New Creative Science and Environment Book 7

Approved by the Government of Nepal, Curriculum Development Centre (CDC),
Sanothimi, Bhaktapur as an additional material

New Creative

SCIENCE

AND

ENVIRONMENT

Class 7

This book has been updated, revised and edited by
Janak Raj Pant
Rajani Maharjan

Yuwa Raj Guragain

JBD Publication Pvt. Ltd.

Bhotahity, Kathmandu

New Creative

SCIENCE

and Environment

7

Publisher : JBD Publication Pvt. Ltd.

Bhotahity, Kathmandu, Nepal
Tel: 01-4252371
Email: [email protected]
Web: www.jbdbooks.com

Copyright : Publisher

Edition : Fifth, 2073 BS (2016 AD) [Revised and updated]

: Reprint, 2075 BS [2018 AD]

Layout : Deltrox IT Solutions Pvt. Ltd.

Cover : Bishnu Dev Bhandari

ISBN : 978-9937-544-49-8

Mahabir Offset Press, Tel: 025-521634

As this is the age of science and technology, science is a very important subject
to the students of the present days. Scientific inventions and discoveries have brought
great changes in the world. The study of science and technology for the students of
developing countries like Nepal has great importance to make many achievements.
For this, the students should have good knowledge of science through systematic
as well as properly designed textbooks, instructional materials and good teaching
methods. In schools, textbooks are the most important educational materials. So, the
books designed according to the need and interests of the students are very useful
and appropriate.

Keeping these facts in mind this series of textbooks for the students of classes
6-10 has been prepared by a group of writers namely Basu Sharma, Suman Naupane
and Bharat Bhattarai in 2065 with fresh and fascinating approach to the study of
science at school level. During teaching learning process both teachers and students
have to face problems of what is to be taught and learnt, what should be focused for
the examination, how to learn and write to the point, how to start and how to end.

This series for the first time itself is the best attempt and presentation to solve
these problems. After facing all these problems during teaching, we have designed
and prepared this series as a solution with these salient features.

Each book of this series includes-objectives of the unit in the very beginning,
important questions, clues and memory note as important things, definitions in
separate colour, answer writing skills as model questions and their answer, glossary
of difficult words, lots of activities as creative activities, summary of the unit as
revision, etc.

Further this series has been updated, enlarged, modified, edited and designed
according to the syllabus prescribed by CDC of Nepal. Cognitive and practical
student-friendly, colourful presentation and use of lucid and easy language, both
knowledge and exam-oriented matters in sequential order are the main attractions
of this series.

We are grateful to Mr. Himal Paudel for computer work and JBD publication
for publishing this series.

We are highly indebted to those who provide their positive support,
suggestions and comments for the improvement of the series in further editions.

Authors

Unit-1 : Measurement 1
Unit-2 : Force and Motion 19
Unit-3 : Simple Machines 33
Unit-4 : Pressure 42
Unit-5 : Work, Energy and Power 47
Unit-6 : Heat 60
Unit-7 : Light 74
Unit-8 : Sound 84
Unit-9 : Magnetism 92
Unit-10 : Electricity 99
Unit-11 : Matter 109
Unit-12 : Mixture 118
Unit-13 : Metals and Non-Metals 132
Unit-14 : Some Useful Chemicals 140
Unit-15 : Living Beings 146
Unit-16 : Cell and Tissue 171
Unit-17 : Life Processes 181
Unit-18 : Structure of the Earth 193
Unit-19 : Weather and Climate 200
Unit-20 : Earth and Space 209
Unit-21 : Environment and Its Balance 218
Unit-22 : Environmental Degradation and Its Conservation 238
Unit-23 : Environment and Sustainable Development 251

1 Measurement

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

Æ define measurement, units and standard units.
Æ explain different types of systems of measurement.
Æ measure different types of physical quantities like length,

mass, time, area, volume, etc.

Introduction

We encounter two types of quantities such as measurable and non-
measurable qualities in our daily life. Those quantities which can be measured in
different units are called physical quantities. Length, mass, temperature, volume,
etc. are the examples of physical quantities. Similarly, those quantities which
cannot be measured are called non-physical quantities, such as love, kindness,
hatred, emotions, etc. Physics deals with a lot of physical quantities. So, physics
requires a frequent and precise measurement of these physical quantities for
drawing a correct conclusion.

Measurement is the process of finding the value of any unknown physical
quantity by comparing it with a known and standard quality. The physical
quantities are of two types: (i) fundamental quantity and (ii) derived quantity.

(i) Fundamental quantity

The quantities which do not depend upon others and have their own
identity are called fundamental quantities. For example, length is a fundamental
quantity because it doesn’t depend upon mass or time or temperature or any
other physical quantities. There are seven fundamental quantities. They are:

(i) Length (ii) Mass (iii) Time

(iv) Temperature (v) Amount of substance

(vi) Current (vii) Luminous intensity

(ii) Derived quantity

The quantities which depend upon fundamental quantities and do not have
their own identity are called derived quantities. For example, force is a derived

Measur ement 1

quantity because we know that,

Force = mass × acceleration

= mass × change in velocity
time

As the force depends upon mass, length ( velocity = change in distance/time) and time,

it is a derived quantity. There are hundreds and thousands of derived quantities, e.g.

density, volume, velocity, etc.

Memory Note

Acceleration, force etc. are derived quantities.

Questions

# Why is mass called a fundamental quantity?
# Why is pressure called a derived quantity?
Unit

Many measurements are made by comparing the object with the scale of
units or a measuring tool. People use measuring tools of various kinds like a
beam balance to measure mass, a meter gauge to measure the length of cloth, a
watch to measure time, etc. Every measurement involves two things:

(i) A number

(ii) A unit

A number itself is not a measurement. There would be no meaning in saying
that the length of rod is 6. No one would understand whether the rod was 6 cm
or 6 m or 6 mm long. However, if the rod was described as 6 m long, then the
measurement would have a definite meaning.

Thus, a unit is a standard quantity, with the help of which we do different
measurements by comparing the unknown value with it. As there are two
quantities namely, fundamental and derived quantities there are also two units-
(i) fundamental units and (ii) derived units.

Fundamental units and derived units

The units of fundamental quantities or the units having their own identity
are called fundamental units, such as metre, kilogram, kelvin, etc. The seven
fundamental quantities along with their units are as given below:

2 New Creative Science and Environment; Book 7

S.N. Fundamental quantities Fundamental units Symbols
1. Length Metre (m)
2. Mass Kilogram (kg)
3. Time Second (s)
4. Temperature Kelvin (K)
5. Current Ampere (A)
6. Amount of substance Mole (Mol)
7. Luminous intensity Candela (Cd)

The units of the derived quantities or the units which depend on fundamental
units are called derived units. For example, the unit of the volume (m3) is a
derived unit because it depends on the fundamental unit such as metre. Some
other derived units are kg/m3, m2, m/s, Pascal, Newton, etc.

Questions

# Why is kg a fundamental unit but kg/m3 a derived unit?

Importance of measurement

(i) It helps to find the exact value of any physical quantity.
(ii) It helps to buy and sell goods.
(iii) It helps to draw the exact conclusions from experiments and researches.
(iv) It helps to find the truth etc.

System of Measurement
There are four standard systems of measurement. They are : (i) CGS system,

(ii) FPS system, (iii) MKS system and (iv) SI system.

(i) CGS System

The system of measurement in which length is measured in centimetres,
mass in gram and time in seconds is called the CGS system. It is also known as
the French system of measurement.

(ii) FPS System
The system of measurement in which length is measured in feet, mass in

pounds and time in seconds is called the FPS system. It is also known as the
British system of measurement.

(iii) MKS System

The system of measurement in which length is measured in metres, mass in
kilograms and time in seconds is called the MKS system. It is also known as the
Metric system of measurement.

Measur ement 3

(iv) SI System

It is the standard international system of measurement for all fundamental and
derived units. It is introduced by the Eleventh General conference on weights and
measurement held in France in the year 1961 A.D. It is the improved version of MKS
system as it includes all the units of physical quantities (both fundamental and
derived) whereas MKS system includes only three units of fundamental quantities
i.e. length, mass and time. In both of these systems, length is measured in metre,
mass in kilograms and time in seconds.

S.N. Fundamental quantities Fundamental units Symbols

1. Length Metre (m) (m)

2. Mass Kilogram (kg) (kg)

3. Time Second (s) (s)

4. Temperature Kelvin (K) (K)

5. Current Ampere (A) (A)

6. Amount of substance Mole (Mol) (Mol)

7. Luminous intensity Candela (Cd) (Cd)

Memory Note

The MKS system was also originated in France.

Questions

# Why is the SI system known as the extended version of the MKS
system?

# Differentiate between MKS and SI systems.

Measurement of length
The distance between two fixed points is called length. It is measured by

using measuring tools like a measuring scale, measuring tape, measuring rod etc.
The SI unit of length is metre (m). Other units of length are kilometre, millimetre,
centimetre, inch, foot, light year, parsec, etc.

The units like kilometre, decametre and hectometre which are bigger than
metre are called its multiples. Similarly, the units like centimetre, millimetre and
decimetre which are smaller than metre are called its sub-multiples.

Sub-multiples/sub units of metre

1 metre = 10 decimetres (dm)
1 metre = 100 centimetres (cm)
1 metre = 1000 millimetres (mm)

4 New Creative Science and Environment; Book 7

Multiple of metre

10 metres = 1 decametre (dm)
100 metres = 1 hectometre (hm)
1000 metres = 1 kilometre (km)

Smaller units are used to measure the smaller lengths like the length of a
book, a pen, etc. Similarly, metres and kilometres are used for bigger lengths like
the length of roads, etc. Again, parsec is used to measure large distances like the
distance between heavenly bodies, etc.

Memory Note

One light year is the distance travelled by light in one year. Its value is
9.46 × 1012 km. It is the unit to measure the distance between two heavenly bodies
in the space.

Find the correct measurement of the length of your science book, your
height, the length of your fingers and the length of your pen. What types of
units are suitable for measuring the length of your science book and your
height? And why?

Precautions while measuring the length

During the time of measurement of length, we should be very careful. Even
minor carelessness may lead a serious mistake. So, the following points should
be remembered while measuring length.
1. The measuring scale should be kept in a close contact, and it should be

parallel to the measuring side.

(wrong measurement (wrong measurement as it
due to the slanted scale) does not touch the surface)

Measur ement 5

Correct measurement due to the close contact and parallel to the side
2. The eye of the observer must be in front of the reading just vertically

perpendicular.

(Observing from the left (Observing from the right)
(wrong observation) (wrong observation)

(Correct observation)
3. We should not use a damaged scale.

Wrong measurement (Correct measurement)
(A damaged scale)

6 New Creative Science and Environment; Book 7

4. The average reading should be calculated by taking
many readings.

To measure very short lengths, we should take more
readings and thus the average is calculated.

Objective

To measure the thickness of a paper

Materials required

a scale, a book, etc.

Procedure

1. Take a book without its cover.

2. Count the total number of paper sheets of
the book.

3. Measure the total thickness of the book
by using a scale.

4. Divide the total thickness of the book by

the total number of papers.

Thickness of a sheet = Total thickness of the book
Total no. of sheets

Observation

The total thickness of the book = 10cm

The total number of papers = 1000

Thickness of a sheet = 10 = 0.01 cm.
1000

Conclusion

The thickness of a sheet of paper is 0.01 cm.

Measurement of mass

The total amount of matter contained in a body is called its mass. It is
measured by a beam balance and its SI unit is kg. The units like gram, milligram,
centigram etc. which are smaller than kg are known as the sub-multiples of kg and
the units like quintal, tonne, etc. which are bigger than kg are called its multiples.

Measur ement 7

Sub-multiples of kg
1 kg = 10,00,000 mg
1 kg = 1,00,000 cg
1 kg = 10,000 dg
1 kg = 1000 g
1 kg = 100 dag
1 kg = 10 hg

Multiple of kg
100 kg = 1 quintal
1000 kg = 1 tonne

Precautions while measuring the mass

1. A correct beam balance should be used. In a correct beam balance, the
pointer coincides with the mark and the beam should be horizontal to the
surface.

A defective beam balance A correct beam balance

2. Correct and standard weight blocks should be used. In standard weight
blocks, there should not be any scratch but an official stamp of the metrology
department should be marked.

A defective weight block A correct weight block

8 New Creative Science and Environment; Book 7

Measurement of time

The interval or duration between two events is called time. The SI unit
of time is a second. Time is also measured in minutes, hours, nanoseconds,
microseconds, days, weeks, months, years, etc. The units like microseconds,
milliseconds, nanoseconds, picoseconds, etc. are smaller than a second and are
called its sub-multiples whereas, days, hours, minutes are its multiples.

Sub-multiples of second (s)
1s = 1000 milliseconds
1s = 10,00,000 microseconds
1s = 1,00,00,00,000 nanoseconds

Multiple of second (s)
60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day

365 days = 1 year

Time is measured by a clock or a watch. Nowadays, many digital and atomic
watches have been invented, which give us the correct time.

Memory Note

■ The time taken by the earth to complete one rotation around its own
axis is called one solar day. One solar day is equal to 24 hours.

■ The science of locating events in time is called chronology.

■ The study of the devices used to measure time is called horology.

Regular and Irregular body

The bodies having a fixed geometric shape or length, breadth and height
are called regular bodies, and the bodies having no fixed geometric shape or
length, breadth and height are called irregular bodies.

(A) Measurement of an area

The space occupied by a surface is called an area. Its SI unit is a square metre
(m2). Its sub-multiples are cm2, mm2, dm2 and its multiples are km2 and Hm2.

Sub-multiples of m2

1 m2 = (100 × 100) cm2 = 10000 cm2

1 m2 = (1000 × 1000) mm2 = 10,00,000 mm2

Measru ement 9

Multiples of m2
(1000 × 1000) m2 = 1 km2
10,00,000 m2 = 1 km2

(a) Measurement of the areas of regular objects
The area of regular objects can be calculated by using a formula. For example:
For a rectangular surface,
Area = length × breadth
A=l×b
For a square surface
A=l2
Area = (length)2
Area of sphere = 4 p (radius)2 = 4 p r2
Thus, for finding the area of a rectangular object, we can measure its length

and breadth and use the above formula i.e. A = l × b.
(b) Measurement of the area of irregular objects

We can measure the area of an irregular object by the graphical method.
For this, we should trace the surface of the irregular object such as a broken
glass, a leaf, etc. on the graph paper as shown in the figure.

Measurement of the area of irregular object

Then, the numbers of complete squares are counted at first. Let the number
of complete squares be ‘a’. Then the numbers of those squares are counted, which
are covered into half or more than half the size by the outline. Let their numbers
be ‘b’, whereas those squares which are covered into less than half the size by the

10 New Creative Science and Environment; Book 7

outline are neglected. Now, the total numbers of squares (a + b) is multiplied by
the area of one small square of the graph paper, and this is how its total area is
calculated.

Find the area covered by your science book by using the formula and also
find the area of a leaf by using the graphical method.

(B) Measurement of Volume

The total space occupied by an object is called its volume. Its SI unit is cubic metre
(m3). Its sub-multiples are mm3, cm3, dm3 etc. and km3, dam3 etc. are its multiples.
Sub-multiplies of m3

1m3 = (100 × 100 × 100) cm3 = 10,00,000 cm3
1m3 = (1000 × 1000 × 1000) mm3 = 1,00,00,00,000 mm3
Multiples of m3
(1000 × 1000 × 1000) m3 = 1km3
1,00,00,00,000 m3 = 1 km3

(a) Measurement of the volume of regular solids

For the regular solids, the volume can be found by using a simple formula. For
example,

* For rectangular block

Volume = length × breadth × height

V=l×b×h

* For cube

Volume = (length)3

V=l3

* For sphere r

Volume = 4 p(radius)3 Measurement of the volume of
3 regular solid (football)
4
V= 3 pr3

* For cylinder

Volume = p × (radius)2 × (height)

V = pr2h

Measru ement 11

Find the volume of your science book.

(b) Measurement of the volume of irregular solid objects

The volume of irregular objects can be found out by using a measuring
cylinder applying the water displacement method.

Objective : To measure the volume of a piece of stone.
Materials required: an irregular object (stone), a measuring cylinder, water, and

thread
Procedure
1. Pour some water in the measuring cylinder for the stone to sink into it

completely.
2. Note the water level in the measuring cylinder. Let it be ‘x’ ml.
3. Now, tie the stone with a thread and make it immerse in the water

completely as shown in the figure:

Measurement of the volume of irregular solid (stone)

Observation
The water level rises. Let its final reading after immersing the stone into

the water be ‘Y’ ml.
Calculation

The volume of the stone can be calculated directly as:
Volume = (Y – X) ml

Conclusion
Hence, the volume of an irregular object can be directly measured by

using the water displacement method.

12 New Creative Science and Environment; Book 7

(c) Measurement of the Volume of Liquids

The volume of liquids can be directly calculated by using various measuring
vessels such as a measuring cylinder, measuring glass, pipette, etc. The volume
of liquids is generally measured in litres (l).

Various devices used to measure the volume of liquids

We must be careful while measuring the volume of liquids. There are two
kinds of liquids. For example one which wets the wall of the vessel and another
which doesn’t wet the wall of the vessel.

Water which wets the wall of the vessel forms upper concave structure and
we should take the reading of lower meniscus.

Mercury which does not wet the wall of vessel, forms upper convex structure
and we should take the reading of upper meniscus.

(Water) (Mercury)
(Taking lower meniscus) (Taking upper meniscus)
Memory Note

The volume of a broken piece of a glass can be found out by using the water
displacement method.

Measru ement 13

Find the volume of your rubber (eraser) first by using the formula and
then by using the water displacement method. Compare these two results.

1. The unit of pressure (pascal) is a derived unit, why?

Ü The unit of pressure (pascal) depends on three fundamental units such as
kg, metre and seconds as,

Force
Pressure =

Area
\ pascal = kg m/s2

m2

= kg m–1 s–2

So, pascal is a derived unit.

2. Why is the SI system called the extended version of the MKS system?

Ü In the SI system, as in the MKS system, length is measured in metres, mass
in kg and time in seconds. Here MKS includes only these three fundamental
units but the SI system includes all the fundamental as well as derived units.
So, SI system is the extended version of the MKS system.

3. Kilogram is a fundamental unit, why?

Ü Kilogram is a fundamental unit because it does not depend upon other units
and has its own identity.

4. Why do we need standard units?
Ü We need standard units to bring uniformity in measurement all over the

world.

5. Covert 7.15 mm into metre.

Solution, 7.15

7.15 mm = 1000 m [\ 1000 mm = 1m]

\ 7.15 mm = 0.00715 m

6. Convert 1 day into seconds.

Solution,
1 day = 24 hours

14 New Creative Science and Environment; Book 7

= (24 × 60) minutes [\ 1 hr = 60 min]
= (24 × 60 × 60) seconds [\ 1 min = 60 s]
\ 1 day= 86,400 seconds

7. If the area of a room is 16m2 and its length is 4m, find its breadth.
Solution,

Given, area (A) = 16 m2
length (l) = 4 m
breadth (b) = ?

We have, from the formula,

A =l×b

or, 16 =4×b

or, b 16
b =4
=4m

Hence, the breadth of the room is 4 m.
8. The length, breadth and height of a book are 20cm, 15cm and 25 mm

respectively. Find its volume.
Solution,

Given, length (l) = 20 cm
breadth (b) = 15 cm
height (h) = 25 mm = 2.5 cm
volume (v) = ?

We have, v = l × b × h
= 20 × 15 × 2.5
= 750 cm3

\ The volume of the book is 750 cm3.

F The quantities which can be measured are called physical quantities and
those which cannot be measured are called non-physical quantities.

F The physical quantities may be fundamental and derived.
F The quantities which do not depend upon others are called fundamental

quantities and those which depend upon fundamental quantities are
called derived quantities.

Measru ement 15

F There are seven fundamental quantities.
F Measurement is the process of finding an unknown value by comparing

it with a known value.
F The systems of measurements are CGS, FPS, MKS and SI systems.
F The SI system of units is the extended version of the MKS system.
F The total amount of matter contained in a body is called its mass. Its SI

unit is kg.
F Length is the distance between two points. Its SI unit is the metre.
F Time is the interval between two events. Its SI unit is the second.
F The area of a regular object can be calculated by using a formula whereas

the area of an irregular object is found out by using the graphical method.
F The volume of regular objects can be calculated by using a formula

whereas the area of irregular objects is found out by using the water
displacement method.

A. Fill in the blanks.

1. Every measurement involves two things; a _______ and an _______.
2. A cubic metre is the SI unit of _________.
3. Kg/m3 is the SI unit of _________.
4. ___________ method is used to find the area of irregular objects.
5. __________ method is used to measure the volume of irregular objects.

B. Write T for true and F for false statements.

1. A surface area is expressed in m3.
2. 1 day equals to 86,400 seconds.
3. Mass is a fundamental quantity.
4. In the MKS system, length is measured in foot.
5. Temperature is a derived quantity.

16 New Creative Science and Environment; Book 7

C. Choose the best answer.

1. The SI unit of an area is

(a) m (b) m3

(c) m2 (d) m/s2

2. The volume of a cube is given by :

(a) (length)2 (b) (side)2

(c) (length)3 (d) none of these

3. The fundamental units involved in kg/m3 are:

(a) kg and second (b) kg and metre

(c) second and metre (d) metre and kelvin

4. 1 day is equals to :

(a) 24 hrs (b) 86,400 seconds

(c) 1440 minutes (d) all of these

5. Which is not a fundamental unit:

(a) metre (b) ampere

(c) kelvin (d) metre per second

D. Define:

(a) MKS system (b) Physical quantity
(c) FPS system (d) Fundamental quantity

(e) CGS system (f) Derived quantity

E. Answer the following questions.

(a) Define measurement and write its importance.
(b) Differentiate between fundamental and derived units.
(c) What does it mean by “the length of an object is 5 metres”?
(d) Define mass. How is it measured? What is its SI unit?
(e) Why is the volume of irregular objects measured by using a measuring

cylinder?
(f) Why are MKS and CGS systems widely used?
(g) Why is m3 a derived unit?

Measur ement 17

(h) How do you find the volume of irregular objects?

(i) You are given a rectangular block of a brick. How do you calculate its
volume?

Numerical Problems

1. Convert the following:

(a) 300 cm into metre (Ans: 3m)

(b) 46 kg into g (Ans: 46000g)

(c) 3 days into seconds (Ans: 2,59,200 s)

2. The length, breadth and thickness of a brick are 18 cm, 8 cm and 5cm
respectively. Find its (i) the area of the largest face and (ii) volume.

(Ans: 144 cm2 and 720 cm3)

3. The area of a brick is 120 cm2 and the height is 50 mm. Find its volume.
(Ans: 600 cm3)

4. If a piece of paper has the length of 20cm and the breadth of 15cm, find its

surface area. (Ans: 300 cm2)

5. In the given figure, what is the volume of the object immersed in the water?
Why?

250 cm3

200 cm3

Fundamental – basic
Immerse – to put something into a liquid so that it is completely covered
with the liquid
Volume – total space occupied by an object
Density – the ratio of mass and volume of a substance



18 New Creative Science and Environment; Book 7

2 Force and Motion

After the completion of this unit, students will be able to:
Æ define force.
Æ explain rest and motion
Æ define various types of force.
Æ do simple calculations of force.

Introduction

In our daily life, we perform different kinds of work. While doing all those
type of work, we use force. To open a window, we either push or pull it. Similarly,
when we draw a bucket of water from a well, we pull the rope. In the above
mentioned examples, we either pull or push to do the work. So, this pull or push
is called force.

Hence, force is an external agency that changes or tends to change the state
of rest or motion of a body. The direction in which the body is pulled or pushed
is called the direction of force.

Types of force

There are different types of force. Some of them are listed below.
1. Pulling force
2. Pushing force
3. Centripetal and centrifugal force
4. Magnetic force
5. Gravitational force
6. Frictional force
7. Electrostatic force
1. Pulling force
While withdrawing water from a well, we pull the bucket upward. To
displace the table, we pull it. To open the door, we should pull it. Similarly, a
labourer pulls a cart while transporting goods. The above mentioned activities

Force and Motion 19

are the examples of pulling force.

Examples of pulling force
Pulling force is defined as a force that pulls or tends to pull an object. We
should apply less force to pull light objects and more force to pull heavy objects.
2. Pushing force
While playing football, we kick the ball to pass it to a player and to score. In
each action, we push the ball. Similarly, we push a bicycle on an inclined road. In
all the above mentioned actions, we use pushing force.

Examples of pushing force
A pushing force is defined as the force that pushes or tends to push an
object.
3. Centripetal and centrifugal force
The force applied to a body moving in a circular path, which tries to pull
the body towards the center of the circle is known as a centripetal force.
Similarly, the force applied to a body moving in a circular path, which
tries to move it away from the center of the circle is known as a centrifugal force.

20 New Creative Science and Environment; Book 7

To show centripetal and centrifugal forces.
Materials required

stone, rope
Procedure

1. Tie a stone with a thread.
2. Rotate the stone as shown in the

figure. What do you feel?
Observation

A continuous force should be applied in
order to keep the stone in motion and as the
speed increases, it tries to move away.

Two kinds of forces act while rotating a
stone. One tries to pull the stone towards the center of the circle and the other
tries to move away from the center of the circle.

The force applied to a body moving in a circular path which tries to pull
the body towards the center of the circle is known as a centripetal force.

Similarly, the force applied to a body moving in a circular path which
tries to move it away from the center of the circle is known as a centrifugal
force.

Memory Note

Æ When a body moves in a circular path both centripetal and centrifugal forces
act on it.

Æ If centrifugal force exceeds centripetal force, the body goes away from the
center of the circle by breaking the thread.

Questions

‰ Give 5/5 examples of pulling and pushing force.
‰ When does a body moving in circular path go away from the

center of the circle?
4. Magnetic force

If a magnet is brought near iron materials, the iron
materials are pulled towards the magnet. This confirms
that magnet is applying a force to the iron materials.

Hence, the force exerted by a magnet is called a
magnetic force.

Force and Motion 21

Magnet is capable of attracting metals like iron, nickel, cobalt and steel. So,
these substances are called magnetic substances.

Memory Note

Æ The magnetic force helps to separate magnetic substances from the
mixture of non-magnetic substances.

5. Gravitational force

An apple when detached from a tree falls
on the ground. A stone when dropped from
a roof falls on the ground. Why is it so? It is
because the earth pulls them with its pulling
force, which we call force of gravity. This force
always acts towards the center of the earth.
When we lift a bucket full of water from a well,
we apply a force against the pulling force of the
earth.

The force of attraction of the earth towards
its center is known as force of gravity or simply
gravity.

But in this universe, another kind of force also exists. That is the force of
attraction between any two bodies. Due to this force, planets are moving around
the sun. This force is called the gravitational force.

Hence, the force of attraction between any two bodies in the universe is
known as gravitational force.

Moon revolves round the earth in a fixed path because of the gravitational
force between the earth and the moon.

Memory Note

Æ The force of gravity gives weight to every object.
Æ Weight is a force.
Mathematically,

W = mg, m = mass of an object
g = acceleration due to gravity (9.8 m/s2).

22 New Creative Science and Environment; Book 7

6. Frictional force

A rolling football comes at rest after
certain time. A moving bicycle comes at rest
if not pedalled. What makes the bicycle come
at rest? There is a force which stops the bodies
from moving. This opposing force between
two surfaces is called a frictional force.

The resisting force developed between
two surfaces when one slides over another is
known as frictional force.

In the above mentioned examples, the ball and bicycle come at rest because
of the frictional force between the surfaces of the ball and the ground or tyres of
the bicycle and the road. Friction always opposes motion.

Questions

‰ Why does a moving ball stop after certain time?
‰ Are the weight and mass the same terms?

7. Electrostatic Force

The force of attraction which is developed Pen
in a non-conductor body due to change in Bits of paper
number of electrons is called electrostatic force.
For example, when a plastic pen is rubbed over
the dry hair, a force of attraction is developed
on the comb. As a result of this, pen attracts the
pieces of paper.

Rest and motion

If any body changes its position with respect to the surrounding objects or
a reference point, then the body is said to be in motion.

If any body doesn’t change its position with respect to the surrounding
objects or a reference point, then the body is said to be at rest.

From these definitions, it is clear that for defining rest and motion, there
should be a comparison between the bodies and the surrounding. Nobody will
be in motion if there is no body at rest and vice versa. So, we can say that rest and

Force and Motion 23

motion are the relative terms.

Discuss whether the walls of your classroom are at rest or in motion.

Question

# Nobody is perfect rest, why?

Uniform and variable motion

Consider the motion of the car as shown in the figure.

(a) (b) (c) (d) (e)
10 m 10 m
15 m 18 m

Variable motion

The car travels,
a) 10 m from (a) to (b) in 1 sec
b) 15 m from (b) to (c) in 1 sec
c) 18 m from (c) to (d) in 1 sec
d) 10 m from (d) to (e) in 1 sec
Here, the car doesn’t cover an equal distance in an equal interval of time. So,
the motion of the car is non-uniform or variable.
Therefore, if a body doesn’t cover an equal distance in an equal interval of
time then the motion of the body is said to be variable or non-uniform motion.

24 New Creative Science and Environment; Book 7

Instead, if the car covers a distance of 10m in every one second as shown in

the figure below then the car is said to be in uniform motion.

0 sec 1 sec 2 sec 3 sec 4 sec

(a) (b) (c) (d) (e)

10 m 10 m 10 m 10 m

Uniform motion

Therefore, if a body covers an equal distance in an equal interval of time,
then the motion of the body is said to be uniform motion.

Most of the bodies have non-uniform motion like the motion of human
beings, vehicles, animals, etc. But the motion of planets, satellites, heavenly
bodies and machines are the examples of uniform motion.

On a ground, make five points at an interval of 1m each. Roll a ball slowly
from one end to the marked area. With the help of a stopwatch, measure the
time taken by the ball to pass each mark. Write down the conclusion of this
activity.

Vectors and scalars

The quantity having magnitude and a fixed direction is called a vector
quantity, such as displacement, velocity, acceleration, force, etc. They are
represented by a letter with an arrowhead that shows the direction of the vector.
The vectors may be positive, negative or zero.

Suppose a body is travelling from A to B and let length from A to B is 5m.

If we take the direction A to B as positive, we should take the direction B to
A as negative.

A 5m B

Negative and positive direction

The quantity having magnitude but not a fixed direction is called a scalar
quantity, such as mass, distance, density, etc. They are represented by a number
with a proper unit, such as 1 kg, 2m, 1000 kg/m3 etc. The scalars are always
positive.

Force and Motion 25

Differences between scalars and vectors Vectors
Scalars
They have both the magnitude and
1. They have only magnitude but 1. a fixed direction.
not a fixed direction.
Their sum may be positive, negative
2. Their sum is always positive. 2. or zero.

3. Scalars can be added by a 3. Vectors can be added only by
simple algebraic rule. certain vector rules.
Examples: distance, mass,
density Examples: displacement, force,
velocity etc.

Distance and Displacement

The total length between two fixed points is called the distance between
them. Its SI unit is metre and it is a scalar quantity.

Suppose a car travels from A to B with a length of 5m and back to A
following the same path. Then in this condition, the total distance covered by the
car;

= AB + BA

= 5m + 5m

= 10m

But in this case, displacement is zero as a total displacement

= AB + BA
= AB – AB = 5m – 5m = 0

So, displacement is the shortest distance between the initial and final
positions. Here the initial and final positions being the same, the displacement is
zero. Its SI unit is also metre and it is a vector quantity.

Let us take another example. Suppose an object starts from O, travels 5 km
towards east, represented by OA, 5 km towards north, represented by AB and 10
km towards west, represented by BC. The total distance is

= OA + AB + BC

= 5km + 5km + 10km = 20 km

But, here, the displacement is only the magnitude
of OC which is

(OC) = OD2 + DC2
= (5)2 + (10 – 5)2 = (5)2 + (5)2 = 5 2 km

26 New Creative Science and Environment; Book 7

Memory Tips

Automobiles are fitted with a device that shows the distance travelled. Such
a device is known as an odometer and the speed by speedometer.

Question

# Can you state a condition in which displacement of a body is
zero but distance covered is not zero ?

Walk from one corner of your ground to the opposite corner along its side.
Measure the distance covered by you and the magnitude of the displacement.

Speed and velocity

Objects may travel fast or slow. For example, we may observe that an
aeroplane travels very fast while a bicycle is slow. What, precisely do we mean
when we use the terms ‘fast’ and ‘slow’?

When we say a body moves fast or slow we refer to its speed.

The speed of an object is defined as the total distance travelled in per unit time

i.e. Speed (s) = Distance travelled (d)
Time taken (t)
d
\s = t

Its SI unit is m/s. It is a scalar quantity.

The rate of change of displacement of a body is called its velocity i.e.

Velocity (v) = Displacement covered (d)
Time taken (t)
d
\v = t

The SI unit of velocity is m/s and it is a vector quantity.

The speed and velocity have the same meaning if a body is moving in a
straight line in a fixed direction but if the body is moving in a curved path, it is
better to say its motion by speed than that of velocity.

The ratio of total displacement covered by a body and the total time taken
is called its average velocity i.e.

Average velocity (Av)= Total displacement covered (d) 27
Total time taken (t)

Force and Motion

\ Av = d .......................(1)
t

Mathematically, average velocity is the sum of initial velocity (u) and final

velocity (v), divided by 2.

i.e. Average velocity (A.v.) = Initial velocity (u) + final velocity (v)
2
i.e. (A.v.) = u + v ......................(2)
2

From equation (1) and (2)

d = u+v
t 2

\ d = u + v × t .................(3)
2

This shows that the distance covered is equal to the product of average
velocity and time taken.

Memory Tips

Speed of a body is measured by speedometer.

Question

# Discuss with an example that a body moving with a uniform
speed may have variable velocity.

Make a distance of 100m on your school ground. Measure the time you
take to cover this distance and calculate your speed. Now, run and find out the
time you take to complete the same distance and calculate your speed. Compare
the results.

Acceleration

The rate of change in velocity is called acceleration i.e.

Acceleration (a) = Change in velocity
Time taken
= Final velocity (v) – Initial velocity (u)
Time taken (t)
v-u
\ a= t

The SI unit of acceleration is m/s2 and it is a vector quantity. The negative
acceleration is called retardation. For example, if the acceleration of a body is

28 New Creative Science and Environment; Book 7

-2m/s2 then we can say that its retardation is 2 m/s2.

In case of uniform velocity, acceleration (a) of the body is zero.

When the velocity of a body changes at a uniform rate, it is said to have
uniform acceleration, such as the motion of a body falling freely under the action
of gravity, rolling down of ball in a smooth inclined plane, etc.

Similarly, when the velocity of a body changes at a non-uniform rate then
the acceleration is said to be variable or non-uniform.

Solved Numericals

1. A vehicle starts to move from the rest. If its velocity after 5 seconds is 100
m/s, calculate its acceleration.

Solution: Given, initial velocity (u) = 0 [ the vehicle starts from rest]

Final velocity (v) = 100 m/s

Time taken (t) = 5 s
Acceleration (a) = ?

We have, a= v – u = 100 – 0 = 100 = 20 m/s2
t 5 5

\ The acceleration of the vehicle is 20 m/s2.

2. A boy leaves home on a car and reaches his school in 30 minutes. If the
school is 25 km away from his home, calculate the average speed of his
car.

Solution: Given, time (t) = 30 min = 30 × 60s = 1800s

Distance (s) = 25 km = 25 × 1000 m = 25000 m

Average speed (v) = ?

We have, v = d = 25000 = 13.89 m/s
t 1800

\ The average speed of his car is 13.89 m/s.

3. If a car covers a distance of 2.4 km in 10 minutes, how much distance does
it cover in 1 second?

Solution: Given, the distance covered (s) = 2.4 km = 2400 m

Time taken (t) = 10 min = 10 × 60 = 600 s

We have, an average speed (v) = d
t

Force and Motion 29

= 2400 = 4 m/s
600

The average speed of the car is 4 m/s. It means that it covers 4 metres of

distance in one second.

1. What is force? What are the effects of force?

Ü Force is an external agency that changes or tends to change the state of rest
or motion of a body.

The effects of force are: ii) effect on motion
i) effect on shape iv) effect on direction
iii) effect on speed

2. Define gravitational force of attraction.

Ü The force of attraction between any two objects of the universe is called
gravitational force.

3. Why is heat produced when we rub our hands?

Ü Heat is produced due to friction when we rub our hands.

4. Mathematically show one newton force.

Ü We know that, F = m.a

or, F = 1 kg. 1m./s2

or, F = 1 kgm/s2

\ F = 1 N.

5. Give an example where there is uniform velocity but certain acceleration.

Ü A body moving in a circular path with the uniform velocity has acceleration
due to change in direction.

F Force is an external agency that changes or tends to change the state of
rest or motion of a body.

F The force that pulls or tends to pull an object is called pulling force.
F The force that pushes or tends to push an object is called pushing force.
F The force exerted on a body, moving in a circular path which tries to pull

the body towards the center of the circle is called centripetal force.

30 New Creative Science and Environment; Book 7

F The force exerted on a body moving in a circular path which tries to
move it away from the center of the circle is called centrifugal force.

F The force exerted by a magnet is called magnetic force.
F The force of attraction on an object exerted by the earth towards its center

is known as gravity.
F The force of attraction between two bodies in the universe is called

gravitational force.
F The opposing force developed between two surfaces when one slides

over another is known as frictional force.
F The force of attraction which is developed on a insulating body due to

change in the number of elections is called electrostatic force.
F Force is calculated by using the formula:

Force = mass × acceleration

A. Fill in the blanks.

1. Pull or push is called ______________.

2. Pushing force tends to _____________ an object.

3. ___________ force acts away from the center.

4. Friction is ______________ of an area of contact.

5. An object falls towards the centre of the earth due to ____________.

B. Match the following.

1. Force pulling force

2. Gravitational force external agent

3. Rough surface force between two bodies

4. Opening door high friction

C. Write True or False.

1. Force is an internal agency.

2. Friction produces heat.

3. Force charges the shape of a body.

4. Friction never opposes motion.

5. Everybody falls towards the centre of the earth.

Force and Motion 31

D. Define. 2. Pulling force
1. Force 4. Frictional force
3. Magnetic force
5. Electrostatic force

E. Differentiate between.
1. Pulling and pushing force
2. Centripetal and centrifugal force
3. Magnetic and frictional force

F. Give reasons.
1. A ball rolling along the ground stops after some time.
2. Lubricant is applied to the wheels of a bicycle.
3. It is easier to push a car on a smooth road than on a rough road.
4. It is difficult to walk on a smooth floor.
5. There are grooves on the surface of tyres.

G. Answer the following questions.
1. What is force? Write its SI unit.
2. Define centripetal and centrifugal force with an example.
3. State two/two advantages and disadvantages of friction.
4. Why is friction called a necessary evil?
5. What is gravity? Which force exists between two bodies in the universe?

Numerical Problems

1. A bus is moving with an initial velocity 10 m/s. After 2 seconds, the velocity
becomes 20 m/s. Find the acceleration and distance taken by the bus. (Ans:
5 m/s2, 30m)

3. If the velocity of a bicycle is 10 m/s, how long will it take to cover a distance

of 18 km? (Ans: 1800 s)

3. A man runs 28m in a straight line in 4 seconds. Find his velocity. (Ans: 7 m/s)

Perform - do work
Squeezing
Pedalled - pressing inside
Detached
- turned or pressed the pedal

- not joined

‰‰

32 New Creative Science and Environment; Book 7

3 Simple Machines

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

Æ define simple machines.
Æ state various types of simple machines.
Æ explain the principle of a lever.
Æ calculate simple numericals related to a lever.

Introduction

A machine is a device that changes the amount, speed or direction of a
force for a useful purpose. We use different kinds of machines in our daily life
to make our work easier and faster. Scissors are used to cut a piece of cloth.
Wheelbarrows are used to move a pile of bricks. A bottle opener is used to open
a bottle. The scissors, the wheelbarrows, the bottle opener are simple machines.
Machines make it easier for us to do any work.

A simple machine is a simple device that helps us to do our work easily in
a short time.

There are some machines that are made up of a large number of simple
machines. These are called complex machines. Bicycles, sewing machines, buses,
trucks, etc. are the examples of machines.

• How does a simple machine help to do work in an easy way?
Ü The following are the ways how a simple machine makes our work easier.

§ It magnifies the applied effort.
§ It performs work in a short time.
§ It changes the direction of force.

Types of simple machines

The following are the types of simple machines.

1. Lever 2. Pulley 3. Wheel and axle

4. Inclined plane 5. Screw 6. Wedge

Simlp e Machi nes 33

1. Lever

A lever is a rod which can turn

about a fixed point called the fulcrum. E.D. L.D.
It helps to lift heavy loads by applying

less force.

The mass to be lifted or moved
by the lever is called load. The force applied to move the load is called effort.

The distance from load to fulcrum is known as load distance while the
distance from the fulcrum to the effort is called effort distance.

Principle of the lever

The principle of the lever states that for an ideal lever the product of load
and load distance is equal to the product of effort and effort distance i.e.

Load × Load distance = Effort × Effort distance

or, L × L.d = E × E.d
This shows that by making effort distance longer than load distance, we can
lift a heavy load by applying less force.

We use a lever in our daily life as a simple machine to perform work
easily in a short time. Some of the examples of the lever are a see-saws, scissors,
wheelbarrows, nutcrackers, spades, bottle-openers, fishing rods, axes, etc.

Types of lever

On the basis of the position of load, effort and fulcrum, there are three kinds
of levers. They are as described here.

i. First class lever

The lever in which fulcrum lies
between the load and effort is known
as the first class lever.

See-saws, dhiki, scissors, beam
balances, crowbars, nippers, etc. are
some examples of the first class lever.

34 New Creative Science and Environment; Book 7

ii. Second class lever
The lever in which load is

between the fulcrum and the effort is
called the second class lever.

In the second class lever, the
load distance is always less than the effort distance. So, it is very efficient in
multiplying the applied effort.

Wheelbarrows, nutcrackers, lemon squeezers, mango cutters, bottle openers
etc. are some examples of the second class lever.

iii. Third class lever
The lever in which effort is

between the load and the fulcrum is
called the third class lever.

In the third class lever, load
distance is always greater than effort
distance. So, the third class lever cannot multiply the applied effort.

Spades, brooms, shovels, forceps, forearms, tongs, etc. are the examples of
the third class lever.

Simlp e Machi nes 35

2. Pulley
A pulley consists of a circular disc made of metal or wood, having a groove

along its rim. It is a simple machine that is used for lifting a load by applying
effort in a suitable and convenient direction. It is widely used in our daily life
to withdraw water from a well. It is also used by motor mechanics to lift heavy
objects like the engine of a vehicle. It is also used in cranes and lifts.
Types of pulley

The following are the types of pulleys.
i. Fixed pulley: It is similar to the first class lever. It is used to change the

direction of the force. In this pulley, both load and effort move through the
same distance. So, the effort to be applied is equal to the load to be lifted.

(Fig : Fixed pulley)
ii. Movable pulley: It is capable of moving up and down along with the load.

In this pulley, effort applied is always half of the load to be lifted because
load distance decreases along with the increase in effort distance.

36 New Creative Science and Environment; Book 7

(Fig : Movable pulley)
iii. Combined pulley: It is a system of the pulley consisting of both fixed and

movable pulley. It is capable of both magnifying effort as well as changing
the direction of force.

Fixed pulley

Movable pulley

(Fig : Combined pulley)

3. Wheel and axle

A wheel and axle consists of two cylinders of different diameters fixed
together. The larger cylinder is called a wheel and the smaller one is called an
axle. Both the cylinders are wound with a thread.

Simlp e Machi nes 37

Effort is applied from a wheel and load is lifted by an axle. It is much more
efficient in magnifying effort. The knob of a door, a string roller, the wheel of a
sewing machine, the steering of vehicles, etc. are the examples of a wheel and
axle.

Wheel and axle
4. Inclined plane

An inclined plane is a slanted surface which is used to raise heavy loads
easily by moving along with it.

Labourers use an inclined wooden plank to load heavy goods upon a truck.
They do so, because they have to apply less effort by using an inclined plane in
comparison to lift the load directly.
5. Screw

A screw is a special case of an inclined plane that has been rolled up. It is
generally used to hold things together. It is made by cutting spiral grooves on the
surface of a metal rod.

38 New Creative Science and Environment; Book 7

A jackscrew is used to lift heavy vehicles like cars, buses and trucks.
6. Wedge

A wedge is a simple machine having two inclined planes combined together.
One end of the wedge is sharp while another is blunt. It is especially used for
cutting. Axes, knives, khukuris, needles, etc. are the examples of a wedge.

1. What is a lever? State the principle of a lever.
Ü A lever is a rod which can turn about a fixed point called the fulcrum.

The principle of a lever states that for an idea lever load × load distance =
effort × effort distance.
2. What is a pulley? Why can a fixed pulley not magnify effort?
Ü A pulley is a circular disc made of wood or iron, having a groove at its rim.
In a fixed pulley, both load and effort move through the same distance. So,
the effort to be applied is equal to the load to be lifted. Hence, a fixed pulley
cannot magnify effort.

Simlp e Macih nes 39

3. The roads of mountain region are not made straight, why?
Ü The roads of mountain regions are not made straight so that there occurs

inclination. In such a road it is easy to move.

F A simple machine is a simple device that helps us to do our work easily
in a short period of time.

F A lever is a rod which can turn about a fixed point called a fulcrum.
F The principle of the lever states that in an ideal machine,

Load × Load distance = Effort × Effort distance
F The first class lever has a fulcrum between the load and effort.
F The second class lever has a load between the fulcrum and effort.
F The third class lever has effort between the fulcrum and load.
F A pulley consists of a circular disc made of metal or wood having a

groove along its rim.
F A wheel and axle consist of two cylinders of different diameters fixed

together. The larger cylinder is called a wheel and a smaller one is called
an axle.
F An inclined plane is a slanted surface which is used to raise a heavy load
easily by moving along with it.
F A screw is a special case of an inclined plane that has been rolled up.
F A wedge is a simple machine having two inclined planes combined
together.

A. Fill in the blanks.
1. The first class lever has __________ in the middle.
2. A fixed pulley changes the _____________ of force.
3. The point of support in a lever is called _____________.
4. An inclined plane is a __________ surface.
5. _______________ is a simple device.

40 New Creative Science and Environment; Book 7

B. Match the following. holds objects
1. Wheelbarrow change the direction of force
2. Screw the first class lever
3. Pulley the second class lever
4. Shovel the third class lever
5. Beam balance

C. True or False.

1. A simple machine is difficult to construct.

2. The third class lever has a load between the effort and fulcrum.

3. A screw is a special case of an inclined plane.

4. A simple machine cannot magnify effort.

5. A pulley changes the direction of force.

D. Answer the following questions.

1. What is a simple machine? How does a simple machine make our
work easier?

2. What is a lever? What are the types of levers?

3. State the principle of a lever.

4. Draw a diagram of a wheel and axle.

5. Differentiate between a fixed and movable pulley.

6. Write down the uses of an inclined plane and a screw.

7. What is a pulley? Draw a fixed pulley that is used to lift a load.

8. What is a wedge? Write its two uses.

Pile - a number of things placed on top of each
Efficient - doing something well
Convenient - easy
Plank - a long flat piece of wood

‰‰

Simpl e Macih nes 41

4 Pressure

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

Æ define pressure with its unit.
Æ differentiate force and pressure.
Æ explain the application of pressure.

Introduction

Have you ever wondered why a camel can run in a desert while a girl in
high-heeled shoes feels it difficult? Why has a tractor wide tyres? Why have
cutting tools sharper edges?

We should introduce the term, ‘Pressure’ in order to find the answers of
these questions.

Pressure
The force acting normally on per unit area of the surface is called pressure.

I.e. pressure (P) Force (F)
= Area (A)

F
\P= A
Its S.I unit is Nm–2 or pascal (Pa). Other common units of pressure are mm
of Hg, atmosphere, bar, torr etc.

1 atmosphere = 105Nm–2 or pascal (Pa)

= 760 mm of Hg

From the above relation, it is clear that,

Pressure (P) ∝ Force (F) [when area is kept constant]

1 [when force is kept constant]
P ∝ Area (A)

It shows that pressure increases with the increase in force and decreases

with the increase in a surface area. So, a girl high-heeled shoes exerts more

pressure on the sand as it has a less area and makes more depression. But on the

other hand, the elephant’s weight is distributed among its four legs (larger area).

So, the pressure exerted is less and it makes less depression.

42 New Creative Science and Environment; Book 7

If 1N force is applied to the surface area of 1m2 then the pressure exerted on
that surface is known as one pascal pressure.

The above figures apply different pressures on the given surfaces.
Memory Tips
In honour of scientist Blaise Pascal, the SI unit of pressure is called pascal (Pa).

Questions

# All cutting and piercing tools have sharp edges. Why?
# It is difficult to walk on sand. Why?

To show the pressure of a brick in different conditions
Take a piece of foam and a brick. Keep the brick as shown in figure (a)
with its larger surface area over the foam and observe the depression on the
foam. Now, keep the brick as shown in figure (b) with its smaller surface area
over the foam and again observe the depression.

The foam is depressed more in the latter case than the former. It is

because, in condition (b) the area of the brick which is in contact with the foam

is less than in condition (a). As we know pressure ∝ 1 the brick exerts more
pressure and the foam gets more depressed.
A

Pressur e 43

Solved Numerical

The force of 106N is acting on a wall of an surface area 10m2. Find the pressure
acting on the wall.
Solution: Given, force (F) = 106N,

Area (A) = 10m2
Pressure (P) = ?

F 106
We have, P = A = 10 = 105 Pa
\ The pressure exerted on the wall is 105 pascal.

Differences between force and pressure

Force Pressure
1. The pulling and pushing 1. The amount of force applied to

factor is called force. per unit area of the surface is
called pressure.
2. Its SI unit is newton (N).
2. Its SI unit is N/m2 or pascal
3. The formula for force is (Pa)
f = m.a.
3. The formula for pressure is
F

P= A

1. All cutting instruments have sharp edges, why?

Ü A sharp edge makes the cross-sectional area of the cutting instruments
small and as we know,

F force will get distributed over small areas thereby increasing
Ppr=essAures.oT, houisr makes cutting easier. So, cutting instruments are provided

with sharp edges.

2. A girl in high-heeled shoes put much more depression on the sand than
an elephant. Why?

Ü The high-heeled shoes of the girl has less area and as we know,

F exerted by the girl will be more on the sand than
P= elAephsaon, tt.hHeepnrcees,ssuhree will exert more depression than the elephant.
the

44 New Creative Science and Environment; Book 7

F The pulling and pushing factor is called force.

F The amount of force applied to per unit area of the surface is called
pressure.

F The SI unit of force is newton (N) and the SI unit of pressure is N/m2 or
pascal (Pa).

F F
Pressure is measured by using formula P = A .
F Pressure increases as we increase force and decrease the area.

F All the cutting instruments have a sharp surface area to apply more
pressure and to cut thing easily.

F The base of a building, a dam and a tank is made wider so that area
increases and pressure decreases.

A. Fill in the blanks.
1. The _______________ and _______________ factor is called force.
2. The SI unit of pressure is called _______________.
3. Pressure increases when we _______________ a surface area.
4. Pressure decreases when we _______________ the applied force.

B. Given reason.
1. The base of a building is made wider.
2. A sharp knife cuts things easily.
3. A tractor has big and wide tyres.
4. The wall pin has a sharp and pointed end.

C. Answer the following questions.
1. Define force and pressure with their formula and SI unit.
2. Write any two differences between force and pressure.
3. Explain how force and surface area affect the applied pressure.
4. Explain the application of pressure in our daily life.

Pressur e 45

D. Solve the following nemericals.

1. Calculate pressure when a box of 250 N weight is placed on the surface
of 25 m2 area. (Ans: 10 Pa)

2. A person with a weight of 500 N is standing on the surface applying
100 Pa pressure. Calculate the surface area that is covered by him.
(Ans: 5m2)

3. A body weighed 30 kg is placed on the square surface of 10 m length.
Calculate the pressure applied to the surface assuming the value of
acceleration due to gravity 10m/s2. (Ans: 3 Pa)

Pascal - surname of sientist Blaise Pascal

Sharp - having a very less surface area

Acceleration - change in velocity.

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46 New Creative Science and Environment; Book 7