Green Science and Environment 7 Final (2076)

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

Green

Health and Physical Education

7

Editor
Dr. Deepak Chand

M.Sc. (TU, Kirtipur, Kathmandu)
Ph.D. (University of Idaho, USA)

Author
Bishnu Prasad Bhatt

M.Sc. (TU, Kirtipur, Kathmandu)

Lalitpur, Nepal, Tel: 977-1-5529899
e-mail: [email protected]
www.greenbooks.com.np

Publisher: Green Books 7

Copyright: Author (2074 BS)

All rights reserved. No part of this book may
be reproduced, stored in a retrieval system,
or transmitted in any form or by any means
without prior permission in writing from the
author and editor.

Edition
First : B.S. 2074 (2017 AD)
Reprint : B.S. 2075 (2018 AD)
Revised : B.S. 2076 (2019 AD)
Revised : B.S. 2077 (2020 AD)

Illustrator
Prakash Samir

Layout
The Focus Computer
[email protected]

Printed in Nepal

Preface

It gives me an immense pleasure in presenting this book-
Green Science and Environment for class 7. This book
is written specially to meet the requirements of the new
syllabus introduced by the Government of Nepal, Ministry
of Education, Curriculum Development Centre, Sano Thimi,
Bhaktapur, Nepal.

My aim and effort while writing this book has been to help
students understand, enjoy and appreciate the fascinating
subject of Science and Environment by making the process of
learning enjoyable and stimulating. I have attempted to present
the subject matter covering the entire prescribed syllabus in a
simple language and interesting style with a large number of
illustrative examples for easy understanding and application
of the fundamental principles of science. Each unit of the book
has been carefully planned to make it student-friendly and
present the subject matter in an interesting, understandable and
enjoyable manner. A Structural Programme Learning Approach
(SPLA) has been followed and exhaustive exercises are given
at the end of each unit to test knowledge, understanding and
applications of concepts taught/learnt.

The text is supplemented with weighting distribution, learning
objectives, word power, teaching instructions, sample test
papers and a large number of well-labelled accurate pictures. I
sincerely hope that this book will serve its intended purpose and
be received enthusiastically by both the students and teachers
concerned.

I wish to express my sincere gratitude to Green Books Team for
publishing this book. My hearty thank goes to Focus Computer
for excellent type setting and layout.

I also wish to acknowledge my great indebtedness to many
teachers for their valuable suggestions and advice concerning
the textbook. I am confident that as result of their suggestions
this book will be more useful than any other textbooks. However,
sympathetic criticisms and constructive suggestions for further
improvement of the book, if any, will be welcomed and with
warm regards incorporated in the subsequent editions.

Author

Kathmandu, Nepal
September 2016

Contents

Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Force and Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3. Simple Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5. Energy, Work and Power . . . . . . . . . . . . . . . . . . . . . . . . . 56
6. Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
7. Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8. Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
9. Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
10. Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
11. Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
12. Mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
13. Metal and Non-Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
14. Some Useful Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . 152

Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
15. Living Beings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
16. Cell and Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
17. Life Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

Geology and Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
18. Structure of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
19. Weather and Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
20. The Earth and Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

Environment Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
21. Environment and Its Balance . . . . . . . . . . . . . . . . . . . . . 247
22. Environmental Degradation and Its Conservation . . 267
23. Environment and Sustainable Development . . . . . . . . 286

Physics

UNIT Measurement

1

Weighting Distribution (Approximate) Teaching periods : 5 Marks (in %): 1

Before You Begin

Physics is a science of measurement. We take measurement of
various things in our daily life. Measurement is essential for buying
and selling goods, making medicines, sewing clothes, performing
experiments and so on. To measure a physical quantity, we
compare the given physical quantity with a known standard
quantity of the same kind. This process is called measurement. So,
measurement is called the comparison of an unknown physical
quantity with a known standard quantity of the same kind. The
quantities that can be measured are called physical quantities.
Length, mass, area, volume, temperature, etc. can be measured.
So these quantities are called physical quantities. Different devices
are used to measure different physical quantities.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to measurement
i. introduce measurement and tell its importance.
• SI system
ii. introduce the SI system of measurement and its use.
• Length, mass and time
iii. calculate the surface area of regular and irregular
objects. • Regular and irregular objects

iv. explain the method for measuring the volume of • Introduction to area and
liquids and calculate their volume. volume

v. solve simple numerical problems related to area and • Measurement of area and
volume. volume

Glossary: A dictionary of scientific/technical terms

• Introduction to measurement
• SI system
• Length, mass and time
• Regular and irregular objects
• Introduction to area and volume
• Measurement of area and volume

GREEN Science and Environment Book-7 5

Measurement

Measurement is an important and useful process in our day to day life. When
you go to a tailor to prepare your school uniform, s/he takes measurement
of your body by using a measuring tape. Similarly, when you go to buy
sugar in a shop, the shopkeeper compares the sugar with a standard mass
by using a physical balance. Thus, there is a necessity of measurement. To
know the magnitude of a physical quantity, we compare the quantity with a
known standard quantity of the same kind. So, measurement is the process of
comparison of an unknown physical quantity with a known standard quantity
of the same kind.

Fig.

1.1

We measure different physical quantities like length, weight, mass, volume,
area, temperature, pressure, etc. The quantities which can be measured are
called physical quantities. Different measuring devices are used to measure
various physical quantities. For example, a watch is used to measure time and
thermometer is used to measure temperature.

SI system

In October 1960, the 12th General Conference of Weight and Measures in
France, agreed to use international system of units to bring uniformity in
scientific measurement throughout the world. This system is called SI system.
SI means 'system international de units' in French. SI system is the standard
international system of units.

There are seven fundamental units of seven physical quantities in SI system
which are as follows:

S.No. Physical quantity Fundamental unit Symbol
meter m
1 Length kilogram kg

2. Mass

6 GREEN Science and Environment Book-7

3. Time second s
4. Temperature kelvin K
5. Electric current amper A
6. Luminous Intensity candela Cd
7. Amount of substance mole mol.

These fundamental units are also known as standard units of measurement as
they are accepted, applied and valid all over the world.

Length and its Measurement

We measure the length of different objects by using a measuring tape or scale.
Length is the distance between any two points. For example, the length of a
stick is 2m means that the distance between the upper tip and lower end of
the stick is 2 times longer than the length of a metre rod. In our practical life,
we use different forms of length like breadth, thickness, depth, radius, height,
diameter, etc. We use different measuring devices such as scale, inch tape,
metre rod, measuring tape, etc. to measure length.

Do You Know

Fig. There are seven fundamental physical
quantities in SI system. They are length, mass,
1.2 time, temperature, electric current, luminous
Ruler intensity and amount of substance.

Measuring tape

Length is a fundamental physical quantity which is measured in metre (m)
unit in SI system. While measuring length, the given object is compared with
the standard length of a scale, metre rod or a measuring tape.

Activity 1

Make a list of any five measuring devices and write their uses.

S.N. Names of measuring devices Uses

1.
2.

3.

4.

5.

GREEN Science and Environment Book-7 7

In SI system, length is measured in metre (m). It may be too big for measuring
the length of very small objects. So, submultiples of metre like centimetre,
millimetre, micrometre, nanometre, etc. are used. Metre is too short for the
measurement of large length/distances. So multiples of metre like kilometre,
light year and astronomical units are used. The various units of length and
their interrelationship are given below.

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

Among different units of length, millimetre, centimetre, metre and kilometre
are widely used.

Ways to take the correct and exact reading of length
i. We should place our eyes vertically above the point where the measurement

is to be taken.

ii. We should keep the scale exactly along the length to be measured.

iii. The ends of the ruler must not be damaged.

Correct

Incorrect Incorrect

Fig.

1.3

Activity 2

Measure the given objects by using your scale and fill the table with their
measurement.

S.N. Objects Length Breadth Height
1. Science book
2. Chalk box
3. Brick
4. Blackboard
5. Instrument box

8 GREEN Science and Environment Book-7

Measurement of diameter of spherical objectsFig.
Fig.
The diameter of spherical objects like volleyball, football, etc. is measured
by using two similar solid blocks. In this process, the spherical object whose
diameter is to be measured placed on top of a plane surface. Two similar
blocks are placed on the either side of the spherical object. The lower surface
of the blocks should be adjusted along the ruler. The reading of the position
of two faces one of each block touching the spherical object, is taken. Then the
difference between these two readings is calculated which is the diameter of
the spherical object.

Solid block Spherical object (Football)

Solid block

1.4

Mass

We buy sugar by measuring its mass. Similarly, we buy potatoes and onions
by measuring their mass. Mass is the quantity of matter contained in a body.
In SI system, mass is measured in kilogram (kg). Mass is also measured in
milligram (mg), gram (g), quintal, metric ton, etc.
Mass of a body is measured by using a beam balance or a physical balance.
A beam balance is a physical balance which has a horizontal beam supported
at its centre. It is fixed by a needle. Two identical pans are supported at equal
distances from the needle. Needle indicates balance of mass on both pans of
the beam balance.

1.5

Beam balance Spring balance Grocer's balance

The object whose mass is to be measured should be placed on the right pan of

the beam balance. The standard mass are kept on the left pan. We should add

or remove the masses until both the pans are balanced which is indicated by a

straight needle of the beam. When both pans of the balance are at equilibrium,

GREEN Science and Environment Book-7 9

the total mass of the object is calculated by adding the masses of the weights
kept on the left pan.

Units of Mass

The SI unit of mass is kilogram (kg). Milligram, gram, kilogram, quintal, etc.
are other units of mass.

The mass of light objects is measured in gram (g) and milligram (mg) whereas the
mass of the heavy objects is measured in kilogram (kg), quintal, metric ton, etc.

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 3

Measure the mass of some objects like small stone, book, brick, pen,
instrument box, etc. by using a beam
balance. Note down their mass in your
science copy.

Fig.
Fig.
1.6

Time

Ram takes 30 minutes to reach school from home. Hari takes 10 minutes to
reach home from school. This duration is called time. The interval or gap
between two events is called time. The SI unit of time is second (s).

Time is measured by a clock. There are different types of clocks like pendulum
clock, quartz clock, mechanical clock, atomic clock, electronic clock, wristwatch,
etc.

1.7 Wrist watch Stop watch
Pendulum clock

10 GREEN Science and Environment Book-7

We measure time in second, minute, hour, day, week, month, year, decade, etc.
A short duration of time is measured in second, minute and hour. Similarly, a
long duration of time is measured in decade, century, millenium, etc.

The multiples and sub-multiples of time are given below:

60 seconds = 1 minute 60 minutes = 1 hour
24 hours = 1 day 7 days = 1week

365 days = 1 year 10 years = 1 decade

100 years = 1 century 10 decades = 1 century

1000 years = 1 millenium

Regular objects

Those objects which have a fixed geometrical shape and proper dimension
are called regular objects. Book, chalk box, pencil, brick and chalk are some
examples of regular objects.

1.8Fig. Fig.
Book Pen Ball

Irregular objects

Those objects which do not have a fixed geometrical shape and proper
dimension are called irregular objects. A piece of stone, leaf, a piece of paper
and a piece of broken glass are some examples of irregular objects.

1.9 Leaf Broken glass
Piece of a brick
GREEN Science and Environment Book-7 11

Activity 4
Collect any eight objects from your surroundings.
Observe their shape and classify them as regular and irregular objects.

Area

The total surface possessed by a body is called its area. The SI unit of area is
square metre (m2). It is also measured in cm2, km2, etc.

We can measure the area of regular plane surfaces by using the given formulae.

1. Area of a rectangular object (A) breadth
= length (l) × breadth (b)
length
\A=l×b

2. Area of a square (A) = (length)2 length

\ A = l2 radius

3. Area of a circle (A) = p × (radius)2

A = pr2 qwhere, p = 22 r
7

Worked out Example: 1

The length of a rectangular room is 10 metre and its breadth is 8 metre.
Calculate its area.
Solution
Length (l) = 10 m
Breadth (b) = 8 m
Area (A) = ?
We know that,
Area of a rectangular body (A) = length (l) × breadth (b)
= 10 m × 8 m
= 80 m2
\ Area of rectangular body (A) = 80 m2

Activity 5

Calculate the area of your science notebook by measuring its length and
breadth.

Calculate the area of your classroom by measuring its length and breadth.

12 GREEN Science and Environment Book-7

The area of an irregular body cannot be measured by using different formulae
because they have no proper dimension for the measurement of length,
breadth, height, etc. So the area of an irregular body can be measured by using
graph paper.

The irregular object (say a leaf) is placed on a
graph paper and its outline is drawn by using a
sharp pencil. Then the leaf is removed and the
number of complete squares and squares which
are half and more than half within the outline is
counted. However, the squares which are less
than half are ignored. By multiplying the total
counted squares and area of unit square of a
graph paper, the approximate area of the given
irregular object is calculated.

Activity 6

Calculate the area of a leaf by using a graph paper.

Calculate the area of a piece of stone by using a graph paper.

Volume

Volume of a body is defined as the total space occupied by the body. Its SI unit
is cubic metre (m3). The volume of a solid object is measured in mm3, cm3, m3,
etc. Similarly, the volume of liquid substances is measured in millilitre (ml),
litre(l), etc.

The volume of regular solid objects can be calculated by the given formulae:

i. Volume of a rectangular solid or a cuboid (V) Fig. Fig. h
= length (l) × breadth (b) × height (h)

\V=l×b×h

1.10 l b

ii. Volume of a cube (V) = (length )3
\ V = l3

l
1.11

GREEN Science and Environment Book-7 13

iii. Volume of a sphere (V) = 4 p (radius)3 r
3
r
\ V = 4 pr3 qwhere p = 22 r Fig. Fig. h
3 7
1.12

iv. Volume of a cylinder (V) = p × (radius)2 × height (h)
\ V = pr2h

Worked out Example: 2 1.13

The radius and height of a cylinder are 7 cm and 25 cm respectively. Calculate
the volume of the cylinder.

Solution:

Radius of the cylinder (r) = 7 cm

Height of the cylinder (h) = 25 cm

Volume (V) = ?

We known,

V = pr2h

or, V = 22 × (7)2 × 25
7

= 3850 cm3

\ The volume of the cylinder (V) = 3850 cm3.

Measurement of the volume of a liquid 100 ml Wrong
90 ml Correct
The volume of liquids is measured by 80 ml Wrong
different measuring devices. In the 70 ml
science laboratory, volume of different Fig. Water 60 ml
liquids is measured by using a measuring 50 ml
cylinder. Measuring cylinders are of 40 ml
different capacities like 10, 25, 50, 100, 30 ml
250, 500, 1000 millilitres. Volume of a 20 ml
liquid is measured in millilitres (ml) 10 ml
or cubic centimetre (CC) and litre (l).
To measure the volume of a liquid, the

1.14

14 GREEN Science and Environment Book-7

liquid is poured into a measuring cylinder. The reading at the surface of the
liquid is noted which is the volume of the liquid.

Different liquids form different surfaces in a measuring cylinder. Liquids
like water, alcohol, kerosene, oil, etc. form a concave surface and liquids like
mercury form a convex surface in the cylinder. The eye should be kept in the
level with the bottom of the meniscus in the concave surface and eye should
be kept at the upper meniscus in the convex surface of liquid in the measuring
cylinder to avoid parallax error.

Fig. Lower meniscus Upper meniscus
Water Mercury

1.15

Measurement of the volume of irregular solid objects

The volume of irregular solid objects can be measured by displacing water (liquid)
using a measuring cylinder. When an irregular body is completely immersed in
the liquid (water), it displaces liquid (water) equal to its volume and the level of
water increases. The increase in the volume of water is the volume of that object.
This method is called a water displacement method. It is applicable for irregular
solid objects which are insoluble in liquid (water) and can be handled by using a
measuring cylinder.

Experiment: 1

To measure the volume of a piece of brick by a water displacement method
Materials Required: Measuring cylinder, a piece of brick, thread, water

Procedure
• Take a measuring cylinder and fill it in half with water.

• Record the level of water in the cylinder. Let it be Vi which is the initial
volume of water.

• Now, tie the given piece of brick with a thread and immerse it into the
water in the measuring cylinder. The level of water rises in the cylinder.

• Record the level of water rise. It is the final volume (Vf) of water.

GREEN Science and Environment Book-7 15

Fig. 100 ml 100 ml
90 ml 90 ml
1.16 80 ml 80 ml
70 ml 70 ml
60 ml 60 ml
50 ml 50 ml
40 ml 40 ml
30 ml 30 ml
20 ml 20 ml
10 ml 10 ml

Observation
Initial volume of water in the cylinder (Vi) = 50 ml
Final volume of water in the cylinder (Vf) = 80 ml
Volume of given irregular body (V) = Vf - Vi = 80 ml - 50 ml = 30 ml
\ Volume of the piece of brick = Volume of the water displaced
= 30 ml
= 30 cm3 [\ 1 ml = 1cm3]

Precautions
1. Measuring cylinder should be placed on a plane surface and water should be

at rest.

2. The reading of level of the water should be taken at the bottom of the menisus.

Activity 6

Take a piece of stone and measure its volume by water displacement method.

Key Concepts

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

2. SI system is developed to bring uniformity in measurement all over the
world.

3. Those quantities which can be measured are known as physical quantities.

16 GREEN Science and Environment Book-7

4. The things having a fixed geometrical shape and proper dimension are
called regular objects.

5. The things which do not have a fixed geometrical shape are called irregular
objects.

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

7. Mass is the total quantity of matter contained in a body. Its SI unit is kilogram
(kg).

8. The interval between any two events is called time. Its SI unit is second (s).

Exercise

1. Put a tick (√) for the correct statement and a cross (×) for the incorrect one.
a. Physical quantities cannot be measured.

b. In SI system, mass is measured in metre.

c. The duration between any two events is called time.

d. The volume of a solid object is measured by a measuring cylinder.
e. The area of an irregular object can be measured by using a

graph paper.

2. Fill in the blanks with appropriate words
a. The ......................... which can be measured are called physical
quantities.

b. In SI system, amount of substance is measured in .........................

c. The interval between any two events is called .........................

d. ......................... is the SI unit of temperature.

e. The volume of ....................... is measured by using measuring cylinder.

3. Tick (√) the best answer from the given alternatives.
a. The SI unit of mass is .......

newton kelvin

kilogram metre

GREEN Science and Environment Book-7 17

b. The total quantity of matter contained in a body is called ..........

time mass

length measurement

c. SI unit is accepted throughout the ........

Nepal Asia

World France

d. The volume of a sphere can be measured by the formula .......

pr2h 2prh
l×b
4 pr3
3

4. Answer the following questions.
a. What is measurement? Define physical quantity.

b. What is SI system of measurement?

c. Name any five physical quantities.

d. Define length. Write its SI unit.

e. What is meant by the mass of a body? Write its SI unit.

f. What is the duration between any two events called? Write down its SI
unit.

g. Define regular body with any three examples.
h. What is area? How is the area of a leaf measured?
i. Define volume. Write its SI unit?

5. Differentiate between
a. Area and Volume
b. Regular Body and Irregular Body
c. Mass and Volume

6. Give reason.

a. Standard physical quantity is used for correct measurement of the
given body.

18 GREEN Science and Environment Book-7

b. Graph paper is used to measure the area of an irregular body.

c. A measuring cylinder is used to measure the volume of an irregular
solid object.

7. Numerical problems

a. The length of a white board is 80 cm and its breadth is 65 cm. Calculate

its area. (Ans: 5200 cm2)

b. Study the given figure and calculate the area of the leaf.

Area of each square
= 1 cm2

c. The length, breadth and height of a box is 8m, 6m and 3m respectively.

Calculate its volume. (Ans: 144 m3)

d. A cylindrical body has 6m height and its radius is 2m; calculate its

volume. (Ans: 75.428 m3)

e. The radius of a volleyball is 0.05 m, calculate its volume.
(Ans: 0.00052 m3)

GREEN Science and Environment Book-7 19

UNIT Force and Motion

2

Weighting Distribution (Approximate) Teaching periods : 4 Marks (in %): 1

Before You Begin

Force can be defined as the pull or push which changes or tends
to change the position of a body. In our daily life, force is used for
pulling, pushing, pressing, lifting, stretching, twisting, etc. Force
can change the shape and size of a body. Force can stop a moving
body and change its direction. Similarly, force can bring a body
from rest to motion. The SI unit of force is newton (N). Force is a
vector quantity as it has both magnitude and direction. A body is
said to be in motion when it changes its position with respect to
the objects in its surroundings.

Learning Objectives Syllabus
After completing the study of this unit, students will be able to:
i. introduce force and explain various types of force. • Force and its types
• Distance and displacement
ii. introduce distance and displacement. • Scalar and vector quantity
• Speed and velocity
iii. define speed and velocity and explain uniform and • Acceleration
non-uniform velocity.

iv. define acceleration.

v. solve some simple numerical problems related to
force and motion.

Glossary: A dictionary of scientific/technical terms

force : the pull or push which changes or tries to change the position of a body
centripetal : acting towards the centre
centrifugal : the force acting between any two bodies due to their masses
acceleration : the rate of change in velocity

20 GREEN Science and Environment Book-7

Force and Motion

In our every day life, we need force to do a variety of works. For example,
pulling a bucket from a well, opening (pushing) and closing (pulling) doors
and windows, kicking a football, lifting different loads and so on. We push the
door while opening and pull the door while closing. The pulling or pushing
process of an object brings a change in the position and motion. So, the pull
or push which changes or tends to change the position of an object from rest
to motion or motion to rest is called force. Force is used for pulling, pushing,
lifting, pressing, twisting and so on. Force is an external agent which is applied
on a body. Force has magnitude as well as direction. So, it is called a vector
quantity. In SI system, force is measured in newton (N). In CGS system, it is
measured in dyne.
Force acting on a body can do the following things:
i. Force can change the shape of a body.
ii. Force can change the position of a body.
iii. Force can change the direction of a moving body.
iv. Force can stop the moving body.
v. Force can change the speed of a moving body.

Types of force

There are different types of force. Among them, the main types of force are
given below:
i. Pulling force
ii. Pushing force
iii. Centripetal force and centrifugal force
iv. Gravitational force
v. Magnetic force
vi. Electrostatic force
vii. Frictional force
viii. Muscular force

GREEN Science and Environment Book-7 21

i. Pulling force Fig. Fig.2.1

The force which is used to pull any object
is pulling force. We can pull lighter objects
more easily than heavier ones. A force is
used to pull a cart on the road. A force
is used to pull a door to close it. A force
is used to reduce the speed of a moving
object. These examples explain the pulling
force. However, pulling force may or may
not change the position of the body pulled.

ii. Pushing Force 2.2

The force which is used to push an object
is a pushing force. We can push lighter
objects more easily than heavier ones. We
push a door to open it. Similarly, kicking
a football, throwing a ball, squeezing a
lemon, etc. are acts of pushing. We use
pushing force to push different objects in
our daily life.

iii. Centripetal force and Centrifugal force

When a body moves in a circular
path, a force acts towards the centre
which is called centripetal force and
another force acts away from the
centre which is called centrifugal
force.
Fig.
When a cyclist turns a bicycle in

a circular path, the cyclist bends 2.3
towards the centre due to centripetal

force. Similarly, when a bus is moving in a bend, the passengers experience a

force which acts away from the centre, that force is a centrifugal force. 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. A body moves in a circular motion, if there is balance between

centripetal and centrifugal force.

22 GREEN Science and Environment Book-7

Centrifugal
force

Centripetal
force

Fig.
Fig.
2.4
Fig.
Activity 1
Take a small tin can and tie it with a rope of a length of about 100 cm. Hold
the can in your hand firmly and rotate it in
a circular path as shown in the given
figure. Can you experience centripetal
and centrifugal force?

2.5

iv. Gravitational force
If we throw a stone upwards, it returns to the ground. Similarly, flowers and
fruits of plants fall on the ground. Water flows from high to low land. These
activities are caused due to the force of attraction exerted by the earth on all the
bodies towards its centre which is called gravity. Like the earth, other planets
and satellites have their own gravity. The planets of the solar system revolve
around the sun. A kind of force exists between the planets and the sun. The
force of all attraction between any two objects due to their masses is called
gravitational force.

Do You Know

Some effects of gravity of the earth are rainfall,
blowing of wind, flowing of river, falling of
fruits towards the earth's surface, etc.

2.6

GREEN Science and Environment Book-7 23

Activity 2

Take a ball and throw it upward. What do you observe after a while? After
sometime, the ball returns to the ground.

v. Magnetic force

The force exerted by a magnet to the
magnetic substances is called a magnetic
force. Iron, cobalt, nickel, etc. are called
magnetic substances. The magnetic force
cannot be exerted on non-magnetic
substances. When iron nails are kept near
by a magnet, it attracts the nail towards its
surface or poles. Such types of force is
called magnetic force.
Fig.2.7
Fig.
S Attraction N Repulsion

N NS S
2.8 S N

Magnetic force is used in our daily life

in many devices. It is used to make Do You Know

different electrical and electronic Like poles of magnets repel and unlike
devices. Similarly, magnetic force is
poles attract each other.

used by doctors to remove iron dust The force exerted by a magnet is called

from the eyes of patients. It is also magnetic force.

used to shift and lift heavy loads of

iron and steel in industries and used to separate magnetic substances from

garbages.

Activity 3

Take some iron nails and spread them on a sheet of paper. Bring a bar magnet
near the nails. The nails will move towards the poles of the magnet. The clips
get drawn towards the magnet due to the influence of magnetic force. Such
types of force is called magnetic force.

24 GREEN Science and Environment Book-7

vi. Electrostatic force

The force exerted by an electrically charged

object is called electrostate force. If you Rubber pen
bring a plastic pen near the pieces of papers,

nothing happens. But when we rub the pen

in dry hair several times and bring it near Fig. Attraction
the pieces of paper, they are drawn towards Pieces of paper
the pen. Here pen became charged and it

attracted the uncharged pieces of paper. So, 2.9

an electrically charged object can exert an

electrical force on uncharged object or another charged object.

Activity 4

Take a piece of paper and cut it into many small pieces. Keep those pieces on
a table. Now, rub the comb for several times in dry hair and bring it near the
pieces of paper. What do you observe? Why?

vii. Frictional force

The force which opposes the motion of one body over another when they are in
close contact is called a frictional force. It depends on the extent of roughness
and smoothness of the surfaces in contact. If the contact surfaces are rough,
the friction will be more. But if contact surfaces are smooth, the friction will
be less. Frictional force also depends on the weight of the sliding body on the
surface of another body. If weight is more, frictional force will be more and if
weight is less, frictional force will be less.

Advantages of frictional force

1. Friction enables us to write on paper with a pen.
2. Friction enables us to apply brakes and stop a moving vehicle.
3. Friction enables us to walk on the ground without slipping.
4. Friction enables nails and screws to hold things together and nuts to be tied,

to cut wood, to burn a matchstick, etc.

Disadvantages of frictional force

1. Friction reduces the efficiency of machines.
2. The moving parts of machines get heated up due to friction.
3. Friction gradually tears away the sole of our shoes and tyres of vehicles.
4. Friction slows down the motion of moving parts of a machine.

GREEN Science and Environment Book-7 25

Methods of reducing friction

1. The friction between movable parts of a machine can be reduced by using
lubricants like oil or grease.

2. The friction between moving parts of some complex machines can be reduced
by using ball bearings between the moving surface.

3. The friction of rough surface can be reduced by polishing of surface.

Activity 5

Take a wooden log and tie it with a rope and drag it with the help of a rope
on the ground. Now put some pieces of cylindrical wood below the log and
drag it again on the ground.

How much effort do you apply in both of the cases?

What do you conclude from this activity?

viii. Muscular force

The force exerted by the muscles of a
body is called muscular force. A horse
uses muscular force to pull a cart. Human

beings use muscular force for pulling, Fig.

pushing, lifting, pressing, running,

jumping, fighting, etc. 2.10

Scalar and Vector Quantities

A physical quantity which is described Do You Know
completely by its magnitude only is
called a scalar quantity. Thus, a scalar The sum of scalar is always positive but
quantity has only magnitude but no the sum of vectors may be positive, zero
direction. Length, distance, time, area, or negative.
temperature, speed, mass, energy,
power and volume are some examples Scalars are added by the rules of simple
of scalar quantities. algebra but vectors are added by the rules
of vector algebra.
A physical quantity which requires
Scalars are not written in a special way but
vectors are written in a special way, e.g.

both magnitude and direction for vector PQ is denoted by PQ .

its complete description is called a

vector quantity. Thus, a vector quantity has both magnitude and direction.

Displacement, velocity, force, acceleration and weight are some examples of

vector quantities.

26 GREEN Science and Environment Book-7

Differences between Vector and Scalar quantity

Vector quantity Scalar quantity

1. Vector quantity has both 1. Scalar quantity has only

magnitude and direction. magnitude.

2. The sum of vectors may be 2. The sum of scalars is always

positive or zero or negative. positive.

Speed and Velocity

The speed of a moving body is defined as the distance covered by it per unit
time, i.e.

Speed = Distance covered
Time taken

The SI unit of speed is metre per second (m/s) and CGS unit is centimetre per
second (cm/s). Speed is a scalar quantity. The speed of fast moving bodies
like car, bus, motorcycle, aeroplane, etc. is expressed in kilometre/hour (km/h).
The speed of a moving body may be uniform or non-uniform.

The velocity of a moving body is defined as the distance covered by a body per
unit time in a fixed direction. It is also called the rate of change of displacement.

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

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

Difference between Speed and Velocity

Speed Velocity

1. The rate of change of distance of a 1. The rate of change of displacement

moving body is called speed. of a moving body is called velocity.

2. Its magnitude is always positive. 2. Its magnitude can be negative,
zero or positive.

3. It is a scalar quantity. 3. It is a vector quantity.

GREEN Science and Environment Book-7 27

Worked out Example: 1

A bus travels a distance of 400 km in 6 hours; calculate the speed of the bus.

Solution

Given,

Distance covered by the bus (s) = 400 km

Time taken by the bus (t) = 6 hr

Speed of the bus (v) = ?

According to the formula

v = s
t

= 400
6

= 66.67 km/hr

\ The speed of the bus is 66.67 km/hr.

Worked out Example: 2

A car is moving in a straight line towards the north. If it covers a distance of 8
km in 40 minutes, calculate the velocity of the car.

Solution

Displacement (s) = 8km

= 8 × 1000 [ 1km = 1000 m]

= 8000 m

Time taken (t) = 40 minutes

= (40 × 60) seconds

= 2400 s

According to the formula,

Velocity (v) = s
t

= 8000
2400

= 3.34 m/s

\ The velocity of the car is 3.34 m/s.

28 GREEN Science and Environment Book-7

Distance and Displacement

Distance

The total length of the path travelled by a body is called distance. In SI system, it
is measured in metre (m). For example, if a motorbike travels from Chapagaon
to Lagankhel, the distance covered by the motorbike is the total length of the
road from Chapagaun to Lagankhel.

Suppose, a motorbike travels from a place A to B (5 km), B to C (2 km) and C to
D (10 km), then the total distance travelled by the motorbike is AB + BC + CD
= 5km + 2 km + 10 km = 17 km.

Therefore, the distance travelled by the motorbike is 17 km.

C D
2 km
10 km

Fig.2.11 A5 km B
Fig.
Distance is a scalar quantity as it has magnitude but no direction.

Displacement

Displacement is the shortest distance between the initial position and the final
position travelled by a moving body in a particular direction. The SI unit of
displacement is metre (m).

It is also measured in centimeter (cm), kilometre (km), etc. For example, if the
motorbike travelled from Chapagaoun to Langakhel, the displacement of the
bike is given by straight distance between Chapagaoun to Lagankhel towards
north.

When a motorbike travels, from a place A to place D, the distance covered by
the motorbike is 17 km but the displacement is only 15 km.

C 10 km D

2 km 2 km

2.12 A 5 km B 10 km E

Displacement is a vector quantity as it has both magnitude and direction.

GREEN Science and Environment Book-7 29

Uniform velocity

If a body travels in a straight line and moves equal distance in equal interval of
time, then the velocity is called uniform velocity. In practical life, a body rarely
has uniform velocity.

Fig. Fig.10 m/s 10 m/s 10 m/s 10 m/s 10 m/s 10 m/s
East 4s 3s 2s 1s West

2.13 5s 0s

In the given figure, a car is travelling from west to east, the car covers 10m in
every one second so the velocity of the car is uniform.

Variable velocity (non-uniform velocity)

If a body travels in a straight line and does not travel equal distance in equal
interval of time, then the velocity of the body is called variable velocity or non-
uniform velocity. Variable velocity is very common in our practical life.

East 3 m/s 5 m/s 4 m/s 6 m/s 10 m/s
2.14 5s 4s 3s 2s 1s West

0s

In the given figure, a car is moving from west to east. The car travels different
distances in every one second. So, the velocity of the car is called variable
velocity.

Average velocity

Average velocity of a moving body is the arithmetic mean of the initial
velocity and the final velocity of a moving body is called average velocity. If
'u' is the initial velocity and 'v' is final velocity of a body, then average velocity

(vav) = Initial velocity (u) + final velocity (v)
2
u + v
\ Average velocity (vav) = 2

Acceleration

Let us consider a car is at rest at the place 'A' (i.e, initial velocity 'u' = 0 m/s),
when it starts moving on the road. Its velocity goes on increasing and reaches
40 m/s after 20 seconds until it attains an uniform velocity from B to C place.

30 GREEN Science and Environment Book-7

0 m/sFig. 40 m/s 40 m/s
A B C
20 sec. 20 sec.

2.15

The velocity of the car changes while moving from place A to place B. But the
velocity of the car does not change while moving from place B to place C. So
the change in the velocity of the car, i.e. acceleration is given by:

Acceleration (a) = 40 m/s
20 s

= 2 m/s2

The acceleration of a body is defined as the rate of change of velocity.

i.e. Acceleration (a) = Final velocity (v) – initial velocity (u)
Time taken

= v – u
t

\ a = v – u
t

The SI unit of acceleration is metre/second2.

i.e. a = v – u
t

= m/s
s

= m/s × s

a = m/s2
Acceleration is a vector quantity as it has both magnitude and direction.

Worked out Example: 3

A bus starts from rest and gains a velocity of 25 m/s towards west after 15
seconds. Calculate the acceleration of the bus.
Solution
Given,
Initial velocity (u) = 0 m/s [ The bus starts from rest.]

GREEN Science and Environment Book-7 31

Final velocity (v) = 25 m/s

Time taken (t) = 15 s

Acceleration (a) = ?

According to the formula,

a = v – u
t

= 25 – 0
15

= 1.66 m/s2

\ The acceleration of the bus is 1.66 m/s2.

Activity 6

Make a track of 50 metre in your play ground with the help of a measuring
tape and lime powder. Make a group of five students and run along the track
one by one and record the time taken by each student to cover the distance
of 50 m with the help of a stop watch. Calculate the velocity of each student.

Key Concepts

1. The physical quantity which changes or tends to change a body from rest to
motion or motion to rest is called force.

2. Forces are of various types. The main types of forces are:

i. Centripetal force ii. Gravitational force

iii. Magnetic force iv. Electrostatic force

v. Frictional force

3. The length of the path covered by a moving body is called distance.

4. The shortest distance covered by a moving body in a fixed direction is called
displacement.

5. The distance covered by a body per unit time is called the speed of a body.

6. The arithematic mean of initial velocity and final velocity of a moving body
is called average velocity.

7. The physical quantity which has only magnitude but no direction is called
a scalar quantity, e.g. time, speed, distance, length, etc.

8. The physical quantity which has both magnitude and direction is called a
vector quantity, e.g. force, displacement, acceleration, etc.

9. The rate of change of velocity of a body is called acceleration. Its SI unit is
m/s2.

32 GREEN Science and Environment Book-7

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect one.
a. Force can change the direction of a moving body.

b. Rainfall occurs on the earth due to effect of gravity.

c. The SI unit of displacement is m/s.
d. The force of attraction between any two bodies of the universe is

called centripetal force.

e. Vector quantity has magnitude but no direction.

2. Fill in the blanks with appropriate words.
a. The force which acts towards the centre is called ........................... force.
b. An electrically charged body exerts ........................... force.
c. An apple falls towards the earth's surface due to ...........................
d. The shortest distance between any two points is called ...........................
e. The SI unit of velocity is ...........................
f. The rate of change of ........................... is called accelration.

3. Write differences between:
a. Centripetal force and centrifugal force
b. Distance and displacement
c. Scalar and vector quantity
d. Speed and velocity
e. Velocity and acceleration

4. Tick (√) the best answer from the given alternatives.
a. In SI system, force is measured in ...........................

pascal newton

dyne metre

GREEN Science and Environment Book-7 33

b. The force that acts away from the centre is called ...........................

centripetal force centrifugal force

gravitational force electrostatic force

c. The force used by a man to pull a bucket from a well is ...........................

pushing force muscular force

pulling force gravitational force

d. The SI unit of velocity is ...........................

m/s km/s

ms m

e. The rate of change of velocity of a body is called ...........................

speed velocity

acceleration motion

5. Answer the following questions.
a. What is force? Name any four types of force.
b. What is centripetal force? Write with example.
c. Define gravitational force. Give any two examples.
d. How can you demonstrate the magnetic force? Write with an example.
e. How does frictional force produce? Write down any two effects of
friction.
f. What is electrostatic force? Give one example.
g. Define vectors and scalars with any three examples of each.
h. What is meant by distance and displacement?
i. What do you mean by average velocity? Write its SI unit.
j. What is acceleration? Write its formula and SI unit.

6. Give reason:
a. A magnet attracts iron nails.
b. The moon revolves around the earth.

34 GREEN Science and Environment Book-7

c. Displacement is called a vector quantity.
d. If we drop a stone from the roof of a house, it falls downwards.
e. Grease is applied to the moving parts of a machine.

7. Match the following:

A B

Centripetal force Force exerted by a magnet

Gravitational force Force exerted by a charged body

Magnetic force Force acting towards the centre

Electrostatic force Force which opposes motion

Frictional force Force acting between any two objects

8. Numerical problems

a. A bicycle covers a distance of 100 metres in 5 seconds. Calculate the

speed of the bicycle. [Ans: 20 m/s]

b. A car is moving with the velocity of 20 m/s. Calculate the distance

covered by the car in 10 seconds. [Ans: 200 m]

c. The initial velocity of a micro van is 15 m/s. It gains a velocity of 40 m/s

in 10 seconds. Calculate the average velocity and acceleration of the

van. [Ans: 27.5 m, 2.5 m/s2]

d. A racing car starts from rest. If it gains an acceleration of 5m/s2 in 10

seconds, calculate the final velocity. [Ans: 50 m/s]

GREEN Science and Environment Book-7 35

UNIT Simple Machines

3

Weighting Distribution (Approximate) Teaching periods : 4 Marks (in %): 1

Before You Begin

We use a variety of machines like knife, scissors, pulley, screw, axe,
beam balance, crow-bar, etc. to make our work easier and faster.
These machines have simple structure. So they are called simple
machines. Simple machines help us to work more efficiently. They
help us to perform mechanical work using our muscular energy.
Simple machines help to multiply force, change the direction of the
force and increase the speed of work. We can make our life easier by
using a variety of simple machines.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to simple
i. introduce simple machines with examples. machines

ii. explain types of simple machines with examples. • Types of simple machines
- Lever and its types
iii. explain the utilities of simple machines and utilize - Pulley
them in their daily life. - Wheel and axle
- Inclined plane
- Screw
- Wedge
• Utilities of simple machines in

our daily life

Glossary: A dictionary of scientific/technical terms

machine : a piece of equipment with moving parts that is designed to do a particular
work
load
effort : the force exerted by a machine after application of effort
convenient : the force applied on a machine while doing work
input work : easy or quick to do
output work : the work done on a machine
ideal : the work done by the machine
fulcrum : perfect, most suitable
screw : the point on which a lever turns or is supported
wedge : a modified inclined plane with grooves cut in it
: a simple machine having two or more sloping surfaces

36 GREEN Science and Environment Book-7

Simple Machine

We use different types of machines in our daily life to make work easier, faster
and comfortable. A pulley is used to pull a bucket of water from a well. A
crowbar is used to overcome a heavy stone. A knife is used to cut vegetables. A
nail cutter is used to cut nails. A bottle opener is used to open the lid of a coca
cola bottle. A screw driver is used to unscrew or tighten the nuts and so on. So,
a simple machine is a device having simple structure which makes our work
easier, faster and more convienent.

Fig.

3.1 Bottle opener Knife Crowbar
Nail cutter

We use simple machines for different purposes which are as follows.

• Simple machines multiply the Do You Know
force applied.
The machines which make our work easier,
• Simple machines change the faster and more convenient and simple in
direction of the force applied structure are called simple machines. For
example pulley, crowbar, nail cutter, bottle
• Simple machines help to apply opener, screw driver, etc.
force at a convenient point.

Types of Simple Machines

Simple machines are classified into six types on the basis of their structure and
use which are as follows:

1. Lever 3. Wheel and axle 5. Screw

2. Pulley 4. Inclined plane 6. Wedge

1. Lever

We use simple machines like scissors, beam balance, crow bar, wheel-barrow,
forceps, etc. These simple machines are called lever. A lever is a straight or
bent rigid bar which moves freely about a fixed point. The fixed point of a
lever is called fulcrum. In a lever, effort is applied at one point to lift a load
kept on another point. A lever consists of three parts. They are: fulcrum, effort
and load.

GREEN Science and Environment Book-7 37

Effort (E) Fulcrum (F) Load
3.2 Effort arm (L)
Fig.
Fig. Load arm

The distance between the fulcrum and the point where effort is applied is
called effort arm or effort distance. The distance between the fulcrum and the
point where load acts is called load arm or load distance.

When lever is ideal and in balanced Do You Know
condition, input work is always
equal to the output work. It is called The work done on a machine is called input
principle of lever. work and the useful work done by the
machine is called output work.

i.e. Input work = Output work

or, Effort × Effort arm = Load × Load arm

Types of lever

Lever is classified into three types on the basis of position of fulcrum, effort
and load. They are as follows:

i. First class lever

ii. Second class lever

iii. Third class lever

i. First class lever Effort
In scissors, the
Load
fulcrum lies in Fulcrum Load arm
Effort arm
between load and

effort. It is called first 3.3
class lever. The lever

in which the fulcrum lies between

the load and the effort is called first Do You Know

class lever. Scissors, see saw, beam In first class lever, effort arm is longer than the
load arm.
balance, crowbar, pliers, etc. are

examples of first class lever.

38 GREEN Science and Environment Book-7

Fig.3.4 See-saw Beam-balance
Fig. Scissors

ii. Second class lever

In a wheel-barrow, Effort
load lies in between
Load
fulcrum and effort. Fulcrum

It is called second Effort arm

class lever. The Load arm

lever in which the 3.5

load lies between

the effort and the fulcrum is called Do You Know

second class lever. Bottle-opener, In second class lever, effort arm is always
longer than the load arm. So second class lever
paper-cutter, lemon-squeezer, wheel always multiplies the effort applied on it.

barrow, nut-cracker, etc. are some

examples of second class lever.

Fig.3.6 Nut-cracker Lemon-squeezer
Fig. Wheel-barrow

iii. Third class lever Effort

In a fishing rod, the Load Fulcrum
effort lies in between
load and fulcrum. It is 3.7
called third class lever.
The lever in which the

GREEN Science and Environment Book-7 39

effort lies between the fulcrum and the Do You Know
load is called third class lever. Shovel,
spade, forceps, fire tongs, fishing rod, In third class lever, effort arm is always shorter
etc. are examples of third class lever. than the load arm. Therefore, third class lever
increase the speed of work done, but it can't
overcome the heavy loads.

Fig.3.8Fishing rodShovel
Fig. Fire tongs

Activity 1

Collect different types of lever present at your home.
Identify load, effort and fulcrum in these levers one by one.
Classify these levers in terms of first class, second class and third class.

2. Pulley

Have you seen people lifting water

from a well using a pulley? Have you

observed the structure of a pulley? Wooden disc

It consists of a circular disc having a Groove
groove over which a rope is passed.

A simple machine having a groove Rope

in circular metallic disc or wooden

disc over which a rope passes is

called pulley. In a pulley, the load is 3.9 Effort Load
connected to one end of the rope and

effort is applied at another end. When the rope moves, the disc rotates.

Types of pulley Do You Know

There are two types of pulleys on the Pulley is commonly used to lift heavy loads.
basis of mobility. They are as follows: Pulley makes our work easier by changing the
direction of force applied.
i. Fixed pulley

ii. Movable pulley

40 GREEN Science and Environment Book-7

i. Fixed pulley

If a pulley remains at a fixed point and does
not move up and down along with load, it
is called a fixed pulley. The wheel of a fixed
pulley rotates about an axle fixed to a support
and does not move as the load is raised.

In a single fixed pulley, a rope passes over

the groove of the pulley, one end of which Fig. Fig. Load
Fig.
is connected to the load and effort is applied

at the other end of the rope. Effort is applied 3.10 Effort
downward and the load can be raised

upward. So, it is used to change the direction of applied force. A fixed pulley

is generally used to lift water from a well and in a flag pole to raise the flag.

ii. Movable pulley Effort
Rope
A movable pulley is the pulley which moves
up and down along with the load and is not Pulley
fixed at a point. In this pulley, one end of rope
is fixed at a point and the effort is applied at the
another end. The load is connected to the axle
of the pulley.

Do You Know 3.11 Load

The effort applied to lift a load is shared
equally by both parts of a rope supporting the
pulley. So, we can lift load two times heavy by
applying an effort on a single movable pulley.

In a movable pulley, downward effort can be used to overcome the load and
pull upward.

3. Wheel and axle Wheel
Axle
You might have seen simple machines having Rope
two cylinders of different radius like screw
driver, knob of a tap, the steering of a car, Effort Load
bobbin of a kite, paddle of bicycle, etc. These
simple machines are examples of wheel and
axle. Wheel and axle is a arrangement of two
co-axial cylinders of different radius attached

3.12

GREEN Science and Environment Book-7 41

to each other. The cylinder which has longer radius is called a wheel and that
having a shorter radius is called an axle. The rope is wound around the wheel
and axle.

If we rotate the wheel, the axle also rotates and when the wheel completes one
rotation, the axle also completes the same. But distance covered by the wheel
is more than the axle due to different radius. So the effort is applied to the free
end of a rope wound around the wheel and the load is connected to the free
end of the rope wound around the axle. The effort applied on the wheel is
magnified and a heavy load of axle will be overcome by a small effort applied
on the wheel.

Fig.3.13
Fig. Fig.
Steering of a car Screw driver Bobbin of a kite Handle of sewing
machine
4. Inclined plane

We know that it is very difficult to climb

up hilly areas by vehicles without winding

roads. Similarly, it is very difficult to load

a heavy drum of kerosene into a truck. But

it can be lifted easily by using a wooden

plank. So a plane (a wooden plank) which

makes an angle with the horizontal and

is used to push things upward is called 3.14
inclined plane. It is considered as a simple

machine because it makes work easier and comfortable by carrying heavy

loads.

3.15 Steep road Children slide
Ladder

42 GREEN Science and Environment Book-7

5. Screw Do You Know

A screw is a simple machine which

is used to lift very heavy loads and Jack screw is used to lift heavy automobiles.

tighten the nuts. Actually, screw is Screws and bolts are used to fasten two or

a modified form of inclined plane more pieces of wood or metal.

with grooves cut in it. It looks like a

nail having winding edges called a

thread. The distance between two screw thread is called a pitch. The force is

applied to the head of screw with the help of a screw driver. Examples: screw

nail, driller, jack screw, etc are some examples of screws.

Fig.3.16DrillerScrew nails
Fig. Fig. Jack screw

6. Wedge Effort
(Blunt surface)
A wedge is a simple machine which has
two or more sloping surfaces that taper
to form a sharp edge or pointed edge.
Examples: axe, knife, sickle, nail, etc.

Wedges are used for splitting, cutting Load Load
and drilling holes. A wedge consists of (Slope)
a blunt and a sharp or pointed end. We

should apply effort to the blunt end of

the wedge to work with the sharp or 3.17 (Sharp or pointed surface)
pointed end.

3.18 Knife Sickle
Axe

GREEN Science and Environment Book-7 43

Importance of Simple Machines

Simple machines are very important for human beings. We do a variety of work
in our day to day activities. We need to apply more effort to do difficult work.
Such type of work can be done by using simple machines. Simple machines
make our work easier, faster and comfortable. They help to multiply the force
applied and makes our work easier and faster.

Activity 2

Identify the given simple machines. Write down their name, type and uses
in the given table.

Fig.

3.19 Type Use
Lever
S.N. Name of simple machine To cut things like
1. Scissors paper
2.

Write conclusion of your activity.

Key Concepts

1. A machine having a simple structure which makes our work easier, faster
and convenient is called a simple machine.

2. Simple machines are used to change the direction of force applied, multiply
the force applied and to apply force at a convenient point.

3. A lever is a rigid bar which moves freely about a fixed point called fulcrum.

44 GREEN Science and Environment Book-7

4. First class lever is the lever in which fulcrum lies between effort and load.
5. Second class lever is the lever in which load lies between fulcrum and effort.
6. Third class lever is the lever in which effort lies between fulcrum and load.
7. Pulley is a simple machine having a grooved circular metallic disc or wooden

disc over which a rope passes.
8. Wheel and axle is a simple machine having two co-axial cylinders of different

radius.
9. Inclined plane is a plane which makes an angle with the horizontal and is

used to push things upward.
10. Screw is a modified form of inclined plane with groove cuts in its edges.
11. Wedge is a simple machine having two or more sloping surfaces that tapers

either to form a sharp edge or pointed edge.

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect statement.
a. Simple machines make our work easier and faster.
b. Wheel-barrow is an example of a second class lever.
c. In a movable pulley, pulley moves along with load.
d. In wheel and axle, load is connected at the wheel.
e. Jack screw is used to lift heavy automobiles.
f. An axe is a good example of wedge.

2. Fill in the blanks with appropriate words.

a. Beam balance is an example of ............... class lever.

b. ............... is a fixed point about which a lever can rotate.

c. Nut cracker is an example of ............... class lever.

d. A ................. is a simple machine which changes the direction of applied
force.

e. Knife and axe are examples of ...............

f. In third class lever, ............... is situated between fulcrum and load.

GREEN Science and Environment Book-7 45

3. Answer the following questions:
a. Define simple machine with any five examples.
b. How many types of simple machines are there? Write their names.
c. What is a lever? Write the name of various types of lever.
d. Write down the principle of lever.
e. What is a first class lever? Write any three examples of it.
f. What is a second class lever? Write any three examples of it.
g. What is a third class lever? Write any three examples of it.
h. What is a pulley? Why is it used in our daily life?
i. Define fixed pulley with an example.
j. What is a movable pulley? Draw its figure.
k. What is a wheel and axle? Write any three examples.
l. What is an inclined plane? Write any three examples.
m. What is a screw? Write any three examples.
n. What is a wedge? Write any three examples.

4. Differentiate between:
a. First class lever and Second class lever
b. Pulley and Wheel and axle
c. Fixed pulley and Movable pulley
d. Input work and Output work

5. Match the following

A B

First class lever Axe

Second class lever Bicycle paddle

Third class lever Beam balance

Wedge Fire tongs

Inclined plane Wheel-barrow

Wheel and axle Steep road

46 GREEN Science and Environment Book-7

6. Tick (√) the best answer from the given alternatives.
a. Lemon squeezer is an example of ............

wedge first class lever

second class lever Third class lever

b. The point from which lever can rotate is called ..........

load effort

fulcrum all of the above

c. A useful work done by a machine is called ....

fulcrum input work

output work effort

d. A wedge is a simple machine used to ......
apply force at a convenient point

multiply the force applied

multiply the load

cut and split things

e. Bobbin of a kite is an example of .... fixed pulley
wheel and axle

screw wedge

7. Give reason.
a. A beam balance is called first class lever.
b. An axe is called a wedge.
c. A screw driver is used to tighten the screw.
d. A pulley is called a simple machine.

GREEN Science and Environment Book-7 47

8. Classify the given levers.

i. Shovel ii. Broom iii. Fire tongs
vi. Crow-bar
iv. Nut cracker v. Wheel-barrow ix. Bottle opener

vii. Khukuri viii. Fishing rod

9. Draw the diagram of following simple machines.

a. Nut cracker d. Fixed pulley

b. Shovel e. Moveable pulley

c. Beam balance f. Axe

10. Numerical problems

a. A load of 2000 N can be lifted by applying an effort of 500 N. If the

load arm is 50 cm, calculate the effort arm. [Ans: 200 cm]

b. An effort of 40 N is applied to lift a load. If the load arm and effort arm

are 30 cm and 75 cm, calculate the load. [Ans: 100 cm]

c. Study the given figure and calculate the effort distance. [Ans: 70 cm]
200 N

?

700 N
20 cm

d. An effort of 10 N is applied to lift a load of 50 N. If the effort arm

is 10 cm, calculate the load arm, input work and output work

[Ans: 2 cm, 100 Nm, 100 Nm]

48 GREEN Science and Environment Book-7

UNIT Pressure

4

Weighting Distribution (Approximate) Teaching periods : 3 Marks (in %): 1

Before You Begin

We feel difficult to chop vegetables with a blunt knife. So we prefer a
sharp knife. The cutting surface of the blunt knife has more area than
that of a sharp knife. The force of our hand falls over more area of the
blunt knife and exerts less pressure. The force acting perpendicularly
on a unit area of a surface is called pressure. It can also be defined as
the thrust acting per unit area of a surface. Pressure exerted by a body
depends on force applied and area over which the force acts.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to pressure
i. introduce pressure with its formula and SI unit.
• Formula and SI unit of
ii. explain the application of pressure in our daily life pressure
with examples.
• Measurement of pressure
iii. solve some simple numerical problems related to
pressure. • Differences between force and
pressure

• Application of pressure in our
daily life

Glossary: A dictionary of scientific/technical terms

pressure : the thrust acting per unit area of a surface
thrust
stud : the force acting perpendicularly on a surface

blunt : the small round piece of something which is attached to the surface of
sharp something

: having a thick edge, not sharp

: having a thin edge that is able to cut things or a fine point that is able to
make hole in things

GREEN Science and Environment Book-7 49

Pressure

Pressure is defined as the thrust acting per unit area of the surface. If 'F' is the
thrust, i.e. perpendicular force acting on a surface area 'A', then the pressure

Thrust (F)
(P) acting on the surface is given by: Pressure (P) = Area (A)

\ F
P= A

SI Unit of Pressure

The SI unit of thrust (F) is N and that of area (A) is m2. So, the unit of pressure
(P) is N/m2 (newton per square metre) or Pa (pascal).

Factors Affecting Pressure

1. Thrust or force acting perpendicularly on a surface

2. Area over which force acts

Pressure is directly proportional Do You Know

to the thrust applied and inversely One pascal pressure is the pressure exerted
proportional to the area that on a surface area 1m2 by a thrust of 1 N.

receives the thrust. Therefore,

same thrust can produce different 1N
pressures depending on the area In short, 1 Pa = 1m2

over which it acts. When a thrust

acts over a large area of a surface,

it produces small pressure. But if the same force acts over a small area, it

produces a large pressure. Therefore, we prefer a sharp knife than a blunt one

to chop vegetables. A sharp knife cuts vegetables better due to its sharp edge.

The force of our hand falls on less area and produces more pressure. It makes

cutting of vegetables easier. When the thrust acting on a surface increases,

pressure also increases.

Measurement of Pressure

Pressure is the total perpendicular force acting per unit area of the object. It

can also be represented as follows:

Pressure (P) = Force (F)
Area (A)

50 GREEN Science and Environment Book-7