Science and Environment 8

Approved by the Government of Nepal, Ministry of
Education, Curriculum Development Centre,
Sanothimi, Bhaktapur as an additional material

Oasis

Enviro&nment

Grade

8

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

Enviro&nment

8

Publisher and Distributor:

Oasis Publication Pvt. Ltd.

Tel: 01-4313205

Author:
Jayananda Kapadi

Language Editors:
Ramesh Lamsal
Bhan Dev Kapadi

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

Copyright 
Publisher

Computer layout:
Oasis Desktop

(Ramesh Bhattarai)

Printed in Nepal

Preface

Oasis School Science and Environment for Grade 8 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. Deepak Bhatt, Mr. Naresh
Budal, Mr. Tek Bahadur Shahi, 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
would8be gratefully acknowledged and incorporated in further editions.

March 2018 Author
Kathmandu, Nepal

Contents

Physics

Unit 1 Measurement .................................................................... 1
Unit 2 Velocity and Acceleration................................................ 14
Unit 3 Simple Machines............................................................... 31
Unit 4 Pressure ............................................................................. 49
Unit 5 Energy, Work and Power................................................. 65
Unit 6 Heat.................................................................................... 82
Unit 7 Light ................................................................................... 95
Unit 8 Sound.................................................................................. 116
Unit 9 Magnetism......................................................................... 127
Unit 10 Electricity............................................................................ 136

Chemistry

Unit 11 Matter................................................................................. 150
Unit 12 Mixture .............................................................................. 181
Unit 13 Metal and Non-metal....................................................... 193
Unit 14 Acid, Base and Salt........................................................... 204
Unit 15 Some Useful Chemicals................................................... 219

Biology

Unit 16 Living Beings .................................................................... 231
Unit 17 Cell and Tissue ................................................................. 257
Unit 18 Life Processes ................................................................... 270

Geology and Astronomy

Unit 19 Structure of the Earth....................................................... 298
Unit 20 Weather and Climate ...................................................... 312
Unit 21 Earth and Space ............................................................... 322

Environment Science

Unit 22 Environment and Its Balance.......................................... 332
Unit 23 Environmental Degradation and Its Conservation...... 351
Unit 24 Environment and Sustainable Development................ 372

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
1 Physics 22.50 32 8

2 Chemistry 20.00 28 7
3. Biology 20.00 28 7

4. Geology and Astronomy 11.50 16 4

5. Environment Science 26.00 36 9

Total 100.00 140 35

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 25.00

2. Chemistry 15.00

3. Biology 15.00

4. Geology and Astronomy 5.00

5. Environment Science 25.00

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
25
Total

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

1uNIt Estimated teaching periods: Th Pr
3 1

Sir Isaac Newton

MeaSureMeNt

Objectives

After completing the study of this unit, students will be able to:
• define fundamental and derived units.
• identify the measurement of mass, weight and time.
Course of Study
• Physical quantities
• Fundamental and derived physical quantities
• Unit, fundamental units and derived units
• Measurement of mass
• Measurement of weight
• Measurement of time
Points to be Focused/Questions to be Discussed
• What are physical quantities?
• What are fundamental and derived physical quantities?
• What are units? What are fundamental and derived units?
• What is mass? What is its SI unit?
• What is weight? What is its SI unit?
• What is time? What is its SI unit?

PHYSICS Oasis School Science and Environment - 8 1

1.1 Introduction

Measurement is one of the essential aspects of our life. It starts at a very early stage of
our life time. We measure various quantities like mass, length, time, etc. We use beam
balance to measure the mass of a body. Similarly, metre rod is used to measure the length
of a piece of cloth. We compare a piece of cloth with a known standard metre rod to
measure its length. Measurement is the comparison of an unknown physical quantity
with a known standard quantity of the same kind. The measurable quantities are called
physical quantities. Length, mass, time, etc. can be measured. So, they are called physical
quantities. Feelings, sadness, happiness, love, desire, experience, etc. cannot be measured.
So, they are not physical quantities. There are two types of physical quantities, viz.
fundamental physical quantities and derived physical quantities.

(a) (b)
Fig 1.1

1.2 Classification of Physical Quantities

a. Fundamental physical quantities

The physical quantities like length, mass, time, etc. are independent of each other
and not definable in terms of other physical quantities. Those physical quantities
which are independent of each other are called fundamental physical quantities,
e.g. length, mass, time, etc. There are a total of seven fundamental or basic physical
quantities, viz. length, mass, time, temperature, electric current, luminous intensity
and amount of substance. All other physical quantities can be obtained from these
fundamental quantities.

Reasonable fact-1

Length is called a fundamental physical quantity.
Length is called a fundamental physical quantity because length is independent
of other and not definable in terms of other physical quantities.

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

fundamental / f ʌ n d ə ˈ m e n t l / - basic, forming the source or base from which everything else is made

2 Oasis School Science and Environment - 8 PHYSICS

b. Derived physical quantities

Some physical quantities like area, volume, force, velocity, density, pressure, etc.
depend on one or more fundamental physical quantities. Those physical quantities
which are derived from the fundamental physical quantities are called derived
physical quantities, e.g. area, volume, speed, velocity, force, work, power, pressure,
etc. These physical quantities can be expressed in terms of fundamental physical
quantities.

Speed is a derived physical quantity

Since, Speed = Distance = Length [In terms of fundamental quantities]
Time taken Time

Speed is formed by the combination of length and time. So, it is called a derived
physical quantity.

Differences between fundamental physical quantity and derived physical quantity.

Fundamental physical quantity Derived physical quantity

1. Those physical quantities which are 1. Those physical quantities which are

independent of each other are called derived from the fundamental physical

fundamental physical quantities. quantities are called derived physical

quantities.

2. There are seven fundamental physical 2. The number of derived physical quantities

quantities. is not fixed.

1.3 unit

A physical quantity is represented by a number followed by a unit. The standard quantity
which is used for the comparison of an unknown quantity is called unit. In other words,
unit is a fixed quantity in terms of which other similar quantities are measured. To find
complete measurement, we must know the following:

i. the proper unit in which the quantity is measured.

ii. the numerical value which expresses how many times the given unit is
contained in the physical quantity to be measured. For example, if the length
of an iron rod is 5 metre, it means that unit of length is metre(m) and this unit
is contained 5 times in the length of the iron rod. Thus, we can say:

Physical quantity = Numerical value × Unit
(5) (m)
(Length)

The unit selected for measuring a physical quantity should have the following properties.
i. It should not change with time and place.
ii. It should be reproducible.

PHYSICS Oasis School Science and Environment - 8 3

Actiiviii.t y 1It should be well defined without ambiguity.

• ivC. olleItc ts hdoiuffledr ehnatv reu ale crosn ovfe nthieen sta smizee .type of scale. Mark any two points on a
v.p apeIrt. sMhoeuasldu rbee t heaes diliys tavnaciel abbeltew aenedn athcceesses tiwbloe .points by using different rulers.
Why do these scales give different measurement?

Types of Units

Units are classified into two groups, viz. fundamental units and derived units.

i. Fundamental units

Those units which are independent of each other are called fundamental units.
Examples: Metre (m), kilogram (kg) and second (s). These units are not definable
in terms of other units. Fundamental units are the units of fundamental physical
quantities. There are seven fundamental units in SI system. They are: metre (m),
kilogram (kg), second (s), kelvin (K), ampere (A), candela (Cd) and mole (mol.)

The fundamental quantities and their SI units with symbols are given below:

S. N. Fundamental quantities SI units Symbol
metre m
1. Length kilogram kg
second s
2. Mass kelvin K
3. Time ampere A
candela Cd.
4. Temperature mole mol.

5. Current

6. Luminous intensity

7. Amount of substance

ii. Derived units

Those units which are formed by the combination of two or more fundamental units
are called derived units. Examples: m2, m3, m/s, m/s2, N, Pa, etc. These units can be
expressed in terms of fundamental units. Following table shows some derived
physical quantities with their SI units, symbols and fundamental units involved.

S. N. Physical Formulae SI units Symbols Fundamental
quantities units involved
length × breadth metre × metre m2 m×m
1. Area m×m×m
2. Volume length × breadth × height metre × metre × metre m3 kg/(m×m×m)
3. Density
mass / volume kilogram / metre3 kg/ m3

acceleration /əkseləˈreɪʃn/ - the rate of change in velocity
momentum /məˈmentəm/ - the quantity of movement of a moving object, measured as its mass multiplied by its speed

4 Oasis School Science and Environment - 8 PHYSICS

4. Velocity displacement / time metre/second m/s m/s

5. Acceleration change in velocity / time metre/second2 m/s2 m/(s×s)

6. Force mass × acceleration kilogram×metre/ kg m/s2 kg m/(s×s)
second2

7. Work/ force × displacement newton-metre Nm or J kg×m×m/(s×s)
Energy

8. Power work / time joule /second or watt J/s or W kg×m×m/(s×s×s)

9. Pressure force/area newton/metre or Nm-2 or Pa kg/(m×s×s)
pascal

10. Momentum mass × velocity kilogram×metre/ kg m/s kg×(m/s)
second

1. The unit of speed is a derived unit.

We know, distance (s)

Speed (v) = time (t)

∴ Unit of speed (v) = unit of distance or length = metre (m)
unit of time second (s)

Thus, the unit of speed can be expressed in the fundamental units of length and

time, i.e. m/s or ms-1. Therefore, the unit of speed is a derived unit.

2. The unit of power is a derived unit.

We know,

Power (P) = worktimdoen(et)(W)
force (F) × displacement(s)
= time (t) [∵ W = F×s]

= mass(m)×acceleration (a)× displacement(s) [∵ F = m × a]
time (t)

Unit of power (P) = units of mass (m) × acceleration (a)×displacement (s)
unit of time (t)

= kg × m/s2 × m
s

Thus, the unit of power, i.e. watt (W) can be expressed in the fundamental units

of mass, length and time, i.e. kg, m and s. Therefore, the unit of power is called a

derived unit.

PHYSICS Oasis School Science and Environment - 8 5

Differences between Fundamental units and Derived units

S.N. Fundamental units S.N. Derived units
1.
Fundamental units are 1. Derived units depend on
2. fundamental units.
independent of each other.
3. They are the units of derived
They are the units of fundamental 2. physical quantities.
physical quantities.
There are many derived units in SI
There are seven fundamental units 3. system.
in SI system.

Examples: m, kg, s, K, etc. Examples: m/s, N, N/m2, W, J, etc.

1.4 Measurement of Mass

Mass is the basic property of matter. The mass of a body is the total amount of matter
contained in it. Mass is denoted by 'm'. The SI unit of mass is kilogram (kg).

Mass is a scalar quantity. The mass of a body remains the same at all places. So, it is
a constant quantity. The mass of a body can be measured by using a physical (beam)
balance or pan balance by comparing it with a known standard mass.

Fig. 1.2

Physical balance Beam balance Grocer's balance
Fig. 1.3

A physical balance or beam balance consists of the two arms of equal length and the two
pans of equal masses. When an object to be weighed is placed in the left pan, the beam
turns in the anti-clockwise direction. Then known standard weights are placed in the
right hand pan till the beam attains equilibrium. In this condition, the mass of the object in

6 Oasis School Science and Environment - 8 PHYSICS

the left pan becomes equal to that of the standard mass kept in the right pan. In this way,
the mass of a body is measured by using a physical balance or beam balance.

Fig. 1.4 Measuring mass of a body by using a beam balance

The standard unit of mass is kilogram (kg). It is also measured in milligram, gram, quintal,
metric ton, etc.

The object to be weighed should be placed in one of the pans of the beam balance. The
standard weights are kept in 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

1000 gram = 1 kilogram

100 kilogram = 1 quintal

10 quintal = 1 metric ton

Activity 2
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.

PHYSICS Oasis School Science and Environment - 8 7

One standard kilogram Fig. 1.5 Spring balance

One standard kilogram is the mass of a platinum-iridium
cylinder having equal diameter and height kept at 00C at
the International Bureau of Weights and Measures, Sevres
near Paris in France.

All other measuring weights throughout the world
are compared to the 'standard kilogram' kept in the
International Bureau of Weights and Measures, Paris,
France. The 'Kilogram weights' used in Nepal should be
equal to the 'standard kilogram' kept in Department of
Weights and Measures, Kathmandu, Nepal

1.5 Measurement of Weight

Weight of a body is the gravity acting on the body. Thus, the Fig. 1.6
weight of a body is defined as the force with which it is pulled
towards the centre of the earth or a planet. It is a variable
quantity. The direction of the weight is towards the centre of a
massive body (like the earth). Weight is measured by using the
spring balance. The SI unit of weight is newton (N).

The weight of a body at a place depends on

i. mass of the body, and

ii. value of acceleration due to gravity at that place.

The weight of a body is directly proportional to the mass of that body. The weight of a
body is calculated by W = m×g. The value of g (acceleration due to gravity) differs from
place to place. Therefore, the weight of a body also differs from place to place.

Differences between Mass and Weight

S.N. Mass S.N. Weight

1. The total quantity of matter 1. The weight of a body is the measure of
present in a body is called its mass. the gravity acting on the body.

2. Its SI unit is kg. 2. Its SI unit is N.

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

4. It is a constant quantity for a 4. It is a variable quantity.
particular body.

The earth attracts all the bodies which are near its surface. When a ball is dropped from a
roof of a house, it falls down on the surface of the earth. Similarly, fruits fall down from a

vector /ˈvektə(r)/ - the physical quantity having both magnitude and direction
century /ˈsentʃəri/ - a period of 100 years

8 Oasis School Science and Environment - 8 PHYSICS

tree. The force that pulls these objects downwards is the gravity of the earth. Thus, gravity
is the force that pulls a body towards the centre of the earth or a planet.

The SI unit of gravity is newton (N). It is a vector quantity. The gravity (F) of the earth or a
planet depends on its mass (M) and radius (R). The gravity of a planet acts towards its centre.

Activity 3

• Bring a spring balance and measure the weight of different objects like book,
geometry box, calculator, science notebook, etc.

Reasonable fact-2

When a ball is thrown upwards from the earth's surface it returns back towards the
earth's surface.
When a ball is thrown upwards from the earth's surface, it returns back towards the
earth’s surface because of gravity.

Reasonable fact-3

The weight of a body varies from place to place.
The weight of a body varies from place to place because the weight of a body depends
on the mass of that body (m) and acceleration due to gravity (g) and the value of g
differs from place to place (i.e. g ∝ 1/r2).

1.6 Measurement of Time

The duration between any two events is called time. In scientific work, we want to know
how long an event lasts. It is called time interval. Clock is used to measure time. Second,
minute, hour, day, week, month, year, decade, century, millennium, etc. are the units in
which time is measured.

1 minute = 60 seconds
1 hour = 60 × 60 = 3600 seconds
1 day = 24 × 60 × 60 = 86400 seconds

Zenith Zenith

The point in the space just above the observer's head is called the zenith. Earth
Fig.1.7 Zenith
One mean solar day: The time taken by the sun to return to the zenith
from the earth's surface is called 1 mean solar day. It can also be defined
as the time required by the earth to complete one rotation around the sun
about its axis.

1
One second is defined as 86400 th part of a mean solar day.

PHYSICS Oasis School Science and Environment - 8 9

Different time measuring devices

1. Mechanical watch

Mechanical watch works on the basis of the oscillation of a

simple pendulum. Due to the climatic condition, the length of

the pendulum becomes long and short. So, it cannot measure

the accurate time. Mechanical watch is also called a pendulum

watch. Fig.1.8 Mechanical watch

2. Quartz watch

Quartz watch works due to the vibration of quartz crystals. It is
more accurate than the mechanical watch.

Fig.1.9 Quartz watch

3. Atomic watch

Atomic watch works due to the emission of radiation by Cs-133
isotopes. It measures the time very accurately.

Fig. 1.10 Atomic watch

Differences between Pendulum watch and Quartz watch

S.N. Pendulum watch S.N. Quartz watch

1. It works on the basis of oscillations 1. It works on the basis of vibrations of
of the simple pendulum. the quartz crystal.

2. In a pendulum watch, the time 2. In a quartz watch, the time given by
given by it fluctuates by a few it may fluctuate by a few seconds in a
seconds to minutes in a day. month.

Fact file-1

Scientists use atomic clock.
Scientists use atomic clock because it measures the time very accurately than any
other clock.

zenith /ˈzenɪθ/ - the highest point that the sun or moon reaches in the sky, directly above the observer's head
oscillation /ˌɒsɪˈleɪʃn/ - a regular movement between one position and another
fluctuate /ˈflʌktʃueɪt/ - to change frequently from one extreme to another

10 Oasis School Science and Environment - 8 PHYSICS

Activity 4

• Take a long thread of about 50 cm length. Tie a metallic bob at the end of the
thread. Note down the time for 20 oscillations with the help of a stop watch.
Calculate the time for 1 oscillation. This is the time period of the pendulum.

SuMMarY

• Measurement is the comparison of an unknown physical quantity with a known
standard quantity of the same kind.

• There are two types of physical quantities, viz. fundamental physical quantities
and derived physical quantities.

• Those physical quantities which are independent of each other are called
fundamental physical quantities.

• Those physical quantities which are derived from the fundamental physical
quantities are called derived physical quantities.

• The standard quantity which is used for the comparison of an unknown quantity is
called unit.

• The mass of a body is the total amount of matter contained in it.

• The weight of a body is defined as the force with which it is pulled towards the
centre of the earth or a planet.

• Gravity is the force that pulls a body towards the centre of the earth or a planet.

• The duration between any two events is called time.

• One second is defined as 1 th part of a mean solar day.
86400

• One mean solar day is the time taken by the earth to complete one rotation around

the sun.

• Atomic watch works due to the emission of radiation by Cs-133 isotopes. It
measures the time very accurately.

PHYSICS Oasis School Science and Environment - 8 11

exercise

1. Choose the best answer from the given alternatives.

a. Which of the following is not a physical quantity?

i. mass ii. volume

iii. love iv. length

b. The SI unit of temperature is ......................

i. 0K ii. 0C iii. 0F iv. K
iv. m
c. Which of the following is a derived unit?

i. kg ii. m3 iii. A

d. The weight of a body is measured by ......................

i. beam balance ii. spring balance

iii. pan balance iv. physical balance

e. How many seconds are there in one day?

i. 84600 seconds ii. 86400 seconds

iii. 48600 seconds iv. 68400 seconds

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

a. The measurable quantities are called physical

quantities.

b. Gram is the standard unit of mass.

c. The unit of acceleration is a derived unit.

d. The weight of a body is the measure of gravity.
e. Scientists use atomic clock to measure time accurately.
3. Fill in the blanks using appropriate words.

a. The physical quantity that is independent of each other is called .................. .
b. There are ...................... fundamental units in SI system.

c. The SI unit of pressure is ...................... .
d. The basic units involved in the unit of acceleration are ...................... .
e. The mass of a body is measured by using ...................... .

12 Oasis School Science and Environment - 8 PHYSICS

4. Answer the following questions.

a. What is measurement? Write down its importance in our daily life.
b. What are physical quantities? Give any three examples.
c. Define fundamental physical quantities with any three examples.
d. What are derived physical quantities? Give any four examples.
e. What is a unit? Give any two examples.
f. Define fundamental units and derived units with any two examples of each.

g. What is mass? Write down its SI unit.
h. How is 'one standard kilogram' defined in SI system?

i. What is weight? Write down its SI unit.
j. What is time? How is one second time defined in SI system?
5. Differentiate between:

a. Fundamental units and Derived units
b. Mass and Weight
c. Quartz clock and Atomic clock

6. Give reason.

a. Length is called a physical quantity.

b. Mass is called a fundamental physical quantity.

c. Velocity is called a derived physical quantity.

d. The unit of force is called a derived unit.
e. The weight of a body changes from place to place on the earth.

7. Convert the following:

a. 2500 kilogram into gram [Ans: 250000 g]

b. 335 hours into seconds [Ans: 1206000 s]

c. 1 day into seconds [Ans: 86400 s]

8. The mass of a girl is 40 kg. Calculate her weight. (g = 9.8 m/s2) [Ans: 392 N]

9. The weight of a stone is 98 N. Calculate its mass. (g = 9.8 m/s2) [Ans: 10 kg]

PHYSICS Oasis School Science and Environment - 8 13

2uNIt Estimated teaching periods: Th Pr
4 1

VELOCITy AND
aCCeleratIoN

Objectives

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

• explain average velocity and relative velocity.
• introduce acceleration and retardation.
• write and apply equations related to velocity and acceleration.
• solve numerical problems related to velocity and acceleration.

Course of Study

• Rest and motion
• Uniform motion and non-uniform motion
• Reference point, vectors and scalars
• Average velocity and relative velocity
• Acceleration and retardation
• Equations related to velocity and acceleration and numerical problems

Points to be Focused/Questions to be Discussed

• What is meant by rest and motion?
• What is a reference point?
• What are uniform and non-uniform motions?
• What are vectors and scalars?
• What is meant by average velocity and relative velocity?
• What is acceleration?
• What is retardation?

14 Oasis School Science and Environment - 8 PHYSICS

2.1 Introduction

In our surroundings, we see many things. Some of them can move from one place to
another while some remain stationary. Birds, animals, vehicles, etc. change their position
with respect to other objects in their surroundings. But house, tree, electric pole, tower,
etc. do not change their position.

Helicopter at rest Fig. 2.1 Aeroplane in motion

If a body is not moving, we simply say it is at rest. If the position of an object does not
change with respect to other objects in its surroundings, it is said to be at rest. If the
position of the object changes with respect to other objects in its surroundings, it is said
to be in motion.

Rest and Motion are Relative Terms

When you are sitting in a moving bus, the

position of your body with respect to other

seats or passengers remains constant. As

compared to another passenger your position

is not changing. So, you are at rest with respect

to other seats or passengers. But distance

between you and any tree near the road is

changing as time passes. So, you are moving Fig. 2.2
with respect to the tree. Thus, the same object

at the same instant can be at rest with respect to one thing and in motion with respect to

some other things. Motion is not absolute, neither is rest. The state of any object depends

on the other objects with respect to which the position of a body is compared.

Let us consider a bus at rest near a tree. When the bus starts to move, it changes its position
with respect to the tree. Similarly, the driver also changes its position with respect to the
tree. But the driver is not changing his position with respect to the bus.

Therefore, the driver is at rest relative to the moving bus but he is in motion relative to
the tree. Similarly, the tree is at rest relative to the moving bus and the moving bus is in
motion relative to the tree. So, rest and motion are relative terms.

stationary /ˈsteɪʃənri/ - not moving, not intended to be moved

PHYSICS Oasis School Science and Environment - 8 15

Activity 1

• Observe the position of any ten objects like house, tree, bird, water, etc. Find out
which are in a state of rest and which are in the state of motion. What can you
conclude from this activity?

2.2 Reference Point

The body at rest with respect to which the state of another body is compared is called
reference point. It is a certain point or place about which the state (rest or motion) of an
object is studied. In the following figure, a car is moving near a tree. The car is in motion
with respect to the tree. Here, the tree is taken as a reference point.

Fig. 2.3 Showing tree as a reference point

Reasonable fact-1

Reference point is required to calculate relative velocity.
To compare the state of a body at rest with respect to another body, reference point is
required. It is impossible to compare the relative velocity between two bodies without
any reference point. So, reference point is required to calculate relative velocity.

2.3 Uniform Motion and Non-uniform Motion

A body is said to be in uniform motion if it covers equal distance in equal interval of time.

12m 1s 12m 12m 12m 4s
0s 2s 3s

Fig. 2.4 Uniform motion

In the above figure, the car covers 12 m in every one second. Hence, the car is said to be
in uniform motion.

reference /ˈrefrəns/ - a standard by which sth can be compared

16 Oasis School Science and Environment - 8 PHYSICS

A body is said to be in non-uniform motion if it does not cover equal distance in equal
interval of time. It is also called variable motion.

8m 20m 10m 22m

0s 1s 2s 3s 4s

Fig. 2.5 Non-uniform motion

In the above figure, the car does not cover equal distance in equal interval of time. So, it is
said to be in non-uniform motion.

Most of the bodies have variable motion, e.g. motion of a boy, motion of a vehicle, motion
of air, etc. But the planets, stars and different machines have uniform motion.

Activity 2

• Observe the motion of any five nearby objects. Watch them carefully and find
out whether they are in a state of uniform motion or non-uniform motion.

2.4 Vectors and Scalars

Some physical quantities such as time, speed, distance, etc. can be described by their
magnitude only but other physical quantities like velocity, force, displacement, etc. need
magnitude as well as direction to describe them.

Those physical quantities which have both magnitude as well as direction are called
vectors or vector quantities, e.g. displacement, velocity, acceleration, force, etc. Vectors
are written in a special way but scalars do not have any special way of writing the letters.
For example: Vector AB is denoted byA→B.

Those physical quantities which have only magnitude are called scalars or scalar
quantities, e.g. distance, speed, mass, time, pressure, work, energy, area, volume, etc.

Differences between Vectors and Scalars

S.N. Vectors S.N. Scalars
1. Scalars have only magnitude.
Vectors have both magnitude 1.
2. and direction. The sum of scalars is always positive.

3. The sum of vectors may be 2. Scalars are added by the rules of
positive or zero or negative. simple algebra.

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

scalar /ˈskeɪlə(r)/ - the physical quantity having magnitude but no direction

PHYSICS Oasis School Science and Environment - 8 17

Reasonable fact-2

Velocity and acceleration are called vector quantities.
Velocity and acceleration are called vector quantities because velocity and acceleration
both have magnitude as well as directions.

Reasonable fact-3

Speed and mass are called scalar quantities.
Speed and mass are called scalar quantities because speed and mass both have only
magnitude but no direction.

2.5 Distance and Displacement

Suppose a person is at place A. He has to reach B and then C. Now, the person starts from
A and travels a distance of 4 km to reach B, and then travels another 3 km from B to C.
Thus, the person goes along the path ABC. The total length AB + BC is the actual length to
be covered. Thus, the actual length travelled by a body is called distance. In this example,
actual path or distance traveled is AB + BC = 4 km + 3 km = 7 km.

C

5 km
3 km

A 4 km B

Fig. 2.6

As the person reached the point C, we can find out how far he is from the initial point A or
the shortest distance between A and C. The shortest distance (or distance of AC) is 5 km.
The distance travelled by a body in a certain direction is called displacement. Distance
is a scalar quantity and is always taken as a positive quantity, whereas displacement is a
vector quantitity and it may have positive, zero or negative value. Displacement does not
depend on the path followed by a moving body but it depends only on the initial and final
positions of a moving body. The SI unit of both distance and displacement is metre (m).

velocity /vəlɒsəti/ - the rate of change of displacement of a moving body

18 Oasis School Science and Environment - 8 PHYSICS

Differences between Distance and Displacement

S.N. Distance S.N. Displacement
1.
Distance is the total length 1. Displacement is the shortest distance
2. covered by a moving body in a between initial and final positions of
3. a moving body.
certain interval of time.
It is a vector quantity.
It is a scalar quantity. 2.
It can be positive, zero or negative.
It is always positive. 3.

2.6 Speed and Velocity

If a car travels a distance of 300 km in 5 hours, the speed of the car is 300/5=60 km/h, that
is the car travels 60 km in every hour. Thus, the speed of a body gives the idea of how fast
a body is moving but it does not indicate the direction of motion of the body. So, speed is
defined as the rate of change of distance.

Speed = Distance (s)
Time taken (t)

The SI unit of speed is m/s and its CGS unit is cm/s. But speed in a particular direction
is called velocity. So, velocity is defined as the rate of change of displacement. Speed is a
scalar but velocity is a vector quantity.

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

The SI unit of velocity is m/s and its CGS unit is cm/s.

Differences between Speed and Velocity

S.N. Speed S.N. Velocity
1.
Speed is the rate of change of 1. Velocity is the rate of change of
2. displacement.
3. distance.
It is a vector quantity.
It is a scalar quantity. 2.
It can be zero.
It cannot be zero. 3.

Worked out Numerical 1

A car travels a distance of 30 km in 20 minutes towards the east. Calculate the velocity
of the car.

Solution:

Displacement (s) = 30 km

= 30 × 1000 m [∵ 1 km = 1000 m]

PHYSICS Oasis School Science and Environment - 8 19

= 30000m

Time taken (t) = 20 min.

= 20 × 60 = 1200 s [∵ 1 min. = 60 seconds]

Velocity (v) = ?

We know,

v = st
30000

= 1200 = 25 m/s

∴ The velocity of the car = 25 m/s.

2.7 Constant Velocity, Variable Velocity and Average Velocity

Constant Velocity
If a body covers equal displacement in equal interval of time, the velocity of the body is
called constant velocity. It is also called uniform velocity.

In the following figure, a car is moving from east to west. The car covers 8 m in every one
second. So, the velocity of the car is uniform velocity.

East 8m/s 8m/s 8m/s 8m/s West
0s 3s 4s
1s 2s

Fig. 2.7 Constant 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.

East 7 m/s 10m/s 8 m/s 12m/s West
0s 2s 4s
1s 3s

Fig. 2.8 Variable velocity

In the above 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.

Worked out Numerical 2

A person walks at 2 m/s for 10 minutes and then 1 m/s for 4 minutes. Calculate his
average speed throughout his journey.

20 Oasis School Science and Environment - 8 PHYSICS

Solution:
For the first case,

Time (t1) = 10 min. = 10 × 60s = 600 s [∵ 1 minute = 60 seconds]
Distance travelled (s1) = speed × time
= 2 × 600

= 1200 m

For the second case

Time (t2) = 4 min = 4 × 60s = 240 s
Distance travelled (s2) = speed×time
= 1 × 240

= 240 m

Average speed = s1 + s2
t1 + t2

= 1200 + 240
600 + 240

1440
= 840 = 1.71 m/s

∴ The average speed of the person is 1.71 m/s.

Average Velocity

The mean of initial velocity and the final velocity of a moving body is called an average velocity.

Average velocity (vav) = initial velocity (u) + final velocity (v)
2

∴ vav = u+v
2

Worked out Numerical 3

A person takes 8 minutes to cover a distance of 15 kilometers in his car. Calculate the
average velocity of his car.

Solution:

Given, Time taken (t) = 8 minutes

= 8×60 seconds [ ∵ 1 min. = 60 seconds]
= 480 s
Distance covered (s) = 15 kilometers
= 15×1000 metres [ ∵ 1 km = 1000 m]
= 15000 m
Average velocity (v) = ?

PHYSICS Oasis School Science and Environment - 8 21

We know, s
t
vav =

= 15000 = 31.25 m/s
480

∴The average velocity of the car is 31.25 m/s.

2.8 Relative Velocity

The velocity of a body with respect to another body is called relative velocity.

a. Two bodies moving along the same direction

Let us consider two cars moving along a straight road in the same direction as shown
in the fig. (a).

XX

Initial position After 1 second

B 25 m/s 25 m B

10m A

A 10 m/s

YY
Fig. 2.9

In this case, the velocity of a body with respect to another body is given by

Velocity of car A with respect to B (vAB) = velocity of car A (vA) – velocity of car B (vB).

i.e. vAB = vA – vB

b. Two bodies moving in the opposite direction

Let us consider two cars A and B are moving in the opposite direction. Let velocity
of the car A be vA and velocity of car B be vB. Since the direction is opposite, so

velocity of car A with respect to car B = vAB
i.e. vAB = Velocity of car A - Velocity of car B.
= vA - (-vB) [ ∵ vB is negative as it moves in opposite direction]

∴ vAB = vA + vB

Initial position After 1 second
X X

25 m/s AB 10 m/s A B 10 m
25 m

Y Y

22 Oasis School Science and Environment - 8 Fig. 2.10

PHYSICS

Thus, when two cars are moving in opposite direction, they appear to move with higher
velocity.

Worked out Numerical 4

Two vehicles X and y are moving in the same direction with the velocity of 12 m/s and
6 m/s respectively. Calculate the relative velocity of X with respect to y. If both vehicles
are travelling in the opposite direction, what will be the relative velocity ?
Solution:

Here, Velocity of X (vx) = 12 m/s
Velocity of Y (vy) = 6 m/s
When they are moving in the same direction,

Relative velocity of X with respect to Y (vxy) = vx – vy = 12 – 6 = 6 m/s.
When they are moving in the opposite direction,

vxy = vx – vy
Here, vy is negative ( ∵ vx is opposite to vy)
So, vxy = 12 – (-6) = 12 + 6 = 18 m/s
∴ Relative velocity is 18 m/s.

Worked out Numerical 5

Two vehicles X and y are moving with the velocity of 20 m/s towards east and 12 m/s
towards west respectively. If they started from the same place and the same time,
calculate the distance between them after 2 minutes.

Solution:

Here, Velocity of vehicle X (v1) = 20 m/s (towards east)


Velocity of vehicle Y (v2) = 12 m/s (towards west)

Time taken (t) = 2 min. = 2×60 s = 120 s

Distance travelled by vehicle X (s1) = v1×t = 20×120 = 2400 m
Distance travelled by vehicle Y (s2) = v2×t = 12×120 = 1440 m
∴ Distance between the two vehicles (s) = s1 + s2 = 2400 + 1440 = 3840 m

retardation /riːtɑːˈdeɪʃn/ - the rate of decrease in velocity of a moving body

PHYSICS Oasis School Science and Environment - 8 23

2.9 Acceleration and Retardation

When a body is moving with an increasing velocity, the body is said to be accelerated.
Suppose a body is moving from rest and the velocity of the body reaches 10m/s in 5
seconds. The change in velocity of the body is 10 m/s – 0 m/s. Thus, in every second

velocity changes by 150 m/s = 2m/s. The body is said to have changing velocity of 2 m/s in

every second. It means that the body is accelerating at a rate of 2m/s2.

The rate of change in velocity is called acceleration. Its SI unit is metre per second per
second (m/s2). The acceleration of a moving body is calculated by the given formula.

Acceleration (a) = Change in velocity

Time taken

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

∴a = v–u
t

The negative acceleration or the rate of decrease in velocity of a moving body is called
retardation. It has the same unit as that of the acceleration, i.e. m/s2. A moving body is said
to be in the state of retardation when its velocity decreases.

Differences between acceleration and retardation

Acceleration Retardation

1. The rate of increase in velocity of a moving 1. The rate of decrease in velocity of a moving

body is called acceleration. body is called retardation.

2. It is positive value. 2. It is negative value.

Reasonable fact-1

The object having uniform velocity has zero acceleration.
The object having uniform velocity has zero acceleration because the object having
uniform velocity has the same initial and final velocity.

Worked out Numerical 6

Calculate the acceleration of a truck if it starts from rest and attains a velocity of 25
m/s in 5s.

equation /ɪˈkweɪʒn/ - a statement showing that two amounts or values are equal

24 Oasis School Science and Environment - 8 PHYSICS

Solution:

Initial velocity (u) = 0 (∵ The truck starts to move from rest.)

Final velocity (v) = 25 m/s

Time (t) = 5s

Acceleration (a) = ?

We have, a = v–u = 255–0 = 5 m/s2.
t

∴ The acceleration of the truck is 5m/s2.

2.10 Equations of Motion

There are three equations which describe the motion of a body moving with a uniform
acceleration. Those equations give the relation among the initial velocity, final velocity,
time taken, acceleration and the distance travelled by moving bodies. The various
equations of motion are derived below:

i. v = u + at

This equation helps us to find the velocity gained by a moving body in time 't'.

Let us consider a body having initial velocity 'u'. Suppose, it is subjected to a uniform

acceleration 'a' so that the final velocity becomes 'v' after time 't’. Now, from the

definition of acceleration, we have Change in velocity
Time taken
Acceleration =

or, Acceleration (a) = Final velocity (v) – Initial velocity (u)
or, a v–u Time taken (t)
= t

or, at = v – u

or, v = u + at ..................... (1)

ii. v2 = u2 + 2as

Let us consider a body moving with an initial velocity 'u'. Let its velocity become 'v'

after time 't' and the distance travelled be 's'. The average velocity is the arithmatic

mean of initial and final velocities. Therefore,

Average velocity = Initial velocity + Final velocity
2
u+v
i.e. vav = 2

Also,

Distance travelled (s) = Average velocity × Time taken

PHYSICS Oasis School Science and Environment - 8 25

or, s = vav × t

or, s = v+2u × t
or, s = v+2u × v–au [a = v–tu × t or t = v–au ]
v2–u2
or, s = 2a

or, 2as = v2 – u2

∴ v2 = u2 + 2as .................... (2)

iii. s = ut + 1 at2
2
Let us consider a body moving with an initial velocity 'u'. Let its velocity be 'v' after

time 't' and the distance travelled be 's'. Then,

Average velocity = Initial velocity + Final velocity
= 2
u+v
i.e. vav = 2
=
Also,

Distance travelled (s) Average velocity × Time taken

or, s v+u × t
2
v+2u × t
or, s = u + (u+at)

or, s = 2 × t [ ∵ v = u + at]

or, s = ut+ut+at2
2

or, s = 2ut + at2
2
22ut at2
or, s = + 2

∴ s = ut + 1 at2 ……………………….(3)
2

Some points to be remembered while solving the numerical problems related to motion

i. If a body starts from rest, its initial velocity is zero, i.e. u = 0.

ii. If a body comes to rest, its final velocity is zero, i.e. v = 0.

iii. If a body moves with a uniform velocity, its acceleration is zero, i.e. a = 0.

26 Oasis School Science and Environment - 8 PHYSICS

Worked out Numerical 7

A car starts from rest and attains an acceleration of 4 m/s2 after 10 seconds. Calculate
the distance covered by the car.

Solution:

Initial velocity (u) = 0 (∴ The car starts from rest.)

Acceleration (a) = 4 m/s2

Time taken (t) = 10 s

Distance covered (s) = ?

We have, u0 t× +1 012 +a t122 × 4 × (10)2
s =
= 200 m
=

∴ Distance covered by the car (s) = 200 m.

Worked out Numerical 8

A bus starts to move from rest and attains an acceleration of 0.5 m/s2. Calculate the final
velocity of the bus after 50 seconds and distance covered by the bus within that time.

Solution:

Given,

Initial Velocity (u) = 0 [ ∵ The bus starts to move from rest.]

Acceleration(a) = 0.5 m/s2

Time taken (t) = 50 s

Final velocity (v) = ?

Distance covered (s) = ?

According to the formula,

v = u + at

= 0 + 0.5×50

= 25 m/s
Now,
s = ut + 1 at2
2

PHYSICS Oasis School Science and Environment - 8 27

= 0 × 50 + 1 × 0.5 × 502
2

= 0 + 625 = 625 m

∴The final velocity of the bus is 25 m/s and the distance covered by the bus within that
time is 625 m.

Practical Work

• Bring a measuring tape and measure the distance of 100 m in the school ground.
Mark the distance by using lime powder.

• Now, ask your friends to cover that distance one by one and measure the time
taken by each of them to cover the distance of 100 m.

• Calculate the velocity of each of the friends.

• Now, ask two friends to cover that distance and calculate the relative velocity.

SuMMarY

• The body at rest with respect to which the state of another body is compared is
called reference point.

• Those physical quantities which have both magnitude as well as direction are
called vectors or vector quantities, e.g. force, velocity, etc.

• Those physical quantities which have only magnitude are called scalars or scalar
quantities, e.g. length, mass, etc.

• The actual length travelled by a body is called distance. Its SI unit is metre (m).
• The distance travelled by a body in a certain direction is called displacement.
• Speed is defined as the rate of change of distance. Its SI unit is m/s.
• Velocity is defined as the rate of change of displacement. Its SI unit is m/s.
• If a body covers equal displacement in equal interval of time, the velocity of the

body is called constant velocity. It is also called uniform velocity.
• The rate of change in velocity is called acceleration. Its SI unit is m/s2.
• The negative acceleration or the rate of decrease in velocity of a moving body is

called retardation.
• The velocity of a body with respect to another body is called relative velocity.

28 Oasis School Science and Environment - 8 PHYSICS

exercise

1. Choose the best answer from the given alternatives.
a. Rest and motion are ..................... terms.

i. absolute ii. relative iii. independent iv. basic
iv. velocity
b. Which of the following is a vector quantity? iv. retardation

i. distance ii. pressure iii. speed iv. u+t v

c. The rate of change of distance is called ..................... iv. cm/s2

i. speed ii. velocity iii. acceleration

d. Which of the following is the average velocity?

i. v+2u ii. u2–v iii. u+2 a

e. Which of the following is the unit of acceleration?

i. ms2 ii. m/s–2 iii. m/s2

2. Tick (√) the correct statement and cross (×) the incorrect one.
a. Vectors have only magnitude but no direction.

b. The rate of change in displacement is called velocity.

c. Speed is a scalar quantity.

d. The SI unit of retardation is m/s.

e. Velocity may be positive, negative or zero.

3. Fill in the blanks with appropriate words.
a. Rest and ..................... are relative terms.
b. Vector quantities have both ..................... and direction.
c. The rate of change in ..................... is called velocity.
d. The mean of initial and final velocity of a moving body is called .............. .
e. The rate of decrease in velocity is called ..................... .

4. Answer the following questions.
a. Define the term rest and motion with one example of each.

b. What is meant by a reference point?

c. Define uniform and non-uniform motion.

d. What are vector and scalar quantities? Give any two examples of each.

e. Define distance and displacement.

PHYSICS Oasis School Science and Environment - 8 29

f. What is velocity? Write its SI unit.
g. What is meant by average velocity? Write down its formula.
h. What is relative velocity? Give one example.
i. Define acceleration and retardation.
j. What does it mean by the fact that the acceleration of a car is 5 m/s2?

5. Differentiate between: b. Vectors and Scalars
a. Rest and Motion d. Constant velocity and Variable velocity

c. Speed and Velocity

e. Acceleration and Retardation

6. Numerical Problems

a. Two vehicles X and Y are moving in the same direction with the velocity of 12
m/s and 8 m/s respectively. Calculate the relative velocity of X with respect to Y.
Also, calculate the relative velocity when they are moving in opposite directions.
[Ans: 4 m/s, 20 m/s]

b. Two vehicles are moving, one with the velocity of 20 m/s towards east and another
with 10 m/s towards west. Calculate the distance between them after 20 minutes if
they are started from the same place and the same time. [Ans: 36000 m]

c. If a body starts from rest and attains a velocity of 20 m/s in 8 seconds, calculate the

acceleration produced on the body. [Ans: 2.5 m/s2]

d. A vehicle is running at a speed of 45 km/h. If it is stopped in 3 seconds by applying

the brakes, calculate the retardation of the vehicle and the distance travelled before

its stopping. [Ans: 4.16 m/s2, 18.75 m]

e. The velocity of a moving body increases from 10 m/s to 15 m/s in 5 seconds.
Calculate its acceleration. [Ans:1m/s2]

f. A body moving along a straight path at a velocity of 20 m/s attains an acceleration
of 4 m/s2. Calculate the velocity of the body after 2 seconds. [Ans: 28 m/s]

g. A vehicle was moving at a speed of 90 km/h. On seeing a baby 20 m ahead on the

road, the driver jammed on the brakes and it came to rest at a distance of 15m.

What is its retardation and how long does it take to come at rest?

[Ans: 20.83 m/s2, 1.2 s]

h. A body starts moving from rest and attains the acceleration of 0.5 m/s2. Calculate

the velocity at the end of 3 minutes. Also, find the distance travelled by it during

that time. [Ans: 90 m/s, 8100 m]

30 Oasis School Science and Environment - 8 PHYSICS

3uNIt Estimated teaching periods : Th Pr
3 1

Fishing rod

SIMPle MaCHINeS

Objectives

After completing the study of this unit, students will be able to:
• introduce lever and state its principle.
• explain the types of lever and introduce mechanical advantage (MA),

velocity ratio (VR) and efficiency (η) of lever.
• calculate MA, VR and η of lever.

Course of Study
• Introduction to simple machines
• Advantages and basic principle of simple machines
• Lever and its types
• Mechanical Advantage (MA), Velocity Ratio (VR) and efficiency(ɳ)

of simple machines
• Numericals related to MA, VR and η of lever

Points to be Focused/Questions to be Discussed
• What are simple machines?
• What are the advantages of simple machines?
• What is a lever? What is its principle?
• What are different types of lever?
• What is mechanical advantage (MA)?
• What is velocity ratio (VR)?
• What is efficiency (ɳ)?

PHYSICS Oasis School Science and Environment - 8 31

3.1 Introduction

Modern age is the age of machines. Today our lives are made easier by a variety of
mechanical devices called machines. We use a variety of machines in our daily life.
Machines help us to perform mechanical work by using muscular energy. A device which
makes our work easier, faster and more convenient is called a machine. We can do various
works with the help of different machines. Some machines are simple in structure while
others are complex. Those machines which are simple in structure are called simple
machines. A particular simple machine helps us to do a particular work only. The devices
which are used to make our work easier, faster and to change the direction of force are
called simple machines. For example, scissors, crow bar, beam balance, fire tongs, pulley,
knife, etc. Simple machines are very useful to us because they help to multiply the force,
apply force in a convenient direction, apply force at a convenient point and to gain speed.

3.2 SAcispsoprslicationsBeoamf balance Wheel barrow Pulley Screw Axe

Fig. 3.1 Some simple machines

3.2 Advantages of Simple Machines Reasonable fact-1

1. Simple machines help to multiply force. Simple machines are widely used
2. They help to change the direction of force.
3. They help to increase the speed of work. in our daily life.
4. They help to do work safely.
Simple machines are widely used
in our daily life because simple
machines make our work easier,
faster and change the direction of
the force applied.

3.3 Technical Terms Related to Machines

a. Effort (E) : The force applied to a machine to do mechanical work is called effort.

b. Load (L): The force applied by the machine on the body on which the work is done
is called load. For example, a crowbar is used to lift heavy objects. In this, load is the
weight of the body to be lifted.

c. Mechanical Advantage (MA): The ratio of the load to the effort is called mechanical

advantage, i.e.

MA = Load(L)
Effort (E)

Since mechanical advantage is the ratio of two similar quantities, i.e. forces, it has no
unit. If the load lifted by a machine is greater than the effort applied, the mechanical

convenient /kənˈviːnɪənt/ - easy or quick to do

32 Oasis School Science and Environment - 8 PHYSICS

advantage is greater than 1 (MA>1). If the load lifted is less than the effort applied,
the MA is less than 1 [MA<1]. MA depends on friction and the weight of a machine.
If friction increases, MA decreases. No machine is frictionless. Due to the presence of
friction, large amount of effort is wasted. Similarly, due to the weight of a machine,
the mechanical advantage becomes less.

d. Velocity Ratio (VR): Velocity ratio is the ratio of the distance travelled by effort to the

distance travelled by load.

VR = Distance travelled by effort = Effort distance
Distance travelled by load Load distance

VR of a machine is not affected by the friction or weight of a machine. It has no unit
since it is the ratio of two similar quantities, i.e. distance.

e. Efficiency (ŋ): The work done by a machine is called output work. It is the product of
load and the distance travelled by load. Similarly, the work done on a machine is
called input work. It is the product of effort and the distance travelled by effort, i.e.

Output work = load × distance travelled by load

Input work = effort × distance travelled by effort

The percentage ratio of output work to input work is called efficiency of a machine.
It is expressed in percentage and denoted by the letter eta (ŋ).

Efficiency (ŋ) = Output work × 100%
Input work

3.4 Relation among MA, VR and ŋ

Efficiency (ŋ) = Output work × 100%
Input work

= Load×distance travelled by load × 100%
Effort×distance travelled by effort

Load
Effort
= Distance travelled by effort × 100%

Distance travelled by load

= MA × 100%
VR

∴ ŋ = MA × 100%
VR

This is the required relation among MA, VR and ŋ.

efficiency /ɪˈfɪʃnsi/ - the ratio of output work to input work of a machine

PHYSICS Oasis School Science and Environment - 8 33

Reasonable fact-2 Reasonable fact-3

The mechanical advantage of a simple The efficiency of a machine is always
machine is always less than the velocity ratio. less than 100%.

The mechanical advantage of a simple Mechanical advantage is affected by
machine is always less than the velocity friction and weight of a machine but
ratio because mechanical advantage is velocity ratio is not affected by the
affected by the friction and weight of the friction. So, mechanical advantage is
machine whereas velocity ratio is not always less than velocity ratio. Therefore,
affected by the friction and weight of the the efficiency of a machine is always less
machine. than 100%.

Efficiency of a machine can also be defined as the percentage ratio of mechanical advantage
(MA) to the velocity ratio (VR) of the machine. Efficiency is the ratio of two works done.
So, it has no unit. MA and ŋ are affected by the friction but not VR. So, MA is always less
than VR and the efficiency of a machine can be increased by reducing the friction. Some of
the ways to reduce friction are using grease, ball-bearings, making the surface smooth, etc.

3.5 Ideal Machine

The machine without friction during its operation is called an ideal (or perfect) machine.
In such a machine, the output work is always equal to the input work.

For an ideal machine,

Output work = Input work

Also, = Output work × 100 %
ŋ Input work

= 100% [∵ Output work = Input work]

The efficiency of an ideal machine is 100%. It is to be noted that in practice no machine
is 100% efficient. The output work of a machine is always less than the input work. A

machine cannot be 100% efficient because of the following reasons:

(i) A part of the work done or energy supplied to the machine is wasted in overcoming
the friction between the movable parts of the machine.

(ii) A part of work done or energy supplied to the machine is wasted in moving the parts
of the machine. Please note that no machine is weightless. Therefore, the efficiency
of a practical machine is always less than 100%.

Meaning of 80% efficiency of a machine

A machine has 80% efficiency means that 80% of the input work is converted to useful
output work and the remaining 20% of the input work is wasted to overcome the friction
and to move the parts of the machine.

34 Oasis School Science and Environment - 8 PHYSICS

Differences between Practical machine and Ideal machine

S.N. Practical machine S.N. Ideal machine

1. The efficiency of a practical 1. The efficiency of an ideal machine is

machine is less than 100%. 100%.

2. In a practical machine, output 2. In an ideal machine, output work is

work is less than input work. equal to input work.

3. In a practical machine, MA is less 3. In an ideal machine, MA is equal to

than VR. VR.

Worked out Numerical 1

A load of 900 N is lifted by a machine applying 300 N effort. If the effort distance and
load distance are 80 cm and 20 cm respectively, calculate the mechanical advantage,
velocity ratio and efficiency of the machine.

Solution:

Given, Load (L) = 900 N

Effort (E) = 300 N
∴
Mechanical advantage (MA) = L = 900 N = 3
E 300 N

Effort distance (E.d.) = 80 cm

Load distance (L.d.) = 20 cm

∴ Velocity ratio (VR) = E.d. = 80 cm = 4
L.d. 20 cm

Now, Efficiency (ŋ) = MA × 100%
VR
3
= 4 × 100%

= 75%

∴ The MA, VR and ŋ of the machine are 3, 4 and 75% respectively.

3.6 Types of Simple Machines

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

1. Lever 2. Pulley 3. Wheel and axle

4. Inclined plane 5. Screw 6. Wedge

PHYSICS Oasis School Science and Environment - 8 35

1. Lever

A lever is a rigid bar which Effort Load

moves freely about a fixed Effort arm Load arm
point called the fulcrum. In

a lever, effort is applied at Fulcrum
one point to lift a load on

another end. A lever has

a fulcrum, effort distance

and load distance. The Fig. 3.2 Parts of a typical lever

fixed point about which a

lever can rotate freely is called fulcrum. The distance between the fulcrum and the

point 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. 3.2).

When a lever is in balanced state input work is always equal to output work. For
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. They
are as follows:

(i) First class lever (ii) Second class lever (iii) Third class lever

a. First class lever: The lever in which fulcrum lies in between load and effort is called
first class lever. For example, crowbar, see-saw, scissors, pliers, nail cutter, etc.

Crowbar See-saw Scissors Pliers Nailcutter

Fig.3.3 Some levers of the first class

First class lever can increase the rate of doing work, change the direction of force and
multiply the force. In this type of lever, the effort arm may be less, equal to or greater
than the load arm. The MA of first class lever may be more than one, one or less than one.

b. Second class lever: The lever in which load lies in between fulcrum and effort is
called second class lever. For example, wheel barrow, nutcracker, bottle-opener, etc.

fulcrum /ˈfʊlkrəm/ - the point on which a lever turns or is supported

36 Oasis School Science and Environment - 8 PHYSICS

In second class lever, effort arm is always greater than the load arm. So a small effort
applied on the second class lever can lift a heavy load. Therefore, MA of second class
lever is always more than one.

Wheel barrow Nutcracker Bottle opener

Fig. 3.4 Some levers of the second class

c. Third class lever: The lever in which effort lies in between load and fulcrum is called
third class lever. For example, shovel, fishing rod, fire tongs, broom, etc. In third
class lever, effort distance is always less than load distance. So, more effort should
be applied to overcome a smaller load. Therefore, MA of a third class lever is always
less than one.

Shovel Fishing rod Fire tongs Broom

Fig.3.5 Some levers of the third class

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 Fulcrum

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 the 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 Effort Stand Load
mass of 100g equals to 1 N effort. Let us consider that mass of the

right side is the load and that of left side is the effort. Keep the load

in different distances from the fulcrum and balance it by the effort. Fig.3.6
Complete the following table on the basis of your experiment.

Left Side Right Side

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

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

PHYSICS Oasis School Science and Environment - 8 37

Reasonable fact-4

The cutting edges of metal-cutting scissors are made shorter but those of
cloth-cutting scissors are made longer.
Metal is harder than the cloth. So, we have to apply greater effort to cut metal than
the cloth. If the cutting edges of metal-cutting scissors are made shorter then the load
distance is shorter. As a result of which greater effort is created on that edge and it is
easy to cut the metal piece. But it is easy to cut cloth even the cutting edges of cloth
cutting scissors are made longer. Therefore, cutting edges of metal-cutting scissors are
made shorter but those of cloth cutting scissors are made longer.

Reasonable fact - 5

It is easier to lift a load when shifted towards wheel in a wheel barrow.
A wheel barrow is a second class lever. The mechanical advantage is increased if the
load is shifted towards the fulcrum, which increases the effort arm. Therefore, it is
easier to lift a load when shifted towards wheel in a wheel barrow.

Reasonable fact - 6

It is impossible to get a perfect machine in practical life.
Due to friction and the weight of a machine, the output work of a machine is always
less than the input work. Therefore, it is impossible to get a perfect machine in practical
life.

Worked out Numerical 2
Study the given figure and calculate MA, VR and ŋ of the lever.

600 N 200 N
60 cm
20 cm

Solution: Effort = 200 N
Load = 600 N
Effort arm = 20 cm + 60 cm = 80 cm
Load arm = 20 cm
= ?
MA

groove /ɡruːv/ - a long narrow cut on the surface of sth hard

38 Oasis School Science and Environment - 8 PHYSICS

VR = ?
ŋ = ?
We have,
MA = Load = 600 = 3
Effort 200

Effort distance = 80 = 4
VR = Load distance 20

Now, MA
ŋ = VR × 100%
3
= 4 × 100%

= 75%

∴ The MA, VR and ŋ of the lever is 3, 4 and 75% respectively.

2. Pulley

A pulley is another simple machine which is commonly used to lift heavy loads.
A pulley is a circular disc having a groove on its circumference over which a rope
can pass. In rural areas, pulley is used to lift water from wells. A pulley makes our
work easier by changing the direction of force and by multiplying the force. In a
pulley, load is attached at one end of the rope and the effort is applied at another
end. There are three types of pulleys. They are:

a. Single fixed pulley

b. Single movable pulley

c. Block and tackle system

a. Single fixed pulley: If a pulley does not move up and down
with the load, it is called single fixed pulley. In a fixed pulley,
distance covered by the effort is always equal to the distance
covered by the load. So, VR of a fixed pulley is always one.

Although there is no gain in MA and VR, yet single fixed Load Effort
pulley is widely used because:
Fig. 3.7
(i) it makes our work easier by changing the direction of
the force applied, and

(ii) the person can use his/her own body weight, while
applying the effort downwards.

b. Single movable pulley: If a pulley moves up and down along with the load, it is
called a single movable pulley. In the movable pulley, one end of the rope is attached
to a fixed hook and effort is applied to another end. In this type of pulley, effort
distance is two times more than the load distance. So, the VR of a movable pulley is

PHYSICS Oasis School Science and Environment - 8 39

2, i.e. number of rope segments that supports the load.

In a single movable pulley, load is equal to twice the Fixed pulley

effort. So, a movable pulley doubles the effort we exert. Rope
Effort
Therefore, the single movable pulley acts as a force Movable pulley

multiplier.

A single movable pulley is usually used in combination Load
with a single fixed pulley. Because a fixed pulley allows
the effort to be applied in the downward direction Fig. 3.8
whereas the movable pulley doubles the effort applied.

Differences between a single fixed pulley and a single movable pulley

S.N. Single fixed pulley S.N. Single movable pulley

1. It does not move up and down 1. It moves up and down along with

along with load. load.

2. It is used to change the 2. It is used to multiply the force as it

direction of the effort to a more doubles the effort we exert.

convenient direction.

3. The MA and VR of a single 3. The MA and VR of a single movable

fixed pulley is 1. pulley is 2.

4. It is used to raise a small load. 4. It is used to lift a heavy load.

c. Block and tackle system fixed pulley block
If two or more pulleys are
used in a combined form, it fixed end of rope T spring balance
is called a block and tackle T T reading the effort force
system. Such type of pulley
system makes our work T E
easier by multiplying the
force and changing the moving pulley block
direction of force. The VR
of a block and tackle system slotted masses load
is equal to the number of A block and tackle with VR=4
pulleys in the system.
Fig. 3.9 Block and tackle system

The block and tackle system of pulley consists of two sets of pulleys, each set
containing two or more pulleys. The set of pulleys fixed to the rigid support is
called block whereas the set of pulleys which is movable and attached to the load
is called tackle.

40 Oasis School Science and Environment - 8 PHYSICS

MA, VR and ŋ of a pulley
Load
MA = Effort

VR = No. of pulleys (except single movable pulley)

= No. of rope segments that support the load
ŋ =
MA × 100%
VR

Activity 2

• Observe different types of pulleys in your science laboratory. Draw their
diagrams and classify them in terms of fixed, movable or block and tackle
system.

Worked out Numerical 3

A load of 1200N N is lifted by using a pulley system having 4 pulleys. If the effort
applied is 350N, calculate the efficiency of the pulley system.

Solution: Load = 1200 N
Effort = 350 N
VR = 4 [ ∵ The no. of pulley is 4.]
MA = ?


η = ? Load =1325000
Effort
MA = = 3.42

η = MA × 100%
VR

= 3.42 × 100
4

= 85.5%

∴ The efficiency of the pulley system is 85.5%. Wheel

3. Wheel and axle Axle

Wheel and axle is a system of two co-axial cylinders Effort Load
having two different radii in which the bigger one is
called a wheel and smaller one an axle. It works like a Fig. 3.10 Wheel and axle
lever. Spanner, paddle of bicycle, steering of vehicles,
screw driver, door knob, etc. are some examples of
wheel and axle. In wheel and axle, load is attached to
the axle and effort is applied to the rope of the wheel.

PHYSICS Oasis School Science and Environment - 8 41

When the wheel completes one rotation, the axle also completes the same.

In a wheel and axle, the common axis of wheel and axle acts as a fulcrum which lies
in between load and effort. So, wheel and axle is considered as the first class lever.

MA, VR and ŋ of a wheel and axle

MA = Load
Effort

VR = Effort distance
Load distance
Circumference of wheel (2πR)
= Circumference of axle (2πr)

∴ VR = Radius of wheel (R)
Radius of axle (r)

ŋ = MA × 100%
VR

Worked out Numerical 4

In a wheel and axle, the radius of the wheel is 25 cm and that of the axle is 5 cm. If a
load of 1200 N is lifted by using an effort of 300 N on it, calculate MA, VR and ŋ.

Solution:

Radius of the wheel (R) = 25 cm

Radius of the axle (r) = 5 cm

Load = 1200 N

Effort = 300 N

MA = ?

VR = ?

η = ?

We have, MA = Load = 1200 = 4
Effort 300
VR
η = R = 25 = 5
r 5
MA 4
= VR × 100% = 5 × 100%

= 80%

∴ The MA, VR and ŋ of the wheel and axle is 4, 5 and 80% respectively.

Activity 3
• Take a wheel and axle system and calculate its MA, VR and ŋ.

42 Oasis School Science and Environment - 8 PHYSICS

4. Inclined plane

An inclined plane is a simple machine which is a sloping surface. It is used for lifting
heavy loads applying less effort. The road which is made on the hilly region is an
example of inclined plane. Similarly, an inclined plank used for loading a truck and
a staircase also act as inclined plane. The length of an inclined plane acts as the effort
distance and the height as the load distance.

Length

Height

Fig. 3.11 Inclined plane

In an inclined plane, the length of the inclined plane is always more than its height.

So, VR of an inclined plane is always more than one. Please note that the smaller the

angle of inclined plane, the smaller is the effort needed to move up a load. In other

words, the longer the inclined plane, the smaller is the effort needed to move up a

load on it.

MA, VR and ŋ of an inclined plane

MA = Load
Effort

VR = Length of inclinded plane (l)
Height of inclinded plane (h)

ŋ = MA × 100%
VR

Worked out Numerical 5

Calculate MA, VR and ŋ of the given inclined plane. 400 N
8 m.
Solution: 15 m.
600 N
Load = 600 N
= 400 N
Effort = 15 m
= 8m
L ength of inclined plane (l)

H eight of inclined plane (h)

screw /skruː/ - a modified inclined plane with a raised spiral line along its surface

PHYSICS Oasis School Science and Environment - 8 43

MA = ?

VR = ?

We have, ŋ = ? Load = 600 = 1.5
MA = Effort 400

VR = l = 15 = 1.8
∴ h 8
1.5
ŋ = MA × 100% = 1.8 × 100% = 83.3%
VR

∴ The MA, VR and ŋ of the inclined plane is 1.5, 1.8 and 83.3% respectively.

5. Screw

A screw is a modified inclined plane

with a raised spiral line along its

surface. The raised spiral line is called

a thread. The distance between any Screw nail Driller Jack screw
two successive threads is called pitch.

Screw is turned and pressed into Fig.3.12 Some screws
wood, metal, etc. to fasten two things

together. Jack screw is used to lift heavy vehicles like truck, bus, etc. Screw nail, jack

screw, driller, etc. are some examples of screw.

6. Wedge

A wedge is a piece of metal, wood, etc. with one thick end and another sharp
pointed end. It is used for cutting and splitting, drilling holes and pushing or
lifting heavy objects. In a wedge, effort is applied from the thick or blunt end. All
tools and weapons with sharp edges like axe, knife, blade, chisel, nail, etc. are
some examples of wedge.

Khukuri Knife Axe

Fig. 3.13 Some wedges

44 Oasis School Science and Environment - 8 PHYSICS