.

Teachers are expected to adopt various teaching methods like observation, experiment, demonstration,
discovery, invention, discussion, question-answer, field visit, etc. to teaching learning process for
Science and Environment. Besides these methods, teachers can adopt explanation or lecture method in
the course of introducing any event, subject matter or result of something. ‘Student Centered Method’
is supposed to be the most appropriate in teaching Science and Environment. In this method, every
student gets chance to think critically in solving his/her problems.

In teaching learning activities, teachers are expected to make the involvement of every student in
‘process skills’ like classification, comparison, putting query, reasoning, keeping record, assessment, etc.
Teaching Science and Environment not only aims at accumulating knowledge but also at discovering
knowledge. Therefore, teaching learning process is expected to be centered on / oriented towards
discovery and invention.

Students should be encouraged to learn things on their own by discovering, inventing, experimenting or
by solving problems. For this purpose, teachers are expected to make the involvement of all the students
more and more in practical activities along with the theoretical knowledge.

Specially, for the successful teaching learning process, teachers are expected to keep the following points
in their mind.
i. Asking the students for the situations or events happening in their surroundings
ii. Encouraging the students to hypothesize in advance about the result or effect of the events or

situations
iii. Encouraging the students in testing their hypothesis
iv. Providing an opportunity to every student to reach his/her own conclusion and to rethink of the

significance of his/her conclusion

For effective teaching learning process teachers are expected to emphasize the use of teaching
learning materials. It is emphasized that the use of teaching learning materials is helpful to make
the concept of each lesson clear for easy understanding of the students. Teachers are expected to
make the optimum use of local teaching learning materials as far as possible according to nature
of the subject matter. For successful teaching learning process in Science and Environment,
teachers are expected to adopt to the following activities.

1. Figure/Picture Observation: The picture(s) or figure(s) related to the subject matter of the lesson
play(s) vital role in making the concept of the lesson clear. Teachers are expected to show or
demonstrate the picture(s) or figure(s) related to the lesson and to make the involvement of the
students in observation. This activity helps the students to take part in discussion and question
answer on the basis of their observation.

Weighting Distribution

Estimated Teaching Periods

S. No. Area Weighting Percentage Theoretical Practical
32 8
1 Physics 22.50 28 7
2 Chemistry 20.00 28 7
3. Biology 20.00 16 4
4. Geology and Astronomy 11.50 36 9
5. Environment Science 26.00 140 35
Total 100.00

2. Project Work: Project work is supposed to be an important activity in enhancing learning capacity
of the students. Teachers are expected to provide project work to the students individually or in
group(s) to be finished in limited time frame. After finishing the project work(s), teachers are
expected to provide an opportunity to the students to present the process and result of the work
in front of the class. This activity helps the students for their further improvement.

3. Practice: It is well known saying that ‘learning without practice is meaningless.’ Therefore, practice
is one of the major components of a successful teaching learning process. In this activity, teachers
are expected to focus on the process of every finding rather than merely focusing on finding
answer(s). This activity provides an opportunity to the students for their further improvement
through the feedback from the teacher(s).

4. Activities: For the positive change in the concept, skill or attitude of the students, performance
of every activity is supposed to be an essential part for every teaching learning process. Teachers
are expected to make more and more involvement of the students in different activities that help
them to experience themselves for their further improvement.

5. Field Visit: Teaching learning process for Science and Environment equally focuses on field visit
according to nature of the subject matter in the lesson. On the one hand, students naturally might
feel boring and monotony in classroom teaching only. On the other hand, nature of the subject
matter in Science and Environment demands for field visit for the familiarity of the students
with the environment and the different activities happening around us. Therefore, teachers are
expected to take the students outside the classroom to break their monotony as well as make
them familiar with surroundings. Teachers are expected to make the frequent visit of the students
to the concerning areas with their active participation. This activity is supposed to be beneficial
to the students that provides them the chance of self observation and evaluation of the matter to
enhance their knowledge in concerning fields.

Evaluation of the Students

Evaluation process for Science and Environment is taken as the interlinked part with the teaching
learning process of this subject. Teachers are expected to emphasize the continuous evaluation
of the students in terms of achieving intended goals rather than merely focusing on the formal
written test. Observation of the students’ activities is supposed to be the best method of evaluation
in Science and Environment.

Teachers are expected to make the involvement of the students in continuous teaching learning
process to achieve the intended goals. The evaluation process is expected to be continued
along with teaching process to identify students’ problems that helps both teachers as well as
students for their further improvement. Teachers can evaluate the students by various means
like evaluation of the class work, homework, project work as well as the evaluation of the change
in behaviour of the students. Specially, in Science and Environment, teachers are expected to
evaluate the students’ procedural skills and keep the systematic record of the achievements.
The marking distribution of “Theoretical Exam” of 75 marks is divided as follows:

S. No. Subject Area Weighting (Marks)

1. Physics 25

2. Chemistry 15

3. Biology 15

4. Geology and Astronomy 5

5. Environment Science 15

Total 75

Similarly, following bases should be taken for practical evaluation.

• Drawing, labelling, collection of materials, observation, identification and explaining

characteristics

• Record of practical work
• Construction of materials and their uses
• Mini project work
• Viva voce

The marking distribution of “Practical Exam” of 25 marks is divided as follows:

S. No. Particulars Weighting (Marks)

1. Drawing/labelling/explaining characteristics 5

2. Record of practical work 5

3. Materials construction and their uses 5

4. Mini project work 6

5. Viva voce 4

Total 25

N.B.: The pass mark of “Theoretical Exam” is 30 and that of ‘Practical Exam’ is 10.



Unit Estimated teaching periods: Th Pr
5 1
1

Watch

Measurement

Objectives

After completing the study of this unit, students will be able to :

• introduce SI system of measurement and its use.
• calculate the surface area of regular and irregular objects.
• calculate the volume of regular and irregular objects.
• describe the method for measuring the volume of liquids and calculate their

volume.
• solve simple numerical problems related to area and volume.

Course of Study

• SI system of measurement
• Area of regular and irregular plane surfaces
• Volume of a liquid
• Volume of regular and irregular solids
• Simple numerical problems

Points to be Focused/Questions to be Discussed

• What is SI system?
• What are regular and irregular objects?
• How can you calculate the volume of a liquid?
• How can you calculate the area and volume of regular and irregular objects?

PHYSICS Oasis School Science and Environment - 7 1

1.1 Introduction

We need to measure various things in our daily life. A shopkeeper measures the
length of cloth to sell; a goldsmith measures the mass of gold, a milkman measures
the volume of milk to sell. To express the magnitude of a physical quantity, we
compare that quantity with the known standard quantity of the same kind. So,
the comparison of an unknown physical quantity with a known standard quantity
of the same kind is called measurement. We measure length, mass, volume, time,
temperature, etc. for different purposes. We measure various things to know their
exact magnitude. Measurement is very essential in our everyday life.

A shopkeeper measures cloth to sell A milkman measures milk to sell
Figure 1.1

When we measure length, we compare the unknown length with the known length
of the scale or metre rod or the measuring tape. Similarly, we compare the unknown
mass with the known mass while measuring the mass of a body. Quantities like
length, mass, time, speed, temperature, etc. which can be measured are called
physical quantities.
The quantities which can be measured are called physical quantities. For example,
length, mass, time, speed, area, volume, pressure, temperature, etc. We use various
devices to measure different physical quantities.

1.2 SI System

The measurement of a certain physical quantity should not differ from one place to
another. But the local systems of measurement vary from place to place. To make
measurements more scientific, convenient and uniform, the 12th General Conference
of Weights and Measures adopted an international system of units called SI units
in October 1960. SI system stands for "System International d' Unites' in French
which is the "International System of Units." This system of measurement is used
throughout the world.
In SI system, there are seven fundamental units of measurement which are shown
in the following table.

measurement /ˈmeʒəmənt/ - the comparison of an unknown physical quantity with the known
standard quantity of the same kind

2 Oasis School Science and Environment - 7 PHYSICS

S. No. Physical quantities SI units
Length
1. Mass metre m
2. Time kilogram
3. Temperature second kg
4. Electric current kelvin s
5. Luminous intensity ampere K
6. Amount of substance candela A
7. mole Cd.
mol.

Above mentioned units are accepted and used all over the world. These units of
measurement are also called standard units of measurement as they are accepted
throughout the world.

1.3 Length

The distance between any two points is called length. For example, the length of a
snake is 2.5m means that the distance between the tip of the head of the snake to its
tail is 2.5m. Breadth, thickness, depth, radius, height, diameter, etc. are also forms
of length. Scale, metre rod, inch tape, measuring tape, etc. are used to measure
length.

Inch tape Ruler Measuring tape

Figure 1.2

Measurement of Length
In SI system, length is measured in metre (m). It is also measured in centimetre,
millimetre, inch, foot, kilometre, mile, etc.
Various devices like scale, inch tape, measuring tape, metre rod, etc. are used
for measuring the length of an object. To measure the length, the given object is
compared to the standard length of a scale, metre rod or a measuring tape.
The scale of half-feet, i.e. 15cm or one foot, i.e. 30cm is used for measuring very short
length. A metre rod is used for measuring longer distance. Similarly, measuring tape
is used for measuring very long distance. Metre is the standard unit for measuring
length. We use geometry box scale to measure and draw small straight lines. We
use metre rod for measuring the length of clothes.

PHYSICS Oasis School Science and Environment - 7 3

Activity 1

• Collect some devices for measuring length. Observe them and write their
uses in the given table.

S. No. Name of devices Uses

1. Metre rod

2. Inch tape

3. Geometry box scale

4. One foot scale

5. Measuring tape

Which of the given devices is used for:
(i) measuring length of cloth?
(ii) drawing straight lines?
(iii) measuring length of a book?
(iv) measuring length of a room?
(v) measuring length of the road?

Units of Length

Metre (m) is the SI unit of length. There are many other units of length like millimetre
(mm), decimetre (dm), decametre (dam), hectometre (hm), kilometre (km), etc. The
various units of length and their relation is mentioned below:

10 millimetre = 1 centimetre
10 centimetre = 1 decimetre
10 decimetre = 1 decametre
10 decametre = 1 hectometre
10 hectometre = 1 kilometre

Among the above mentioned units of length, millimetre, centimetre, metre and
kilometre are widely used.

1 centimetre (cm) = 10 millimetre (mm)
1 metre (m) = 100 centimetre (cm)
1 kilometre (km) = 1000 metre (m)

Precautions While Measuring Length

We must take the following precautions while measuring the length of an object:

(i) The eye should be placed vertically above the end where the reading is to be taken.

(ii) The scale should be kept exactly along the length to be measured.

(iii) The ends of the ruler must not be worn out. In case the ruler is still used, any

other full mark on the ruler should be used.

4 Oasis School Science and Environment - 7 PHYSICS

Incorrect Correct Incorrect

Figure 1.3 Correct measurement

Incorrect (Wrong) measurement
Figure 1.4

Incorrect (Wrong) measurement Correct measurement
Figure 1.5

Activity 2

• The names of some objects are given in the following table. Use your scale
to measure them and fill the table with their measurement. Also, answer
the questions that follow:

Name of the objects Millimetre Measurement Metre
(mm) (m)
Centimetre
(cm)

1. Length of your pen

2. Height of your friend

3. Breadth of your science book

4. Length of your foot step

5. Length of your table

a. What can you conclude from this activity ?
b. When do you use a scale for measuring things ?
c. What do you use (metre or millimetre) to measure the length of your

bench ? Why ?

PHYSICS Oasis School Science and Environment - 7 5

Measuring the Diametre of a Spherical Object
Football, volleyball, cricket ball, tennis ball, orange, etc. are some examples of
spherical objects. To measure the diametre of a sphere or spherical object, the given
object is placed on top of a table or a plane surface. Two solid blocks are placed on
either side of the spherical objects as shown in the given figure.

Football Spherical object
Solid block

Ruler

Figure 1.6

The lower edges of the blocks are adjusted along the scale (ruler). The reading of the
position of two faces, one of each block touching the spherical object, is taken.
The diametre of the given spherical object is the difference between these two
readings.

Activity 3

• Collect different objects as given below and calculate their length, breadth
height/thickness and diameter.

S. No. Objects Length Breadth Height/ Diametre
Thickness
1. Football
2. Book
3. Chalk box
4. Pencil
5. Black board
6. Coin
7. Marble
8. Tennis ball

1.4 Mass

The quantity of matter contained in a body is called its mass. In MKS system, mass is
measured in kilogram (kg). It is also measured in milligram (mg), gram (g), quintal,
metric ton, etc.
Measurement of Mass
Generally, a beam balance is used to measure the mass of a body. A beam balance

6 Oasis School Science and Environment - 7 PHYSICS

consists of a horizontal beam supported at its centre and two similar pans suspended
at equal distances from the centre of the beam. Some shopkeepers use an ordinary
grocer’s balance for weighing sugar, rice, pulses, vegetables, fruits, etc. Goldsmiths
and scientists use a physical balance as it is more sensitive and accurate.

Beam balance Grocer's balance Physical balance
Figure 1.7

The object to be weighed should be placed on one of the pans of the beam balance.
The standard weights are kept on another pan until both the pans are balanced and
the beam is horizontal once again. Then the mass of the object is given by the sum
total of the standard weights used.
The mass of very light objects is measured in milligram (mg) and gram (g) whereas
the mass of heavy objects is measured in kilogram (kg). Similarly, the mass of very
heavy objects is measured in quintal and metric ton. The multiples and sub-
multiples of mass are given below.

1000 milligram = 1 gram k1g
1000 gram = 1 kilogram
100 kilogram = 1 quintal
10 quintal = 1 metric ton

Figure 1.8

Activity 4

To measure the mass of some objects
• Take a beam balance and keep it in a balanced condition.
• Take standard weights of kilogram, gram and milligram.
• Measure the mass of your book, notebook, calculator, tiffin box, pen,

pencil, etc. by using the beam balance.
• Note down the mass of each object in your practical notebook.

horizontal / hɒrɪˈzɒntəl / - going across and parallel to the ground 7
PHYSICS Oasis School Science and Environment - 7

1.5 Time

The interval between any two events is called time. The standard unit of time is second (s).

Measurement of Time
Time is measured in second, minute, hour, day, week, month, year, etc. A clock is
used for measuring time. Clocks are of different types, e.g. pendulum clock, quartz
clock, mechanical clock, wristwatch, etc.
Different types of clocks are given below:

Wristwatch Pendulum clock Stop watch
Figure 1.9

A short duration of time is measured in second, minute and hour. Similarly, a long
duration of time is measured in day, week, month and year. But very long duration
of time is measured in decade, century, millenium, etc. The multiples and sub-
multiples of a second are given below:

60 seconds = 1 minute

60 minutes = 1 hour

24 hours = 1 day

7 days = 1 week

365 days = 1 year

10 years = 1 decade

1.6 Regular and Irregular Objects

A variety of things are found in our surroundings. Among them, only a few have a
fixed geometrical shape while others don’t. The things having a fixed geometrical
shape are called regular objects. Examples: Football, drum, brick, chalk box, book,
pencil, etc.

Football Chalk box Pencil
Figure 1.10 PHYSICS

8 Oasis School Science and Environment - 7

The things which do not have a fixed geometrical shape are called irregular objects.
Examples: A piece of broken glass, a piece of stone, a piece of brick, leaf, a piece of
paper, etc.

Stone Piece of paper Leaf
Figure 1.11

Activity 5

• Collect any ten objects from your surroundings.
• Observe their shape and classify them as regular and irregular objects.

Regular objects Irregular objects
1. 1.
2. 2.
3. 3.
4. 4.
5. 5.

1.7 Area

The area of an object is the total space occupied by the plane surface of that object. The SI
unit of area is square metre (m²). It is also measured in mm², cm², km², etc.

Measuring Area of Regular Plane Surfaces

The area of regular plane surface can be measured by using different formulae.
Some of them are given below:

(i) Area of a rectangular object (A) = length (l) × breadth (b)

∴ A=l×b

(ii) Area of a square (A) = (length)²

∴ A = l²

(iii) Area of a circle (A) = π × (radius)² Where π = 22
7

∴ A = πr²

PHYSICS Oasis School Science and Environment - 7 9

Example 1
The length of a rectangular block is 25 cm and its breadth is 8cm. Calculate its
area.
Solution:
Given,
Length (l) = 25cm
Breadth (b) = 8cm
Area (A) = ?
According to the formula,

A = l×b
= 25 × 8
= 200cm²

∴ The area of the rectangular block is 200 cm².

Activity 6

• Take a ruler. Measure the length and breadth of your science text book.
• Calculate the area of the book.
• Also calculate the area of your classroom.

Measuring Area of Irregular Surfaces Figure 1.12
The area of an irregular plane surface can be measured by
using a graph paper. A graph paper is divided into equal-
sized squares of size 1 cm and 1mm.
The irregular object is placed on a graph paper and its
outline is drawn by using a sharp pencil. Then the number
of squares within the outline is counted. Similarly, the
number of squares that are more than half are counted but
the squares less than half are ignored. By adding the two
numbers, the approximate area of the given object is calculated.

Activity 7

• Pluck a leaf and calculate its surface area by using a graph paper.
• Measure the area of an irregular sheet of paper by using a graph paper.
• Take a piece of stone and calculate its area by using a graph paper.

approximate /əˈprɒksɪmət/ - almost correct or accurate, but not completely so

10 Oasis School Science and Environment - 7 PHYSICS

1.8 Volume

The total space occupied by an object is called the volume of that object. The SI unit
of volume is cubic metre (m³). It is also measured in mm³, cm³, ml, l, etc. The volume
of liquids is measured in ml, l, etc. and the volume of solids is measured in mm³,
cm³, m³, etc. In science laboratory, measuring cylinders are used for measuring the
volume of liquids.

1 ml = 1cm³ or 1cc (cubic centimetre)

1000 ml = 1l (litre)

1000 cm³ = 1l

Measuring the Volume of Regular Solids

The volume of regular solids can be calculated by the following formulae.

(i) Volume of a cubiod (V) = length (l) × breadth (b) × height (h)

∴ V = l × b × h h

(ii) Volume of a cube (V) = (length)³ lb

∴ V = l³ l r
h
(iii) Volume of a sphere (V) = 43π (radius)³ OA

∴ V = 4 πr³ [where π = 27 ] OA=r
3 7

(iv) Volume of a cylinder (V) = π × (radius)² × height (h)
∴ V = πr²h

Example 2

The radius of a volleyball is 6 cm. Calculate its volume.

Solution:

Given,
Radius (r) = 6 cm

Volume (V) = ?

According to the formula,

V = 43πr³ [∵ Volleyball is a sphere.]
4 22
= 3 × 7 × 6 × 6 × 6 = 905.14 cm³

∴ The volume of the volleyball is 905.14cm³.

PHYSICS Oasis School Science and Environment - 7 11

Measuring the Volume of Liquids
The volume of liquids is measured in millilitre (ml) or cubic centimetre (cc) and litre
(l). The volume of a liquid is measured by using measuring cylinder. Graduated
cylinder, milkman's measure, oil dealer's measure, pipette, burette, etc. are some
common devices for measuring the volume of liquids.

Graduated cylinder Milkman’s measure Oil dealer’s measure Pipette Burette
Figure 1.13

To measure the volume of a liquid, the liquid is poured into a measuring cylinder.
The reading at the surface of the liquid gives its volume.

Lower 100 Wrong
meniscus 90 Correct
Wrong
Water 80
70

60
50
40
30
20
10

Figure 1.14

Some liquids like water, oil, alcohol, kerosene, etc. form a concave surface in the
cylinder. While taking the reading of these liquids, the eye should be kept in the
level with the bottom of the meniscus to avoid parallax error. Similarly, liquids like
mercury form a convex surface in the cylinder. For such liquids, reading should be
taken from the upper meniscus.

Wrong 100 Wrong 100
Correct 90 90
80 Upper 80 Lower
Wrong meniscus Correct
70 70 meniscus
60 60

50 50
40 40
30 Mercury Wrong
30

20 20 Water
10
10

Figure 1.15

Measuring the Volume of Irregular Solids
The volume of irregular bodies, such as a piece of stone, brick, glass, etc. can be
measured by using a measuring cylinder. This method is based on the fact that the

parallax /ˈpærəlæks/ - the effect by which the position or direction of an object appears to change
when the object is seen from different positions

12 Oasis School Science and Environment - 7 PHYSICS

volume of an irregular solid is equal to the volume of the water displaced by it when
it is immersed in water. When an irregular solid body is immersed in water, the
level of water rises. The rise in the volume of the water is the volume of that object.
This method can be used to calculate the volume of those irregular solids which
sink in water, do not dissolve in water and can enter into a measuring cylinder.

Experiment 1

Object: To measure the volume of a piece of stone

Materials required: Measuring cylinder, water, thread, a piece of stone

Procedure

Bring a measuring Thread
cylinder and fill
it partially with 50
water. Note down 50 V2
the level of water in 40 Volume of stone
the cylinder. It is the 30 40 Water
initial reading. While 20 30
V1 10 20 Measuring cylinder
0
recording the level Measuring Figure. 1.16 10 Stone
of water, keep the cylinder 0
eye in the level with Water
the bottom of the
meniscus to avoid
parallax error. Let it
be V1.

Now, tie the given piece of stone with a thread and immerse it into water in the
measuring cylinder. The level of water rises in the cylinder. Note down the level of
water carefully. It is the final reading. Let it be V2.

Observation

Initial volume of water in the cylinder (V1) = 30 ml
45 ml
Final volume of water after immersing the stone (V2) = V2 - V1
45 ml – 30 ml
∴Volume of the water displaced (V) =

=

= 15 ml

∴ Volume of the stone = Volume of the water displaced = 15 ml

Precautions
i. While taking the reading, measuring cylinder should be placed on a horizontal

surface and the water in the cylinder should be at rest.

immerse /ɪˈmɜːs/ - to put sth/sb into a liquid so that it is completely covered

PHYSICS Oasis School Science and Environment - 7 13

ii. The liquid surface in a measuring cylinder is curved. It is called meniscus.
The reading should be taken at the bottom of the meniscus.

Activity 8

• Take a piece of stone and calculate its volume by using a measuring
cylinder.

• Take a piece of a brick and calculate its volume by using water displacement
method.

Summary

• The comparison of an unknown physical quantity with the known standard
quantity of the same kind is called measurement.

• The quantities which can be measured are called physical quantities.
• The distance between any two points is called length.
• Scale, metre rod, inch tape, measuring tape, etc. are used to measure length.
• To measure the length, the given object is compared to the standard length of

a scale, metre rod or a measuring tape.
• Metre is the standard unit for measuring length.
• The things having a fixed geometrical shape are called regular objects.
• The things which do not have a fixed geometrical shape are called irregular

objects.
• The quantity of matter contained in a body is called its mass.
• The interval between any two events is called time.
• The total space occupied by a body is called its volume.

Exercise

1 Choose the best answer from the given alternatives.

a. The comparison of an unknown physical quantity with a known standard
quantity is called ________

(i) unit (ii) magnitude

(iii) estimation (iv) measurement

b. SI system stands for ________

(i) System International d' Unites (ii) International System of Units

(iii) System of International Units (iv) System de International

14 Oasis School Science and Environment - 7 PHYSICS

c. The SI unit of temperature is ________

(i) degree Celsius (ii) kelvin

(iii) degree Fahrenheit (iv) degree kelvin

d. The total amount of matter present in a body is called ________

(i) volume (ii) mass

(iii) length (iv) diametre

e. The area of an irregular object can be measured by using a

(i) measuring cylinder (ii) measuring tape

(iii) graph paper (iv) ruler

2 Tick (√ ) the correct statement and cross (×) the incorrect one.

a. SI system is used throughout the world.

b. Kilometre is the SI unit of length.

c. Book and football are regular objects.

d. The volume of liquids is measured in cubic metre.

e. Liquids like mercury form a convex surface in the measuring cylinder.

3 Fill in the blanks with appropriate words.

a. All measurable quantities are ……………
b. In SI system, electric current is measured in ……………
c. The total space occupied by a body is called ……………
d. …………… is the SI unit of time.
e. The volume of irregular solids is measured by using ……………

4 Answer the following questions.

a. Define measurement. What is SI system of measurement?
b. Why was the SI system developed?
c. What is length? Write its SI unit.
d. How is the diametre of a spherical body measured? Explain.
e. What is the mass of a body? Write any three units of mass.
f. What is time? Write down its SI unit.
g. Define regular and irregular objects with any two examples of each.
h. What is area? Write down any two units of area.
i. How is the area of a circle measured?
j. What is volume? Write its SI unit.
k. How is the volume of an irregular solid measured? Explain.

5 Identify the given measuring devices and write down the major use of each.

PHYSICS Oasis School Science and Environment - 7 15

(a) (b) (c)

6 Numerical Problems

a. The length of a book is 20cm and its breadth is 12cm. Calculate its area.
[Ans: 240cm²]

b. The length of a square is 5m. Calculate its area. [Ans: 25m²]

c. The radius of a coin is 0.8cm. Calculate its area. [Ans: 2.01 cm²]

d. Study the given figures and calculate their volume.

3m

8m 4m 5m
(a) 2m (b)

[Ans: (a) 64 m³ (b) 141. 42 m³]

e. The radius of a football is 5cm. Calculate its volume. [Ans: 523.8 cm³]

7 Study the given figure and calculate the volume of the piece of stone.

50 ml

35 ml

(a) (b)

16 Oasis School Science and Environment - 7 PHYSICS

Unit Estimated teaching periods: Th Pr
4 1
2
Moving car

Force and Motion

Objectives

After completing the study of this unit, students will be able to :

• explain various types of force.
• introduce distance and displacement.
• define speed and velocity and explain uniform and non-uniform velocity.
• define acceleration.
• solve simple numerical problems related to force and motion.

Course of Study

• Types of force
• Distance and displacement
• Scalar and vector quantity
• Speed and velocity
• Acceleration

Points to be Focused/Questions to be Discussed

• What is force?
• What are the different types of force?
• What are distance and displacement?
• What are scalar and vector quantities?
• What are speed and velocity?
• What is acceleration?

PHYSICS Oasis School Science and Environment - 7 17

2.1 Introduction

We do a number of works in our daily life. For example, pushing a cart, pulling a
door, lifting water from a well, kicking a football, etc. We open a door by pushing
its handle and we close the door by pulling the handle. Similarly, we pull a bucket
upwards while lifting water from a well and we push a wheel barrow while
carrying load by using it. It means that to move an object, it has either to be pulled
or pushed. A pull or push of a body is called force. So, force can be defined as the
pull or push which changes or tends to change the position (either rest or motion)
of a body. Force can change the shape and size of a body. Force can change the
direction of a moving object. Similarly, force can stop a moving body and it can
change the speed of a moving body. The SI unit of force is newton (N) and the CGS
unit is dyne.
In our everyday life, force is used for pushing, pulling, lifting, pressing, twisting,
stretching and so on. A force has magnitude as well as direction. It is an external
agent which is applied on a body from outside. It is almost impossible to do various
works without force.

Types of Force

There are eight common types of force. They are as follows:
1. Pulling Force

A force is used when we pull a door to close it. A force is used when we pull a
bucket of water from the well. A force is used when we pull a cart. Similarly, a force
is used while reducing the speed of a moving body. We can pull lighter objects
more easily than heavier ones. A force must be applied to pull an object. Such force
is called pulling force. Thus, pulling force can be defined as the force that pulls or
tries to pull an object.

Pulley

(a) (b)
Figure 2.1: Pulling force PHYSICS

magnitude /ˈmæɡnɪtjuːd/ - the value of something

18 Oasis School Science and Environment - 7

Activity 1

• Bring a small wooden block and pull it on a table by using a spring balance.
• Note down the reading shown by the spring balance.
• Bring another heavy wooden block and pull it on the same table. Note

down the reading shown by the spring balance.
• What is the difference in the force applied when you pull a small wooden

block and a heavy one ?
• The magnitude of the force shown by the spring balance is the force

required to pull those wooden blocks. Such force is called pulling force.

2. Pushing Force

We push the door to open it. We push a cart
to carry a load on it. Have you seen a fruiter
pushing a cart? Kicking a football, throwing a
stone, squeezing tooth paste, etc. are the acts of
pushing. In the given picture, a man is pushing
a car by applying a force. Such force is called
pushing force. The fallen leaves of a tree or
kites fly away with wind because the force of Fig. 2.2: Pushing force

wind pushes them. Thus, pushing force can be defined as the force that pushes or
tries to push an object. In other words, the force which is used to push an object is
called pushing force.

3. Centripetal Force and Centrifugal Force

When an object moves in a circular motion, it Orbit

experiences two types of force. One force acts towards Axis
the centre which is called centripetal force and another
force acts away from the centre which is called Centripetal
centrifugal force. When a cyclist turns a bicycle in a force
circular path, s/he experiences a force towards the Velocity
centre. Such force is called centripetal force. Owing to
Fig. 2.3

this force, the cyclist bends towards the centre. Similarly, when a bus is moving in a
bend, the passengers experience a force which acts away from the centre. Such force
is called centrifugal force.

When an object moves in a circular path, the force that acts towards the centre
is called centripetal force and the force that acts away from the centre is called
centrifugal force. In the absence of centripetal force, a vehicle or bicycle cannot
be bent in curved roads. When there is a balance between centripetal force and
centrifugal force, an object moves in a circular motion.

It is to be noted that to maintain the circular rotation of a body, both centripetal and
centrifugal force should be equal. If there is more centripetal force, a moving body
bends towards the centre and if there is more centrifugal force, the moving body
goes away from the centre of the circular path.

PHYSICS Oasis School Science and Environment - 7 19

Activity 2

• Take a small rubber ball and tie it with a rope of
length about 1.5 m. Hold the ball in your hand firmly
and rotate it in a circular path as shown in the given
figure. What do you experience while rotating the
ball?

You can observe that the ball rotates continuously in Figure 2.4

its circular path due to the force exerted by your hand
to the rope which attracts the ball towards the centre of the circular path.
This force is called centripetal force.

While rotating the ball in the circular path, you feel a force that tries to
pull the ball away from your hand. Suddenly if you release the rope, the
ball falls away from the centre. This force which tries to pull the ball away
from the centre is called centrifugal force.

4. Muscular Force
When we lift a stone from the ground, we apply a force. The animals like ox, horse
and camel use a force to pull a cart. Similarly, an elephant uses a force to pull the
heavy logs of wood. Such force is exerted by the muscles of human beings and
animals. Thus, muscular force can be defined as the force exerted by the muscles
of a body. Human beings and animals exert muscular force to perform various
activities like running, jumping, swimming, pulling, pushing, kicking, lifting,
climbing, twisting, pressing, etc.

Figure 2.5 Sun Earth
5. Gravitational Force
Figure 2.6
The planets of the solar system revolve around PHYSICS
the sun. A kind of force exists between the planets
and the sun. Similarly, the moon revolves around
the earth due to the presence of a force. The force
which acts between any two objects because of
their masses is called gravitational force. The

20 Oasis School Science and Environment - 7

gravitational force holds the earth in its orbit around the sun. If we throw a ball
upwards, it returns to the ground due to the force of attraction exerted by the earth
on all bodies towards its centre which is called gravity. Hence, gravity is defined as
the force of attraction exerted by the earth which pulls all objects towards its centre.
It is the gravitational force between the earth and the moon which holds the moon
in its orbit around the earth. Similarly, fruits fall on the ground due to the effect of
earth’s gravity.

Activity 3

• Take a rubber ball and throw it towards the sky. What do you observe
after a while? After sometime, the ball returns to the ground. What is the
reason behind it?
The ball returns to the ground due to the force of attraction of the earth.
The earth pulls the ball towards its centre and hence the ball is pulled
towards the ground. This force of attraction of the earth by which it pulls
every object towards its centre is called gravity.

6. Magnetic Force
If we bring a magnet near the iron nails, they are pulled towards the magnet. A
magnet attracts the things like iron, cobalt, nickel, etc. Those things are attracted
towards the magnet due to the force exerted by the magnet. The force exerted by a
magnet is called magnetic force. However, the magnetic force cannot be exerted on
non-magnetic substances.

N S Iron nails

Figure 2.7

Magnetic force is widely used in our day to day activities. The magnetic strips help
to keep the door of refrigerator closed tightly. Similarly, magnetic force is used to
remove iron dust from the eyes. It is also used to separate magnetic substances from
the non-magnetic substances and to lift heavy loads of iron and steel in industries.

Activity 4

• Spread some iron clips on a table and bring a bar magnet near the clips.
• The clips will move towards the pole of the magnet from their original

position. The clips get drawn towards the magnet due to the influence of
magnetic force. So, the force exerted by a magnet is called the magnetic
force.

strip /strɪp/ - a long narrow piece of something 21
PHYSICS Oasis School Science and Environment - 7

Activity 5

• Bring a bar magnet and suspend it freely in a wooden stand as shown in
the given figure. After some time, the magnet will be still in the north-
south direction.

S Thread NN S
NN S Attractive S
Magnet Repulsive
force force
N
S N
S

Figure 2.8: (a) Showing magnetic force of repulsion (b) Showing magnetic force of attraction

• Take another bar magnet and bring the south pole of this magnet near the
south pole of the suspended magnet. It can be seen that the suspended
magnet moves away as if some force were exerted on it. Such force is called
the magnetic force of repulsion. Please note that like poles of magnet repel
each other.

• Now, take the north pole of the bar magnet near the south pole of the
suspended magnet. The south pole of the suspended magnet moves
towards the north pole of the other magnet. Such force is called the
magnetic force of attraction. Please note that unlike poles of magnet attract
each other.

7. Electrostatic Force
If we bring a plastic comb near the pieces of paper, nothing happens. But when we
rub the comb in dry hair several times and bring it near the pieces of paper, they are
drawn towards the comb. It is because of the development of charges on the comb
by rubbing it in dry hair. The force exerted by an electrically charged object is called
electrostatic force. An electrically charged object can exert an electrical force on an
uncharged object or another charged object.

Wall

+
+
+
+
+ Balloon
+
+

Figure 2.9 PHYSICS

22 Oasis School Science and Environment - 7

Activity 6

• Take a piece of paper and tear it into many small pieces. Keep those pieces
on a table.

• Bring a plastic comb near the pieces of paper and observe it. Nothing
happens.

• Now, rub the plastic comb for several times in dry hair and bring it near
the pieces of paper. What happens?

Comb Comb

No attraction Attraction
Pieces of paper
Pieces of paper
(a)
(b)
Figure 2.10

The pieces of paper are attracted to the comb. In this case, the pieces of
paper get drawn towards the plastic comb due to the development of
electrostatic charges while rubbing the comb on dry hair. So, the force
exerted by electrostatic charge is called electrostatic force.

8. Frictional Force
When we roll a football on a ground, it slows down gradually and stops after
covering some distance without some force being applied on it. Whenever the
surface of an object slides over that of another, each exerts a force on the other
which opposes the motion of the body. Such force is called frictional force. The
force which opposes the motion of one body over another when they are in close
contact is called frictional force. The magnitude of frictional force depends on the
extent of roughness or smoothness of the surfaces in close contact. If the surfaces
are quite rough, the friction will be more. But if the surfaces in close contact are
smooth, the friction will be less. Frictional force also depends on the weight of the
sliding body on the surface of another body. Frictional force increases as the weight
of the body sliding on the surface increases and vice–versa.
It is said that when two surfaces come in direct contact, their molecules attract each
other due to the electrostatic force. They stick to each other at a microscopic level
which opposes the motion of a body over the other. So we have to apply sufficient
muscular force to overcome the friction.
Friction plays a significant role in our day to day activities. It provides a grip or
hold which keeps the surfaces together. In some cases, friction is useful and in
other cases it is harmful and needs to be reduced. Therefore, friction is called
a necessary evil.

grip /ɡrɪp/ - an act of holding something tightly 23
PHYSICS Oasis School Science and Environment - 7

Activity 7

• Bring a ball and roll it with equal force once on a rough floor and later on
a smooth cemented floor. In which case does the ball stop earlier? Why ?
What can you conclude from this activity?
The ball on the rough floor stops earlier than that on the smooth, cemented
floor. Thus, the frictional force is greater on the rough surface.

Effects of Friction
When a body moves on the surface of another body, the frictional force between
them has the following effects:
i. Friction opposes motion: In the above activity, we have noticed that a rolling

ball stops after some time due to the friction between the ball and the surface
of the floor. So friction always opposes the motion of a body moving on the
surface of another body.
ii. Friction produces heat: When a body moves on the surface of another body,
both surfaces rub together. As a result, heat is produced.

Activity 8

• Rub your palms together for a while. What do you feel ? After rubbing the
palms for a few seconds, they become warm. This activity shows that the
friction between any two bodies produces heat.

iii. Friction causes wear and tear: When we walk on the road, there is a friction
between the sole of the shoes and the surface of the road. Due to the friction
between them, our shoes wear out gradually. There are many moving parts
in machines, automobiles, etc. which rub against each other regularly. The
rubbing parts of the machines or automobiles wear out gradually. So friction
causes wear and tear of the parts which rub against each other.

Activity 9

• Compare the sole of your old school shoes with that of new pair of shoes.
If possible, compare the old tyre of your bicycle with the new tyre. What
do you observe? You will notice that the old ones are worn out. What can
you conclude from this activity?

24 Oasis School Science and Environment - 7 PHYSICS

Advantages of Frictional Force

(i) The friction between the ground and our feet enables us to walk on the
ground without falling or slipping. It would not be possible to walk if there
were no friction at all. It becomes difficult to walk on a slippery ground or
polished floor because the frictional force is very less between such surface
and our feet. Similarly, friction enables automobiles to move on the road
without skidding.

(ii) The friction between a paper and the tip of a pen enables us to write and
draw on paper.

(iii) Friction enables us to apply brakes and stop a moving vehicle. The friction
between the brake shoe and the rim of the wheel prevents the wheel from
moving and the vehicle stops. The brakes of vehicles would not work at all if
there was no friction.

(iv) Friction enables us to cut wood with saw, to burn a match stick, to climb a
tree without sliding, etc.

(v) Friction enables the nails and the screws to hold things together and nuts to
be tied.

Disadvantages of Frictional Force

(i) Friction gradually tears away the soles of our shoes and tyres of vehicles.
(ii) In machines and automobiles, the moving parts get heated up due to the

friction and undergo wear and tear.
(iii) Friction reduces the efficiency of machines.
(iv) Friction produces heat which gradually damages the machine.
(v) Friction slows down the motion of moving parts of a machine.
(vi) Friction causes the wastage of energy. A large amount of fuel is wasted

overcoming the friction in machines.
Methods of Increasing Friction

(i) Grooves are made on the tyres of vehicles which help in increasing the friction
with the road. It prevents the tyres from slipping on the road.

(ii) Spikes are made on soles of shoes of athletes and mountaineers. The players
and athletes have to turn and run fast. So, greater friction is required between
the soles of their shoes and the ground to prevent slipping. Therefore, spikes
are made on the soles to increase friction.

Spikes

Grooves

Figure 2.11 Oasis School Science and Environment - 7 25
PHYSICS

(iii) The belts of machines are made up of special materials to increase the friction.
It helps to drive the wheels of machines properly without slipping off the
wheels.

Methods of Reducing Friction
(i) The friction between the movable parts of a machine can be reduced by

applying lubricants like oil or grease. When lubricants are applied to the
moving parts of machines, their rubbing surfaces become smooth. As a result,
the friction between them is reduced.
(ii) Friction can be reduced by polishing of rough surfaces. For example, carrom
boards are highly polished to reduce friction.
(iii) In moving parts of a machine, friction can be reduced by using ball bearings
between the moving surfaces.
(iv) Friction can be reduced by using wheels to move objects. For example,
suitcase is provided with wheels to reduce friction as we pull it. Similarly,
vehicles are fitted with wheels to reduce the friction with the road.
(v) The bodies of aeroplane, ship, boat, submarine, etc. are made streamlined to
reduce friction.

Activity 10

• Take a wooden log and tie it with a rope.
• Drag the wooden log with the help of the rope on the ground.
• Now, put two pieces of cylindrical wood below the log and drag it on the

ground.
• How much effort do you apply in both of the cases?
• What do you conclude from this activity?

2.2 Distance and Displacement

Distance

The distance covered by a moving body is the length of path covered by the body.
In SI system, distance is measured in metre (m). It is a scalar quantity as it has only
magnitude but no direction.

C 2 km D

1 km 1 km

2 km B E
A

Suppose a bus moves from A to B (2 km), B to C (1km), C to D (2km) and D to E

submarine /ˈsʌbməriːn/ - a ship that can travel under water

26 Oasis School Science and Environment - 7 PHYSICS

(1km). Then, the total distance covered by the bus is AB + BC + CD + DE
= 2km + 1km + 2km + 1km = 6km
Therefore, the distance covered by the bus is 6km.
Displacement
The shortest distance covered by a moving body in a fixed direction is called
displacement. In SI system, it is measured in metre (m). Displacement is a vector
quantity as it has both magnitude and direction.

C 2 km D

1 km 1 km

2 km B E
A

When a bus moves from A to E, the distance covered by the bus is 6 km but the
displacement is only 4km, i.e. A to B (2km) and B to E (2km). Please note that the
shortest distance from A to E is 4km.

2.3 Scalar and Vector Quantity

Scalar quantity
The physical quantity having only magnitude but no direction is called a scalar
quantity. A scalar quantity is described completely by its magnitude. Examples:
mass, length, time, distance, area, volume, etc.
Vector quantity
The physical quantity having both magnitude and direction is called a vector
quantity. It is described completely by its magnitude as well as direction. Examples:
displacement, velocity, acceleration, force, weight, etc.

Differences between Vectors and Scalars

S.N. Vectors S.N. Scalars

1. Vectors have both magnitude 1. Scalars have only magnitude.
and direction.

2. The sum of vectors may be 2. The sum of scalars is always
positive or zero or negative. positive.

3. Vectors are added by the rules 3. Scalars are added by the rules of
of vector algebra. simple algebra.

acceleration /əkseləˈreɪʃən/ - the rate of change in velocity of a moving body

PHYSICS Oasis School Science and Environment - 7 27

2.4 Speed and Velocity

Speed: The speed of a body is defined as the distance covered by the body per unit
time. It can be calculated by the given formula:

Speed (v) = Distance travelled (s)
Time taken (t)

Speed is a scalar quantity. The SI unit of speed is metre per second (m/s). The speed
of a fast moving object is more and that of a slow moving object is less. The speed
of fast moving objects like car, motorcycle, aeroplane, train, bus, etc. is expressed in
kilometre per hour (km/h).

Example 1
A car travels a distance of 300 km in 4 hours. Calculate the speed of the car.

Solution:
Given,

Distance covered (s) = 300 km

Time taken (t) = 4 h
Speed (v) = ?

According to the formula,

v= s = 300 km = 75km/h
t 4 h

∴ The speed of the car is 75 km/h.

Velocity

The velocity of a body is the distance travelled by the body per unit time in a fixed
direction. Velocity is the displacement per unit time. The velocity of a body is
calculated by the given formula:

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

Velocity is a vector quantity. The SI unit of velocity is metre per second (m/s).

Example 2
A car is moving in a straight line towards east. If it covers a distance of 3km in 5
minutes, calculate the velocity of the car.
Solution:

Given,

Displacement (s) = 3 km

= 3 × 1000m [∵ 1km = 1000m]
= 3000 m.

28 Oasis School Science and Environment - 7 PHYSICS

Time taken (t) = 5 minutes
= 5 × 60 seconds [∵ 1 minute = 60 seconds]
= 300s

Velocity (v) = ?

According to the formula,

v= s = 3000 = 10m/s
t 300

∴ The velocity of the car is 10m/s.

Differences between Speed and Velocity

Speed Velocity

1. The speed of a body is the distance 1. The velocity of a body is
covered by the body per unit time. the distance covered by the
body per unit time in a fixed
direction.

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

Uniform and Variable Velocity

Uniform Velocity
If a body covers equal displacement in equal interval of time, the velocity is called
uniform velocity. It is also called constant velocity.
In the following figure, a car is moving from east to west. The car covers 12 m in
every one second. So, the velocity of the car is uniform velocity.

12m/s 12m/s 12m/s 12m/s
West
East

0s 1s 2s 3s 4s
Figure 2.12: Showing uniform velocity

Variable Velocity
If a body does not cover equal displacement in equal interval of time, the velocity
of the body is called variable velocity.
In the following figure, a car is moving from east to west. The car covers different
distances in every one second. So, the velocity of the car is variable velocity.

3m/s 6m/s 4m/s 8m/s

East West

0s 1s 2s 3s 4s
Figure 2.13: Showing variable velocity

PHYSICS Oasis School Science and Environment - 7 29

Average Velocity

The mean of initial velocity and final velocity of a moving body is called average
velocity. It can be calculated by the given formula:

Average velocity (vav) = Initial velocity (u) + Final velocity (v)
2

∴ Average velocity (vav) = u+v
2

2.5 Acceleration

Let us consider the example of a school bus which starts from rest at stop ‘A’. When
it starts moving, its velocity increases until it attains a uniform velocity. As stop ‘B’
approaches, its velocity gradually decreases and finally becomes zero at stop ‘B’.
This change in velocity of the bus is described in terms of acceleration. Therefore,
the rate of change in velocity of a body is called acceleration. It can be calculated by
the following formula:

Acceleration (a) = Change in velocity
Time taken

= Final velocity (v) – Initial velocity (u)
Time taken (t)

∴ a = v–u
t

The SI unit of velocity is metre per second and that of time is second. Therefore, the
SI unit of acceleration is metre per second per second (m/s/s) or metre per second
square (m/s²). Acceleration is a vector quantity. While expressing acceleration, we
should specify both magnitude and direction.
When a body moves with a uniform velocity, its acceleration is zero as there is no
change in velocity. Again, when the velocity changes uniformly with time, then the
acceleration of the body is said to be uniform.
Please note
If a body starts from rest, its initial velocity, (u) = 0.
If a body comes to rest or stops, its final velocity, (v) = 0.
If a body moves with uniform velocity, its acceleration, (a) = 0.
Example 3
A motorcycle starts from rest and gains a velocity of 20m/s towards west after 10
seconds. Calculate its acceleration.
Solution
Given,
Initial velocity (u) = 0 [∵ The motorcycle starts from rest.]

30 Oasis School Science and Environment - 7 PHYSICS

Final velocity (v) = 20 m/s

Time taken (t) = 10s

Acceleration (a) = ?

According to the formula,
v–u
a = t

= 201–0 0 = 2 m/s²

∴ The acceleration of the motorcycle is 2m/s².

Activity 11

• Take a measuring tape, a stop watch and some lime powder.
• Make a track of 100m in your playground.
• Make a group of five students and run along the track one by one.
• Record the time taken by each student to cover the distance of 100m.

Calculate the velocity of each student.

Summary

• Force can be defined as the pull or push which changes or tends to change the position
(either rest or motion) of a body.

• Pulling force can be defined as the force that pulls or tries to pull an object.
• Pushing force can be defined as the force that pushes or tries to push an object.
• When an object moves in a circular path, the force that acts towards the centre is called

centripetal force and the force that acts away from the centre is called centrifugal force.
• Muscular force can be defined as the force exerted by the muscles of a body.
• The force which acts between any two objects because of their masses is called

gravitational force.
• Gravity is defined as the force of attraction exerted by the earth or a heavenly body

which pulls all objects towards its centre.
• The force exerted by a magnet is called magnetic force.
• The force exerted by an electrically charged object is called electrostatic force.
• The force which opposes the motion of one body over the other when they are in close

contact is called frictional force.
• The distance covered by a moving body is the length of path covered by the body.
• The shortest distance covered by a moving body in a fixed direction is called

displacement.
• The speed of a body is defined as the distance covered by the body per unit time.
• The mean of initial velocity and final velocity of a moving body is called average

velocity.
• The rate of change in velocity of a body is called acceleration.

PHYSICS Oasis School Science and Environment - 7 31

Exercise

1 Choose the best answer from the given alternatives.

a. The CGS unit of force is _________.

(i) metre (ii) dyne
(iii) pascal (iv) newton

b. The force used by a horse to pull a cart is _________.

(i) muscular force (ii) frictional force
(iii) pushing force (iv) magnetic force

c. The force which helps to move an object in a fixed orbit is _________.

(i) centripetal force (ii) centrifugal force

(iii) frictional force (iv) both (i) and (ii)

d. The force which is exerted by a charged body is _________.

(i) fricional force (ii) magnetic force
(iii) electrostatic force (iv) muscular force

e. Burning of a meteor in atmosphere is _________.

(i) gravitational force (ii) electrostatic force

(iii) frictional force (iv) pulling force

f. Aeroplanes have a special streamlined body to reduce_________.

(i) frictional force (ii) gravitational force
(iii) magnetic force (iv) centripetal force

g. Polishing of carrom boards decreases _________.

(i) heat (ii) friction

(iii) motion (iv) force

h. The shortest distance covered by a body in a fixed direction is called _________

(i) distance (ii) velocity
(iii) displacement (iv) speed

i. Which of the following is a vector quantity?

(i) force (ii) time

(iii) speed (iv) length

j. The SI unit of acceleration is _________.

(i) ms² (ii) m/s²
(iii) km/s (iv) km/s²

2 Tick (√) the correct statement and cross (×) the incorrect one.
a. Force cannot change the position of an object.
b. An apple falling from a tree is an example of gravitational force.

32 Oasis School Science and Environment - 7 PHYSICS

c. Friction helps us to walk more easily on a slippery surface.
d. The SI unit of force is dyne.
e. Friction is always disadvantageous.
f. A charged comb attracts small pieces of paper due to electrostatic force.
g. The force acting towards the centre is called centrifugal force.
h. Scalar quantity has magnitude but no direction.

i. When a body moves with a uniform velocity, its velocity is zero.

3 Fill in the blanks with appropriate words.

a. The force exerted by a magnet is called ................. force.
b. A mango falls towards the earth’s surface due to ..................
c. The force that acts away from the centre is called ............... force.
d. When we lift a heavy object, we use ................. force.
e. An electrically charged body exerts ................. force.
f. Frictional force increases as the ................. in the weight of the sliding body.
g. ................. can be reduced by using oil and grease.
h. The length of the path covered by a body is called .................
i. The SI unit of velocity is .................
j. Acceleration is a ................. quantity.

4 Answer the following questions.
a. What is force? Write down its SI unit.
b. Name any five types of force.
c. What is pulling force? Give any two examples.
d. What is centripetal force? Give one example of centrifugal force.
e. What is gravitational force? Give two examples.
f. What is magnetic force? How is this force useful to us?
g. What is electrostatic force? Give one example.
h. What is frictional force? Write down any two effects of friction.
i. Mention any three advantages and two disadvantages of friction.
j. “Friction produces heat.” Write one disadvantage of this property.
k. Define distance and displacement with illustration.
l. What are vector and scalar quantities? Give any three examples of each.
m. What is meant by uniform velocity? Give one example.
n. What is acceleration? Write down its formula and SI unit.

PHYSICS Oasis School Science and Environment - 7 33

5 Differentiate between:

a. Pushing force and Pulling force

b. Centripetal force and Centrifugal force

c. Gravitational force and Magnetic force

d. Distance and Displacement

e. Scalar and Vector quantity

f. Speed and Velocity

6 Match the following: B
A

Muscular force Force acting towards the centre
Frictional force Force exerted by a charged body
Gravitational force Force exerted by muscles
Magnetic force Force acting to pull a body
Electrostatic force Force acting between any two objects
Pulling force Force which opposes motion
Centripetal force Force exerted by a magnet
Force acting away from the centre

7 Give reason:

a. The earth revolves around the sun.
b. If we drop a ball from the roof, it falls downwards.
c. If we rub a plastic comb in hair, it attracts dust particles.
d. A magnet attracts an iron nail.
e. A piece of chalk wears out as it is used on a blackboard.
f. Oil is applied to the moving parts of a machine.
g. Velocity is called a vector quantity.
h. Time is called a scalar quantity.

8 Numerical Problems

a. A bus covers a distance of 600 metres in 30 seconds. Calculate the speed of
the bus. [Ans: 20m/s]

b. A car is moving with the velocity of 25m/s. Calculate the distance covered by
the car in 12 seconds. [Ans: 300 m]

c. The initial velocity of a bus is 15m/s. It gains a velocity of 25m/s in 5 seconds.
Calculate the average velocity and acceleration of the bus.

[Ans: 20m/s, 2m/s²]

d. A motorcycle starts from rest. If it gains an acceleration of 2m/s² in 5 seconds,
calculate the final velocity. [Ans: 10m/s]

34 Oasis School Science and Environment - 7 PHYSICS

Unit Estimated teaching periods: Th Pr
4 1
3
Scissors

Simple Machine

Objectives

After completing the study of this unit, students will be able to :

• identify various types of simple machines (lever, pulley, wheel and
axle, inclined plane, screw and wedge) and define each of them.

• explain the utilities of simple machines and utilize them in their
daily life.

Course of Study

• Introduction to simple machines
• Types of simple machines (lever, pulley, wheel and axle, inclined

plane, screw and wedge)
• Types of lever
• Utilities of simple machines
• Importance of simple machines

Points to be Focused/Questions to be Discussed

• What are simple machines?
• Why are simple machines used?
• What are different types of simple machines?
• What are the uses of simple machines?
• What is the importance of simple machines?

PHYSICS Oasis School Science and Environment - 7 35

3.1 Introduction

A variety of machines are used in our daily life to make our lives easier and
comfortable. A knife is used to chop vegetables. An axe is used to split firewood.
Fire tongs are used to catch a burning coal. Scissors are used to cut paper. A pulley
is used to lift water from well. Similarly, a screw driver is used to unscrew or tighten
nuts and so on. These machines are simple in structure and help us to work easily
and efficiently.

Scissors Beam balance Nail cutter
Figure 3.1: Some simple machines

Devices like sewing machine, motorcycle, truck, car, etc. are more complicated in
their structure and operation. Their mechanism is very complex which is difficult
to understand and operate. Such machines are called complex machines. Those
machines contain a large number of simple machines of different types.

Sewing machine Motorcycle Car
Figure 3.2: Some complex machines

The machine which is simple in structure and makes our work easier, faster and
more convenient is called a simple machine. Forceps, punching machine, knife,
crowbar, wheel-barrow, staircase, nailcutter, axe, pulley, etc. are some examples of
simple machines. We use simple machines to multiply the force applied, change the
direction of force applied and to apply force at a convenient point.

3.2 Types of Simple Machines

On the basis of structure and use, there are six main types of simple machines. They are:

1. Lever 2. Pulley

3. Wheel and axle 4. Inclined plane

5. Screw 6. Wedge

1. Lever

A lever is a rigid bar which moves freely about a fixed point called the fulcrum.
In a lever, effort is applied at one point to lift a load on another end. A lever has
a fulcrum, effort distance and load distance. The fixed point about which a lever
can rotate freely is called fulcrum. The distance between the fulcrum and the point

36 Oasis School Science and Environment - 7 PHYSICS

at which effort is applied is called effort arm. Similarly, the distance between the
fulcrum and the point at which the load acts is called the load arm (fig. 3.3).

When a lever is in balanced Effort Load

condition, input work is Effort arm Load arm
always equal to the output
work. The work done on a
machine is called the input
work whereas the useful
work done by the machine is
called the output work. For
Figure 3.3: Parts of a typical lever

an ideal lever, the input work is always equal to the output work. It is called the
principle of lever.

When a lever is in a balanced condition,

Input work = Output work

or Effort × effort arm = Load × load arm

Types of Lever
Depending on the position of the fulcrum, effort and load, levers are of three types.
(i) First class lever
(ii) Second class lever
(iii) Third class lever

(i) First Class Lever Effort Load

The lever in which the fulcrum Fulcrum
is situated between the effort Figure 3.4: A typical first class lever
and the load is called the first
class lever. The effort arm of a
first class lever is longer than
the load arm.

Beam balance, see-saw, scissors, pliers, crowbar, wire-cutter, etc. are some examples
of the first class lever.

Pliers See-saw Beam balance Wire-cutter

Figure 3.5: Some levers of the first class

PHYSICS Oasis School Science and Environment - 7 37

(ii) Second Class Lever Effort Load

The lever in which the load is
situated between the fulcrum and
the effort is called the second class
lever. Fulcrum

Wheel-barrow, nut-cracker, bottle- Figure 3.6: Typical second class lever

opener, lemon-squeezar, paper-cutter, etc. are some examples of the second class
lever.

The effort arm in the second class lever is always longer than the load arm. So, we
can lift a heavy load by using the second class lever with a little force applied. The
second class lever always multiplies the effort applied on it.

Wheel-barrow Nut-cracker Bottle-opener
Figure 3.7: Some levers of the second class

(iii) Third Class Lever

The lever in which the effort is Load Effort
situated between the fulcrum and
the load is called the third class Fulcrum
lever. Fire tongs, fishing rod,
forceps, broom, shovel, spade, fore Figure 3.8: Typical third class lever
arm of a person holding a load, a
knife used to cut vegetables, etc.
are some examples of the third
class lever.

The effort arm of the third class lever is always shorter than the load arm. Therefore,
third class lever can increase the speed of work done but it cannot lift heavy loads.

Fire tongs Shovel Fishing rod
Figure 3.9: Some levers of the third class PHYSICS

38 Oasis School Science and Environment - 7

Activity 1

• Take a scale of length 30 cm as shown in the figure. Make a hole at the mid
point of the scale such that the tip of a ball pen can be easily inserted in it.
Keep the scale in the balanced condition by inserting
the tip of a ball pen in the hole and fixing it on the
stand. Also, keep the scale in balanced condition by
different masses on its both left as well as right sides.
Convert the mass into effort. It is to be noted that mass
of 100g equals to 1 N effort. Let us suppose that mass of
the right side be load and that of the left side be effort.
Keep the load in different distances from the fulcrum
and balance it by the effort. Complete the following
table on the basis of your experiment. Fig. 3.10

• If the method of the experiment is accurate, effort x effort arm = load × load
arm. But due to the friction during the experiment, negligible differences
may occur.

Left side Right side

Effort (N) Effort arm Effort × Load (N) Load arm Load ×
Effort arm Load arm

2. Pulley

A pulley is a simple machine having a grooved circular disc Pulley
over which a rope passes. The circular disc of a pulley is
made up of wood or metal with a groove cut along its rim.
A rope is passed around the groove. It prevents the rope
from slipping off the circular disc.

In a pulley, the load is attached to one end of the rope and
the effort is applied at another end. Pulleys are commonly
used to lift heavy loads. A pulley is used to apply the effort
in a convenient direction. So, the pulley makes our work Figure 3.11: Fixed pulley
easier by changing the direction of the force applied. We feel easier to apply an
effort in the downward direction than in the upward direction since we can use our
own weight to apply effort.

negligible /ˈneɡlɪdʒəbəl/ - of very little importance or size, insignificant 39
convenient /kənˈviːnɪənt/ - useful, easy or quick to do

PHYSICS Oasis School Science and Environment - 7

Types of Pulley

On the basis of mobility, there are two types of pulleys.

(i) Fixed pulley

(ii) Movable pulley

(i) Fixed Pulley

If a pulley does not move as the load is raised, it Frame
is called fixed pulley. Such type of pulley remains Axle
fixed at a point and rotates about an axle fixed to a Circular disc

support.

A fixed pulley is used to lift water from well. Rope
Similarly, it is used in a flag pole to raise the flag.
A fixed pulley is kept at the top of the flag pole. Load
When the rope is pulled downwards, the flag is Effort
raised. So a pulley is used to change the direction of
the force applied. Figure 3.12: Fixed pulley

(ii) Movable Pulley

If a pulley moves up and down along with the load, it
is called movable pulley. In a movable pulley, one end
of rope is fixed at a point and the effort is applied at the
Effort other end. Similarly, load is attached to the axle of the
pulley.

In a movable pulley, both load and pulley move upward
when the rope is pulled upward. The effort applied
to lift a load is shared equally by both parts of rope
supporting the pulley. So, we can lift a load two times
heavier by applying an effort on the movable pulley.
Load For example, we can lift a load of 100 N by applying an
Figure 3.13: Movable pulley effort of 50 N. Similarly, the effort moves two times the

distance moved by the load.

In our daily life, we combine a number of Movable Fixed pulley
fixed pulleys and movable pulleys. It makes pulley Rope
our work easier and faster. The pulley system
which consists of a number of pulleys is called Effort
combined pulley system or block and tackle
system. Such pulley system makes our work Load
easier and faster by changing the direction Figure 3.14: Combined pulley system
of the force and multiplying the force. In a
crane, a number of pulleys are used. So, it can
lift a very heavy object by applying a small
effort.

40 Oasis School Science and Environment - 7 PHYSICS

3. Wheel and Axle

In our daily life, we use a number of simple machines having two
cylinders of different radius. For example, the steering of a car,
door knob, knob of tap, windlass, screw driver, etc. Such type of
simple machine is called wheel and axle. Out of two cylinders of
a wheel and axle, the bigger is called a wheel and the smaller is
called an axle. So, a wheel and axle is a simple machine having
two cylinders of different radius.

Wheel Generally, a rope is coiled
around the wheel and the axle. In Figure 3.15: Windlass
this simple machine, a load is attached to the axle
Axle and the effort is applied to the wheel. If we rotate
the wheel, the axle also rotates. When the wheel
completes one rotation, the axle also completes
Rope the same. But the distance covered by the wheel
and axle is different because the radius of wheel is
more than that of the axle. So, the effort applied on
Effort Load the wheel is magnified. As a result, a small effort

Figure 3.16: Wheel and axle applied on the wheel can lift a heavy load.

Drill used by carpenters, steering of vehicles, screw-driver, paddle of bicycle,
bobbin of a kite, knob of water tap, handle of the door, handle of bicycle, etc. are
some examples of wheel and axle.

Knob of water tap Steering of car Screw-driver
Figure 3.17: Some examples of wheel and axle

4. Inclined Plane

Inclined plane is a sloping surface or a wooden
plank which is used to push things upward.
Winding road, staircase, ladder, children’s slide,
etc. are some examples of inclined plane.
Inclined plane

We can push a heavy load upward by applying a Figure 3.18: Inclined plane
little effort with the help of an inclined plane. It is very difficult to load a heavy drum
full of kerosene onto a truck. But it can be lifted easily by using a wooden plank. A little
effort can push the drum upward. Therefore, an inclined plane helps us to multiply the
effort applied.

In hospital, ramps (inclined planes) are provided to push wheel chairs carrying
patients easily. Similarly, winding roads are built in hilly areas so that vehicles can
climb up easily.

PHYSICS Oasis School Science and Environment - 7 41

Steep road Children’s slide Ladder
Figure 3.19: Some inclined planes

Activity 2

• Take a trolley or toy car and measure its Effort
weight by using a spring balance.

Make an inclined plane of height 50 cm Load
by arranging bricks and a wooden plank
as shown in the given figure. Pull the
trolley or toy car along the inclined plane
and measure the effort with the help of
the spring balance. Note down the effort
applied and fill in the given table.

Reduce the height of the inclined plane Figure 3.20
to 40 cm, 30 cm and 20 cm respectively by
removing some of the bricks and repeat the above process.

S. Height of the Load Effort Remarks
No. inclined plane

1. 50 cm

2. 40 cm

3. 30 cm

4. 20 cm

What do you learn from this activity? Write down the conclusion of this
activity.

5. Screw
A screw is a modified inclined plane with grooves cut in it. A screw looks like a nail
having a winding edge called a thread. The thread of a screw is an inclined plane
wrapped around a cylinder or cone. The distance between two screw threads is
called a pitch.
Screws and bolts are used to fasten two pieces of wood or metal. The effort is
applied to the head of a screw. It takes less force to insert a screw into wood than
to insert a nail into the wood. Similarly, jack screw is used to lift heavy vehicles like
truck, bus, car, etc. Screw nail, driller, jack screw, etc. are some examples of screw.

42 Oasis School Science and Environment - 7 PHYSICS