New Creative Science Book 9

Sum of anticlockwise moment = f1 × d1 + f2 × d2 + f3 × d3
According to law of moment, in balanced condition,

f1 × d1 + f2 × d2 + f3 × d3 = f4 × d4 + f5 × d5

| NUMERICALS |

a.. Calculate the weight ‘w’ from the following figure.

Here, about the fulcrum F,

Sum of anticlockwise moment = 50 × 150 + 100 × 50

= 7500 + 500 = 12500 N cm

= 12500 Nm [ 1m = 100 cm]
100

= 125 Nm

Sum of clockwise moment = 30 × 50 + 150 × 50 + 200 × w

= 1500 + 7500 + 200 w = (9000 + 200w) N cm

= 9000 + 200 w Nm [ 1m = 100 cm]
100

= (90 + 2 w) Nm

At equilibrium condition,

sum of clockwise moment = sum of anti-clockwise moment

or, 90 + 2 w = 125

or, 2w = 125 – 90

or, w = 35
2

\ w = 17.5N
Hence, the required weight is 17.5 N.

ANSWER WRITING SKILLS

1. Velocity ratio of a single movable pulley is 2. Why?

Ü In a single movable pulley, the distance travelled by the effort is twice the

distance travelled by the load,

i.e. Ed = 2 Ld

And, VR = Ed = 2Ld =2
Ld Ld

Hence, the VR of a single movable pulley is 2.

New Creative Science, Class 9 | 47

2. MA of the third class lever is always less than one. Why?

Ü In a third class lever, the effort lies between the fulcrum and the load. It means

the effort distance is always less than the load distance, so that more effort is to

be applied to overcome a lighter load i.e.

Effort > load and

MA = Load
Effort

\ MA < 1

Hence, MA is always less than 1 in the third class lever.

3. No machine is perfect. Why?
Ü Machines are said to be perfect when their efficiency is 100%. It is not possible

because of frictional force. Due to friction, the input work is converted into useful
work as well as some wastage work. It means all the input work is not converted
into useful work. Some of them is wasted resulting in the efficiency less than
100%. So, no machine is perfect.

4. A long spanner is used to open a tight knot. Why?
Ü A long spanner means long length. It produces more moment and turns the tight

knot. This is because the moment is directly proportional to the perpendicular
distance from the axis of rotation i.e. moment. So, a long spanner is used to open
the tight knot.

5. During storms, generally tall trees collapse. Why?
Ü The trees are taller. It means they have a longer perpendicular distance from

the line of the action of the force. Thus, it increases the moment. Due to the high
moment during a storm, tall trees collapse whereas shorter trees do not.

6. Winding roads are made on hills. Why?
Ü Winding roads are the examples of inclined planes having longer length. Longer

the length of the inclined plane, the easier it is to do the work and vice-versa.
So, vehicles can easily move in winding roads. Thus, winding roads are made
on hills.

7. The efficiency of a simple machine is 75%. What do you mean by this?
Ü The efficiency of a simple machine is 75%. It means the machine converts 100%

of input work into 75% of output work and the rest 25% is wasted due to friction
in the form of heat and other energies.

48 | Simple Machine

8. Machines are frequently oiled or greased. Why?
Ü In every machine, there occurs frictional force. This frictional force reduces the

mechanical advantage and the efficiency of the machine as well. To increase the
MA and efficiency of the machine, it is necessary to reduce friction. And oiling
or greasing machine is one of the easy ways of reducing friction in a machine. So,
machines are frequently oiled or greased.

SUMMARY

” Simple machines are those devices which make our work easier, faster and
more convenient.

” The mechanical advantage is the ratio of the load to the effort applied.
” Velocity ratio is defined as the ratio of the distance moved by the effort to the

distance travelled by a load.
” The efficiency of any simple machine is a ratio of output work to input work

multiplied by 100 in order to express in percentage.
” The principle of simple machine states, “In the absence of friction, output work

is equal to input work.”
” Levers, pulleys, wheels and axles, inclined planes, screws and wedges are six

types of simple machines.
” The lever is a rigid bar capable to rotate about a fixed point called the fulcrum.
” In first the class lever, a fulcrum is situated between the load and the effort.
” In the second class lever, the load is situated between the effort and the fulcrum.
” In the third class lever, the effort is situated between the load and the fulcrum.
” A pulley is a circular metallic or wooden disc having a groove on its

circumference through which a string or a rope passes.
” A pulley may be single movable, single fixed or combined.
” A wheel and axle is co-axial cylinder consisting of two cylinders- the bigger

one is the wheel and the smaller one is the axle.
” Any slanted surface is called an inclined plane.
” A screw is a cylindrical simple machine having threads around it.
” The distance between two adjacent threads in the cylinder of a screw is called

a pitch.
” Any pointed or sharp instrument is called an wedge. It has one pointed side and

another blunt side.
” The turning effect of force on a body is called a moment.
” The principle of the moment states, “In an equilibrium condition, the sum of

the clockwise moment is equal to the sum of the anti-clockwise moment.”

New Creative Science, Class 9 | 49

EXERCISE

1. What are simple machines? How do they make our work easier.
2. Define the mechanical advantages and the velocity ratio of a simple machine.

3. What do you understand by?

(a) MA of a lever is 2. (b) VR of a lever is 3.

(c) Efficiency of a machine is 70%.

4. What is a lever? Write any five examples.

5. Derive the relationship between, MA, VR and h.

6. What is a pulley? Describe its types in brief.

7. What is a screw? Give examples.

8. Define moment. State its law.

9. Give reason:
(a) The efficiency of a machine is never 100%.
(b) These is no gain in MA of a single fixed pulley. Yet it is used in our daily life.
(c) A wheel and axle is a continuous lever.
(d) MA of second class lever is greater than one.
(e) The chance of breaking the branch of a tree increases as a person moves towards its tip.
(f) A wedge is a double inclined plane.
(g) Scissors used for cutting metals having longer handles.

10. Write the differences between:
(a) The second class lever and the third class lever.
(b) A single fixed pulley and a single movable pulley.
(c) Output work and input work.

11. What are the two factors that affect the moment of a lever?

12. What do you mean by a wheel and axle? Prove that VR of wheel and axle is D/d.

Numerical problems

13. A crowbar of length 1m is pivoted at 20 cm away from the load of 400 N. Calculate
(i) Effort applied (ii) MA (iii) VR (iv) Efficiency (v) Show a diagram of the lever.

14. The diameters of the wheel and axle are 0.5 m and 20 cm respectively. Calculate its VR.

15. A pulley system consists of 6 pulleys in order to lift the load of 300 N applying 60 N force.
What is its efficiency?

16. Study the figure and calculate: (b) MA
(d) Work done on load
(a) Height of inclined plane (f) Efficiency
(c) VR
(e) Work done by effort

50 | Simple Machine

17. You are given a single movable pulley to lift a load of 800 N applying 300 N force.
Calculate the efficiency of the pulley.

18. The length of the wedge is 80 cm and its thickness is 50 cm. Calculate its VR.
19. Draw a pulley system consisting of 5 pulleys indicating the point of application of

the effort and the load. Also, calculate the force required to lift the load of 500 N if the
efficiency of the pulley is 75%.
20. A person exerts the force of 200 N in order to pull a mass of 100 kg up-to the height of 2
m using a ladder with efficiency 80%. What is the minimum length of the ladder?
21. A person lifts a load of 60 kg using a wheel and axle of VR 3. If the diameter of the axle is
5 cm, find the radius of the wheel, MA and effort applied. Also, find its efficiency.
22. Calculate the moment of a force from the given figure.

F = 200N

d = 200 cm

23. Study the given figure and find the missing force.

24. A metre metallic scale of 1m is suspended horizontally in its middle. A load of 400 gm
is placed at one end of the scale. How much force is required to balance it if the force is
applied 80 cm away from the load.

25. From the figure, calculate
(a) Load
(b) VR
(c) Length of inclined plane
(d) MA
(e) Efficiency.

A

B GLOSSARY
C

Frequently : time and again

Groove : a long narrow cut in the surface of something

Windlass : a machine for lifting or pulling heavy objects

Advantages : bene ts



New Creative Science, Class 9 | 51

UNIT

4 WORK, ENERGY AND POWER

About the Scientist Introduction

Albert Einstein Human beings perform different types of activities in
(1879-1955) their day-to-day life. It means we are bounded by different
types of works. To perform these types of works, people
Albert Einstein was a very need energy. For example, we eat food and get energy after
famous scientist who led to its metabolic process. We utilize this energy in different
hundreds of new discoveries activities like playing, singing, reading, writing, shouting,
and added milestones in laughing and so on. It means we use our muscular energy
different branches of science. in each and every step of our life. Similarly, carrying loads,
He propounded the theory lifting heavy loads, pulling something, ploughing the field,
of relativity in 1905, which washing clothes, etc. are also done by people utilizing their
is governed by a simple muscular energy. So, energy helps us to do our work. Thus,
equation. energy is a vital component to do our work.

E = mc2 Work
In this theory, the law of the
conservation of mass and Generally, we suppose all the activities done by us are
the law of the conservation work. Would you be puzzled if I say all the activities are
of energy are met together not work? Of course, the activities done by us are not work.
into one principle. For the work to be done, the effort applied must cover
certain displacement. For example, if you are studying hard
52 | Work, Energy and Power to get a better result, pushing the wall of a room, guarding
a house standing at some places, etc. are not regarded as
work in physics. It is because all the activities do not cover
any displacement although the great effort of force is
applied. Then, what is work? What kinds of activities are
called work?

Work is said to be done when the force applied to a
body produces certain displacement on it. In other words,
work is defined as the product of force applied and the
displacement covered by it. The work is generally done in
the direction of force the applied.

Mathematically, W = F × d
Where, W = Work done

F = Force applied
d = Displacement covered

In the SI system, work is measured in Newton metre (Nm) or Joule (J). In CGS
system, it is measured in erg.

Relation between erg and Joule

We have, 1 J = 1N × 1m

= 105 dyne × 102 cm

= 107 dyne cm

i.e. 1J = 107 erg [ erg = dyne × cm]

Although, work is the product of two vectors i.e. force and displacement, it is a
scalar quantity.

MEMORY TIPS
If the angle between the force applied (F) and the displacement is q and ‘S’ is the
distance covered, then, W = F × S × cosq

One Joule work

One joule work is said to be done when the force of 1N applied to a body to
displace through the distance of 1m. In other words, it is the product of 1N force and
1m displacement i.e.

1J = 1N × 1m

Types of work

Depending upon the direction of the force applied, work is divided into two
types i.e. work against gravity and work against friction.

1. Work against gravity
If the force is applied against the force of
gravity to do any work, it is said to be the
work against gravity. In other words, the
work done against the force of gravity of the
earth is called the work against gravity. For
the work against gravity,

W = mgh

New Creative Science, Class 9 | 53

Where ‘m’ is the mass of body and ‘g’ is the acceleration due to gravity and ‘h’ is
height upto which the body of mass ‘m’ is lifted up.

Lifting an object, carrying loads to a certain height, climbing stairs, etc. are the
examples of the work against gravity.

2. Work against friction
Friction always opposes the motion of any body. So,
the work done in the direction opposite to the friction
is called the work against friction. Dragging a load,
pulling and pushing a cart etc. are some examples of
work against friction.

MEMORY TIPS

If mass ‘m’ and acceleration ‘a’ of a body are given to displace the distance ‘d‘,
then, F = ma and W = ma × d

| NUMERICALS |

1. A boy is pulling a load of 130 N. He drags the load for 13 m. What is his work
done?

Solution: Given, Force applied (F) = 130 N
\ Displacement (d) = 13 m
Work done (W) = F × d

= 130 × 13

= 1690 J

2. A body of mass 80 kg is lifted upto the distance of 1000 cm. Calculate the work
done.

Solution: Given, Mass (m) = 80 kg

Height (d) = 1000 cm
1000
= 100 m = 10 m

Now, F = mg = 80 × 10 = 800 N

And, W = F × d = 800 × 10 = 8000 Nm = 8000 J

3. Find the force required to do the work of 125 J that causes the displacement of
50 cm.

Solution: Given, work done (W) = 125 J
Displacement (d) = 50 cm

54 | Work, Energy and Power

= 50 m = 0.5 m
100

Force required (F) = ?

We have,W = F × d
W 125
i.e. F = d = 0.5

\ F = 250 N

Hence, the force required is 250 N.

Energy

Human beings eat food. Why do we do so? It is because food is necessary to
produce energy in our body. We fill petrol or diesel in the tanks of vehicles. It is done
because petroleum products provide energy for vehicles to move. We use batteries in
a torchlight or a radio. It is because the chemicals present in battery provide energy
for the torchlight or the radio. We connect electrical appliances to an electric circuit.
It is because electrical appliances do the work using electricity. In all these, different
substances are utilized in order to get energy to do work.

Energy is defined as the capacity of any body to do work. It is the ability of doing
work. Higher the energy, the higher will be the work done and vice-versa. It is a
scalar quantity because it has only magnitude but no direction. While doing work, the
energy transforms from one form to another. It means the work is done as the energy
converts from one form to another. Its SI unit is joule (J) and CGS unit is erg. It is also
measured in terms of calorie, kilowatt/hour and electron volt depending upon the
types of energy.

S.N. Unit Symbol Equivalent in Joule
1. Erg erg 10–7 J
2. Calories Cal 4.2 J
3. Kilowatt hour kWh 3.6 × 106 J
4. Electron volt eV 1.6 × 10–19 J

One Joule energy

The energy that can do 1Joule work is called 1J energy i.e.

1J energy = 1J work

MEMORY TIPS

The amount of energy in a body is equal to the amount of work done. For example,
100 J energy of a body can do 100 J work, 500 J energy of a body can do 500 J work
and so on.

New Creative Science, Class 9 | 55

Types of energy

There are several types of energy. Some of them are as follows:

(i) Kinetic energy (ii) Potential energy

(iii) Chemical energy (iv) Electrical energy

(v) Heat energy (vi) Sound energy

(vii) Light energy (viii) Nuclear energy

(ix) Magnetic energy

i. Kinetic energy

The energy possessed by a body by the virtue of its position of motion is called
kinetic energy. For example, a moving bus, a flying bird, a bullet fired from a
gun, kicking a ball, a running athlete, etc. Kinetic energy is calculated by using
the formula.

Example of kinetic energy

KE = 1 mv2
2

Where, KE = kinetic energy
m = mass of a moving body
v = velocity of a moving body

Derivation of KE = 1 mv2
2

Let us consider a body of mass ‘m’ is applied with the force ‘F’ so that acceleration
‘a’ is produced on the body and the body covers the distance of ‘s’ in ‘t’ seconds.

56 | Work, Energy and Power

We have, v2 = u2 + 2as

or, v2 = 02 + 2as

\ a= v2
2s

Now, the force applied to the body is

F= ma =m · v2 = mv2
2s 2s

Then, the work done by the body ‘w’ is given by

W = F × s [ s = d = displacement]

= mv2 × s
2s

i.e. W = 1 mv2
2

Since the work done by a body is equal to the energy possessed by a body, the
body has the kinetic energy as it is in motion. So, the kinetic energy KE of the
body is given by

KE = 1 mv2
2

ii. Potential energy
The energy possessed by a body by virtue of its position or arrangement or
configuration is called the potential energy. For example, water stored in a dam,
stretched bow or catapult, compressed spring, foot ready to kick a ball, bullet
ready to be fired, etc. It can be calculated by the formula,
PE = mgh

Where, PE = potential energy

m = mass of the body

g = acceleration due to gravity

h = height above the earth’s surface

The potential energy possessed by a body as it is lifted to a certain height from
the ground level is gravitational potential energy. For example, water stored in a
dam, a stone kept at certain height, leg lifted to kick a ball, etc.

Similarly, potential energy possessed by elastic substances is called elastic
potential energy. For example, a stretched rubber, compressed spring, stretched
bow, etc.

New Creative Science, Class 9 | 57

Derivation of PE = mgh

Let us consider a body of mass ‘m’ is raised to a height of ‘h’
from the level of the ground. In this case, the weight of the body is the
product of its mass ‘m’ and acceleration due to gravity ‘g’ such that,

Weight = mg

Since this weight is the force for the body and height ‘h’ is the
displacement covered by the body,

Work done = force × displacement

= weight × displacement

= mg × h

\ Work done = mgh

The work done by the body is equal to the energy possessed by the body.
Therefore, the potential energy possessed by the body is given by

PE = mgh

MEMORY TIPS

The sum of the total potential energy and kinetic energy is called mechanical energy.
It means, the energy possessed by a body by virtue of its motion or position or
configuration is called mechanical energy.

| NUMERICALS |

1. Calculate the kinetic energy of a body of mass 2 kg moving with the velocity 4
m/s.

Solution: Given, the mass of a body (m) = 2 kg
Velocity of a body (v) = 4 m/s

\ Kinetic energy = 1 mv2
2

= 1 × 2 × (4)2
2

= 16 J

Hence, the kinetic energy possessed by a body is 16 J.

2. A moving body possesses 168 J energy and its mass is 1680 gm. Calculate its
velocity.

Solution: Given, the kinetic energy of a body (KE) = 168 J

58 | Work, Energy and Power

Mass of the body (m) = 1680 kg = 1.68 kg
1000

Velocity of the body (v) =?
1
We have,KE = 2 mv2

Hence, the velocity of the body is 14.14 m/s.

3. What will be the change in kinetic energy of a body if velocity is doubled?

Solution: In the first case, suppose a body of mass ‘m’ is moving with the velocity

‘v’, then its kinetic energy (KE) is given by
1
KE1 = 2 mv2 .....................(i)

In the second case, the velocity of the body is doubled.

It means the velocity of the body is 2v, then
1
KE2 = 2 m (2v)2

= 1 m × 4v2
2

i.e. KE2 = 2mv2
or, KE2 1
= 4 × 2 mv2

\ KE2 = 4KE1 [From equation first]
Hence, the kinetic energy of the body will increase four times.

4. Calculate the potential energy possessed by a body of mass 50 kg lifted upto the
height of 5 m.

Solution: Here, mass of the body (m) = 50 kg
Height above the earth’s surface (h) = 5 m
Acceleration due to gravity (g) = 9.8 m/s2

Then, potential energy (PE) = mgh
= 50 × 9.8 × 5

\ PE = 2450 J

5. A body possesses 1000 J potential energy to raise a certain height and its mass
is 98 kg. Calculate the height. (g = 9.8 m/s2)

Solution: Given, potential energy (PE) = 1000 J
Mass of the body (m) = 98 kg

New Creative Science, Class 9 | 59

Acceleration due to gravity (g) = 9.8 m/s2

Height (h) = ?

We have,PE = mgh

i.e. h = PE = 1000
mg 98 × 9.8

\ h = 1.041 m
Hence, the height of the body above the ground level is 1.041 m.

iii. Chemical energy

The energy possessed by the chemical substances like food, petrol, diesel, etc. is
called chemical energy. It is produced after chemical change or chemical reaction.
Food, petroleum products, chemicals in a battery, photosynthesis process, etc. are
the sources of chemical energy.

Sources of chemical energy

iv. Heat energy
Heat is a form of energy that provides us the sensation of the warmth to our
sense organs. It is also defined as the sum of kinetic energy of molecules of any
body. The sun, burning firewood, burning heater, etc. are the sources of heat.
The heat energy always flows from hotter bodies to colder bodies.

Solar heat Burning of fire wood

60 | Work, Energy and Power

v. Sound energy
The energy that provides us the sensation of hearing to our ears is called sound
energy. It is the energy produced by the vibration of molecules of any body.
Tuning fork, madal, guitar, sitar, speakers, etc. are some sources of sound energy.
The sound between the range 20 Hz to 20 KHz is called audible sound.

Sources of sound energy

vi. Light energy
The energy that gives us the sensation of
vision to our eyes is called light energy. It is
the energy produced by the bodies at a high
temperature. The sun, stars, glowing bulb,
photocells, solar batteries, etc. are some
sources of light energy.

Sources of light energy

vii. Electrical energy
The energy possessed by a body due to the flow of electrons (charged particle) is
called electrical energy. It is the energy of electricity. Electricity is very important
in our life. It is used to cooking, heating, printing press, manufacturing, official
and administrative work, etc. with the help of electrical appliances. The successful
modern life is the gift of electrical energy. Hydropower stations, batteries,
dynamo, generators, etc. are some sources of electrical energy.

Source of electrical energy

New Creative Science, Class 9 | 61

viii. Magnetic energy Magnet

The energy released by a magnet is called magnetic Iron nails
energy. Electrical energy is the outcome of magnetic
energy. It is used in electric bells, high-speed super Effect of magnet
trains, production of electricity in hydropower
stations, etc. Due to the magnetic energy of a body,
magnetic substances are attracted. A bar magnet,
u-shaped magnet, etc. have magnetic energy.

ix. Nuclear reaction
The energy released during different nuclear reactions (nuclear fusion or nuclear
fission) is called nuclear energy. It is produced by nuclear fusion or nuclear
fission reaction.

The nuclear reaction in which heavy nucleus disintegrate or break down into
smaller nuclei with the release of energy is called nuclear fission. For example,

A chain reaction showing the splitting of uranium atoms
The nuclear reaction in which smaller nuclei fuse or join together with the release
of energy is called nuclear fusion. The best example of nuclear fusion is the
nuclear reaction in the sun to produce heat and light.
For example:

62 | Work, Energy and Power

Differences between nuclear fusion and nuclear fission reactions

S.N. Nuclear fission S.N. Nuclear fusion

1. The nuclear reaction in which a 1. The nuclear reaction in which

heavy nucleus splits into smaller smaller nuclei fuse together to give

nuclei is called nuclear fission. a heavy nucleus is called nuclear
fusion.

2. Comparatively, it produces less 2. The energy produced by this

amount of energy than nuclear process is comparatively more than
fusion. energy produced by nuclear fission.

3. It is highly destructive, e.g. 3. It is comparatively less destructive

explosions of atom bombs. and can be regarded as a constructive
one, e.g. nuclear reaction in the sun.

MEMORY TIPS
Nuclear energy is also called atomic energy.

Transformation of energy

It is clear that one form of energy converts or transforms into another while
doing work. This is transformation of energy. Therefore, the transformation of
energy is a process in which one form of energy is converted into another form.
During transformation, no energy is lost. It can be shown from the energy flow
chart. Some of the transformations of energy are as explained below:

(a) Hydro-power stations Kinetic energy Electrical energy
(Rotation of turbine) (In generator)
Potential Energy
(water stored in dam)

(b) An electric bulb

Electrical Energy Light energy
(Electricity) (As switch is on)

(c) In a dynamo Electrical energy light energy
(Hammering metal) (as bulb glows)
Kinetic energy

(Rotation of dynamo cap)

For all these conversion of energies, certain devices play a role in conversion.
Some of the devices that convert one form of energy into another form is tabulated
below:

S.N. Devices Transformation of energy
1. Battery/Cell Chemical energy into electrical energy
2. Dynamo/Generators Kinetic energy into electrical energy
3. Solar cell Light energy into electrical energy

New Creative Science, Class 9 | 63

4. Microphone Sound energy into electrical energy
5. Heater Electrical energy into heat energy
6. Magnets Magnetic energy into electrical energy
7. Turbine Kinetic energy into electrical energy
8. Electric bulb Electrical energy into light energy
9. Firewood (burning) Chemical energy into heat energy
10. Green leaf Light energy into chemical energy

Principle of conservation of energy

The principle of conservation of energy states, “Energy can neither be created
nor be destroyed but it can be transformed from one form to another.” During the
transformation of energy, energy is never lost but may
be transformed to another type of energy. For example, if
PE = mgh

100 J of chemical energy is stored in our body and 60 J is
converted in kinetic energy, then the rest amount of energy
may be converted into sound energy and heat energy i.e.

Chemical energy = Kinetic energy + Heat/sound energy

(100 J) (60 J) (40 J) KE = 1 mv2
2
Let us consider a body of mass ‘m’ is dropped from a
certain height ‘h’. At this height, it has zero kinetic energy
as the velocity of the body is 0 m/s whereas the potential
energy is maximum i.e.

PE = mgh

During the falling of the body, its potential energy decreases and there is
consecutive increase in kinetic energy. It is because decrease in height decreases the
potential energy and increase in velocity increases kinetic energy resulting in constant
mechanical energy.

If ‘v’ is the velocity by which a body strikes the ground, then

v2 = u2 + 2gh

= (0)2 + 2gh

\ v2 = 2gh

At the ground level, potential energy is zero and its kinetic energy is maximum.
It is because h = 0 at ground level whereas velocity is maximum. Then,

KE = 1 mv2 = 1 m · 2gh
2 2

i.e. KE = mgh

It means, loss in PE = gain in KE (in magnitude).

Hence, one form of energy lost is converted into another form. It means energy
can neither be created nor be destroyed but changed from one form to another.

64 | Work, Energy and Power

Power

Do you know machines are regarded as more powerful than humans? Why do
vehicles run faster than humans? It is because they can convert energy from one form
to another in a shorter period of time. This is called power.

Power is the rate of doing work. In other words, it is the rate of change in energy.

Mathematically,

Power (P) = work done = energy change
time taken time

( )i.e. w = f × d = d  d v
P = t t f × t =f×v t =

Its SI unit is Joule per second (J/s) or watt (W). The unit of power i.e. watt is

kept under the name of James Watt as an honor to him. The other units of power are

megawatt, kilowatt and horse power.

1 KW = 103 W

1 MW = 106 W

1 HP = 746 W

Power is also a scalar quantity.

One-watt power

The power of a body that can do 1J work in 1 second is called one watt power, i.e.

1 W = 1 1J
sec

In other words, the power of a body that converts 1J of one form of energy into

another in one second is called one watt power.

| NUMERICALS |

1. Calculate the power of a body that can do 5000 J work in 5 minutes. Also,
express in horsepower.

Solution: Given, work done (w) = 5000 J

Time take (t) = 5 minutes

= 5 × 60 seconds

= 300 seconds

\ Power (P) = w = 5000 = 16.67 W
t 300

i.e. Power = 16.67 Hp [ 1 Hp = 746 W]
746

= 0.02234 Hp

New Creative Science, Class 9 | 65

2. Calculate power of a crane that lifts a load of 300 kg upto the height of 5 metres
in 10 seconds. Also, express in KW and HP. [g = 10 m/s2]

Solution: Given, mass of the body (m) = 300 kg

Acceleration due to gravity (g) = 10 m/s2

Height (h) = 5 metres

Time taken (t) = 10 seconds

Power (P) = ?
We have,
work done
Power (P) = time taken

=f×d
mtg ×
= t h = 300 × 10 × 5
10

i.e. P = 1500 W

= 1500 KW
1000

= 1.5 KW
1500
Also, P = 1500 W = 750 = 2 HP [ 1 HP = 750 W]

Hence, the power of a crane is 1500 W or 1.5 KW or 2 HP.

3. A coolie of mass 50 kg carries a log of wood of 20 kg climbing a staircase of 20
steps of 20 centimetre each in 2 minutes. Calculate his work done and power.
[g = 9.8 m/s2]

Solution: Given, mass of the coolie (m1) = 50 kg
Mass of the log (m2) = 20 kg
No. of steps (N) = 20

Height of each step (l) = 20 cm

Time taken (t) = 2 minutes

= 2 × 60 seconds = 120 seconds

g = 9.8 m/s2

Now, total mass (m) = m1 + m2
= 50 + 20 = 70 kg

Height climbed (h) = N × l

= 20 × 20 cm = 400 cm
400
= 100 =4m

Then, work done (w) = f × d

= mgh = 70 × 9.8 × 4 = 2744 J

And, power (P) = w = 2744 = 22.867 W
t 120

Hence, the work done by the a coolie is 2744 J and his power is 22.87 W.

66 | Work, Energy and Power

Differences between work and power.

S.N. Work S.N. Power

1. Work is the product of force and the 1. It is the ratio of work done to

displacement covered by a body i.e. the time taken to do the work
w=f×d w
i.e. p = t

2. Its SI unit is Joule. 2. Its SI unit is watt.

3. It is independent to time. 3. It is inversely related to time.

ANSWER WRITING SKILLS

1. Work is the product of two vector quantities but it is itself a scalar. Why?

Ü Work is the product of force ‘f’ and the displacement ‘d’ covered by a body. Here
‘f’ and ‘d’ are vectors but work is a scalar quantity because it does not have any
direction but has only magnitude.

2. A person standing at the gate does not do any work. But he is paid for his
work. Support your answer.

Ü A person standing at the gate does not cover any displacement i.e. d = 0 m.
Then work done by him is
w = f × d or, w = f × 0 = 0 J
Hence, no work is done from the physics point of view but he is paid for guarding.
This is because he pays time, attention and invest muscular energy.

3. Both potential and kinetic energies are mechanical energies. Can you
differentiate them?

Ü The differences between potential and kinetic energies are:

S.N. Potential energy S.N. Kinetic energy

1. It is the energy possessed by a 1. It is the energy possessed by a
body by virtue of its position or body by virtue of its motion.
configuration.

2. It depends upon acceleration due to the 2. It depends upon the velocity by
gravity and height above the earth’s which a body is moving.
surface.

3. It is calculated by PE = mgh 3. It is calculated by KE = 1 mv2
2

4. During a free fall, the potential 4. During a free fall, the kinetic
energy of a body decreases. energy of a body increases.

4. Work and energy have the same unit. Why?
Ü Work is said to be done when one form of energy is converted into another form.

It means the conversion of energy is called the work done. All the energies are
expressed in the unit Joule or erg. So, work and energy have the same unit.

New Creative Science, Class 9 | 67

5. Who has more power a man doing 100 J work in 20 seconds or a bike doing 100
J work in 2 seconds?

Ü Sincetheworkdonebyamanandabikeisthesamei.e.100Jbuttimetakenbythebikeis

less than the time taken by the man. As power is inversely proportional to time taken
1
i.e. P ∝ t (as work is constant), the bike has more power.

SUMMARY

” Work is said to be done when the force applied to a body travels through a
certain displacement.

” The work done in the direction opposite to the frictional force is called the work
against friction.

” The work done in the direction opposite to the gravity of the earth is called the
work against gravity.

” Energy is the capacity of doing work.
” The energy possessed by a body by virtue of its motion is called kinetic energy.
” The energy possessed by a body by virtue of its position or arrangement is

called potential energy.
” The energy produced after chemical change is called chemical energy.
” The energy produced by the flow of electrons is called electrical energy.
” The energy produced by the body at a high temperature is called light energy.
” The energy that gives us the sensation of warmth is called heat energy.
” The energy produced by the vibration of molecules is called sound energy.
” The energy of the magnet is called magnetic energy.
” The energy produced after a nuclear fusion or nuclear fission reaction is called

nuclear energy or atomic energy.
” The process of transforming one form of energy to another form is called the

transformation of energy.
” The principle of conservation of energy states, “Energy can neither be created

nor be destroyed but can be transformed from one form to another.”
” The rate of doing work is called power.

EXERCISE

1. Define work with its SI unit and CGS unit. Also, write the relation between these units.
2. What is chemical energy? Mention its three sources.
3. Define work against gravity and work against friction with examples.
4. What do you mean by heat energy? Mention its three sources.
5. An electric bulb is marked 60 W. What do you understand by this?

68 | Work, Energy and Power

6. The power of a machine is 2000 W. What does this mean?

7. What is light energy? Mention its four sources.

8. Write the differences between:

(a) Work done and power

(b) Energy and power

(c) Potential and kinetic energy

(d) Nuclear fission and nuclear fusion

9. What is the transformation of energy? Show the transformation of energy in a flowchart
for the following.

(a) Hydropower station (b) Electric heater

(c) Battery operated radio (d) Electric fan

(e) A car (f) An electric fan

(g) Electric bulb (h) Television

10. Mention the types of energy possessed by the following:

(a) Water stored in dam (b) Flowing river

(c) Kerosene (d) Burning firewood

(e) Compressed spring (f) Bullet fired from gun

(g) Rice (h) Hot iron rod

(i) Stretched catapult (j) Glowing electric bulb

11. Give reasons:

(a) The potential energy of a notebook on the ground level is zero.

(b) Two different engines may have different power although they can do an equal
amount of work.

(c) Work is a scalar quantity.

(d) Power is a scalar quantity.

(e) Both work and energy have the same unit.

12. What do you understand by 1J work, 1J energy and 1W power?

13. What happens to the kinetic energy of a body if its velocity is increased by 3 times keeping
the mass constant? Show it numerically.

14. State the principle of conservation of energy.

15. What happens to the potential energy if the body of a certain mass is dropped from a
certain height?

16. Prove that

(i) KE = 1 mv2 (ii) PE = mgh
2

New Creative Science, Class 9 | 69

Numerical problems

17. A body of mass 80 kg is lifted to a height of 2 m. What is its potential energy?

18. The power of a heart is 1.25 W that beats 72 times in a minute. Calculate its work done.

19. A bullet of mass 10 gm is fired with the velocity 100 m/s. Calculate the kinetic energy
possessed by it.

20. The power of a 100 kg man is 150 W. Calculate the time taken by him to climb the height
of 20 m.

21. A crane converts 1000 J of energy in 1 second. If it can do any work in 1 minute. Calculate
the work done.

22. How much energy is needed to kick a football of mass 500 gm with the velocity 72 km/
hr?

23. A ball is dropped from a height of 5 m and it rebounds back up-to the height of 3 m.
Calculate its,

(a) KE when it strikes the ground.

(b) KE when it leaves the ground for rebounding.

24. An electric bulb is marked 100 W. How much electrical energy is consumed by it if it is
glowed for 2 hours.

25. A person of mass 60 kg is running with the velocity of 20 m/s. What is his power if he
completes the race in 30 seconds?

A

B GLOSSARY
C

Puzzled : to made somebody confused

Drag : to pull something along with e ort

Administrative : connected with organizing the work of business or an institution

Consecutive : following one after another in a series



70 | Work, Energy and Power

UNIT

5 SOUND

About the Scientist Introduction

Heinrich Rudolph Hertz We hear different types of sounds with our ears. It
(1857-1894) means sound gives us the sensation of hearing. Sound
plays an important role in our life. Why do you hear
Heinrich Rudolph Hertz different music, radio, F.M. programmes, etc.? It is because
was born on 22 February these activities provide sensation to our ears. Can you
1857 in Hamburg, imagine watching television or films in the theatre without
Germany and educated at any sound? It will be ridiculous to watch without sound.
the university of Berlin. Similarly, you buy loudspeakers, music system, sound
He laid the foundation for system, etc. Why do you do so? It is obviously for better
future development of the sound because your ears want the sounds to be heard. So,
radio, telephone, telegraph do you know how sound is produced, how it is transmitted
and even television. The to our ears, how sound is made pleasant and unpleasant.
SI unit of frequency was
named hertz in his honour. When the molecules of any body vibrate, sound is

produced. Therefore, sound is a form of energy that is

produced by the vibration of the molecules of a body. In

other words, sound is a form of energy that provides us

the sensation of hearing to our ears. The sound produced

by its source transmits to our ear in the form of waves. It

can be illustrated as: Medium Receiver (ear)
Source
(longitudinal wave) (transverse wave)
(vibration of molecules)

There are different sources of sound. Some of them
are tuning fork, loudspeakers, telephones, musical
instruments, etc.

Radio Madal Bell Tuning fork

Sound propagates through any medium i.e. solid,
liquid or gas in the form of waves. This is called sound
wave. It propagates after the vibration of molecules and

New Creative Science, Class 9 | 71

lasts as long as the vibration continues. It means when vibration stops, no sound is
produced.

MEMORY TIPS
Sound is a type of energy.

Wave

A wave is produced due to the disturbance in the medium. It does not disturb
others but transforms energy from one point to another. It means the wave motion
causes the movement of energy but there is no actual movement of the medium
molecules. Water waves, sound waves, light waves, etc. are some of the examples of
waves.

Sound is propagated with the help of sound waves. For example, when a radio
is on, it produces sound. It transmits through air without the actual movement of air
molecules and reaches our ears causing vibration. In this way, we hear the sound.

There are two types of waves on the basis of the direction of propagation of
particles. They are:

(i) Transverse wave
(ii) Longitudinal waves
1. Transverse wave
The wave in which the vibration of particles is perpendicular to the direction of
the motion of the wave is called a transverse wave. This kind of wave is generally
seen in solid and liquid mediums. In this wave, the particles vibrate up and down
with respect to the direction of the motion of the particles. Light waves, water
waves, radio waves, waves on rope are some examples of transverse waves. It
forms a crest and a trough.

72 | Sound

Transverse wave on a rope

Regarding a transverse wave, different terms are associated. They are as follows:
(a) Crest: The peak or the maximum point of the transverse wave is called a

crest. It is seen in an upward direction.
(b) Trough: The maximum depressed point of the transverse wave is called a

trough. It is the peak point of the downward direction.
(c) Amplitude (a): It is the maximum displacement of any vibrating particle

from its mean position. It is represented by ‘a’ and is measured in metre (m).
(d) Wavelength: The distance between any two consecutive crests or the troughs

is called a wavelength. It is represented by ‘l’ and is measured in metre (m).
(e) Frequency (f): The number of complete cycles made by a vibrating particle

in one second is called frequency. It is represented by ‘f’ and is measured in
per second (s–1) or Hertz (Hz).
(f) Time period (T): The time taken by any vibrating particle to complete its one
vibration is called the time period. It is represented by ‘T’ and is measured
in second (s).

2. Longitudinal wave
The wave in which the vibration of particles occurs in the direction of motion is
called a longitudinal wave. It transmits with the help of a to and fro movement.
It is generally seen in all the mediums. It transmits in the form of compressions
and rarefactions.

The meanings of several terms associated with longitudinal waves are:

New Creative Science, Class 9 | 73

(a) Compression: The part of the region in which the particles are near to each
other is called compression. In this part, the medium has higher density of
vibrating particles.

(b) Rarefaction: The part of the region in which the vibrating particles are farther
to each other is called rarefaction. In this part, the medium has lower density
of vibrating particles.

(c) Amplitude (a): It is the maximum displacement of any vibrating particle
from its mean position.

Differences between transverse and longitudinal waves:

S.N. Transverse waves S.N. Longitudinal waves

1. It is the wave in which the 1. It is the wave in which the medium

medium particles vibrate in the particles vibrate in the direction of

direction perpendicular to the wave motion.

direction of wave motion.

2. It is produced in solid and 2. It is produced in solid, liquid and gas

liquid mediums. mediums.

3. It propagates by forming a crest 3. Itpropagatesbyformingcompression

and a trough. and rarefaction.

4. The variation in density and 4. The variation in density and pressure

pressure is not observed. is observed.

Mechanical and electromagnetic waves

Those waves that require any material mediums for their propagation are called
mechanical waves. These waves cannot travel without any medium, e.g. sound
waves, water waves, etc.

Those waves that do not require any material mediums for their propagation
are called electromagnetic waves. These waves can even travel through a vacuum.
Example, x-ray, uv rays, light-rays, etc.

MEMORY TIPS

Mechanical waves are also called elastic waves as they depend upon the elastic
nature of any medium.

Important terminologies of wave

(i) Amplitude: The maximum displacement of any vibrating particle of the
medium from its mean position is called amplitude. It is represented by ‘a’
and is measured in terms of metre (m) in SI system. Higher the amplitude,
the higher will be the energy and vice-versa. The wave with high amplitude
can propagate or travel a longer distance.

74 | Sound

(ii) Complete wave: A wave having one compression and one rarefaction or one

crest and one trough is called complete wave.

(iii) Time period: The time taken by a wave to form one complete wave is called

the time period. It is represented by ‘T’ and is measured in second (sec.) in

SI system.

(iv) Frequency: The number of complete waves produced by vibrating molecules

in one second is called frequency. It is the reciprocal of time period i.e.
fIt=isT1represented by ‘f’ and is measured in per second (S–1) or Hertz (Hz) in the

SI system.

(v) Wavelength: The distance between two consecutive ‘crests or troughs’ or

‘rarefaction or compression’ is called a wavelength. It is denoted by ‘l’ and

is measured in metre (m) in SI system.

(vi) Wave velocity: The distance travelled by the wave in one second is called

wave velocity. It is represented by ‘v’ and is measured in metre per second

(m/s) in the SI system.

Mathematically,
l
V = T= wave velocity
Where, V

l = wave length

T = time period
1
Also, T = f where, f = frequency

l

Then, V= 1

f

i.e. V = f.l
This is called wave equation i.e.

Wave velocity = frequency × wave length

| NUMERICALS |

1. Calculate the velocity of sound of frequency 100 Hz having the wavelength of
330 cm.

Solution: Given, frequency (f) = 100 Hz

Wave length (l) = 330 cm

= 330 m = 3.3 m
100

\ Velocity (V) = l× f

= 3.3 × 100

= 330 m/s

New Creative Science, Class 9 | 75

2. The velocity of the sound in air is 332 m/s and the wavelength is 0.017 m.
Calculate its frequency.

Solution: Given, Velocity of sound (V) = 332 m/s

Wave length (l) = 0.017 m

Frequency (f) =?

We have,V = l× f

\f = V = 330
l 0.17

= 19529.41 Hz
= 19.53 KHz

MEMORY TIPS
103 Hz = 1 KHz; 106 Hz = 1 MHz

3. If 50 complete waves are formed in 2 seconds, calculate its frequency and the
time period.

Solution: Here, no. of waves (N) = 50

Time taken to form 50 complete waves (t) = 2 seconds

\ Frequency = No. of waves in 1 second

= 50 = 25 Hz
2

And, time period = 1 = 1 = 0.04 seconds.
f 25

Types of sound

The frequency of sound generally ranges from 1Hz to 108Hz. This range of
frequency of sound is called frequency spectrum. Depending upon the frequency
spectrum, sound is categorized into infrasonic, audible and ultrasonic sound.

i. Infrasonic sound

The sound wave with the frequency less than 20 Hz is called infrasonic or
infrasound or sub sound. This type of sounds is not heard by the human ear.
It can be felt because of vibration. For example, the sound produced during an
earthquake. Some of the animals like elephant use this type of sound.

ii. Audible sound
The sound waves with the frequency ranging between 20 Hz to 20,000 Hz (20

76 | Sound

KHz) is called audible sound. These are called audible sounds as we can hear
these sound waves. The vocal cords of human beings produce the sound of
frequency from 60 Hz to 13 KHz.

iii. Ultrasonic sound
The sound waves with the frequency more than 20,000 Hz (20 KHz) is called
ultrasonic sound. These sounds cannot be heard by human beings. Ultrasounds
play an important role as these are used for several purposes. Some of the
practical uses of ultrasounds are as follows.

(i) It is used to determine the depth of the sea and the ocean.
(ii) It is used by some animals like bats, dolphins, etc. for the location and

obstacles.
(iii) It is used to kill bacteria.
(iv) It is medically important to examine the foreign particles in the body.
(v) It is used to detect the metal or rock quality.

MEMORY TIPS

Finding the depth of the sea using ultrasound is called echolocation or SONAR
(Sound Navigation and Ranging)

| NUMERICALS |

a. The speed of ultrasound is 1500 m/s and the sound is received back after 5
seconds as the sound transmits inside the sea. What is the depth of the sea?

Solution: Given, velocity of sound (V) = 1500 m/s

Time taken to reach the depth of sea (t) = 5 = 2.5 seconds
2
Depth of sea (d) = ?

We have,V = d
\ d =t
V × t

= 1500 × 2.5

= 3,750 m

Hence, the depth of sea is 3750 m.

Speed of sound in different mediums

The speed of sound depends upon the medium through which it propagates.
The speed of sound is maximum in solid and it is minimum in gaseous mediums. For
example, two boys are sitting apart by taking two boxes tied to a thread. The sound in
this case propagates through the rope faster than through the air.

New Creative Science, Class 9 | 77

In solids, the molecules are tightly packed so that the vibration produced in one
molecule is quickly transferred to another. It means sound wave is quickly propagated.
In liquids, the molecules are comparatively loosely packed than in solids. So, the
vibration produced in one molecule takes some time to propagate to other. It means,
the speed will be comparatively less. In gas, molecules are very loosely packed so that
it takes few more time for the propagation of vibration from one molecule to another.
As a result, the speed of sound is the least.

Speed of sound in different mediums

S.N. Medium Example Speed of sound
330 m/s
Air (0°C) 343 m/s
1280 m/s
1. Gas Air (20°C) 258 m/s
Hydrogen (0°C) 1450 m/s
1210 m/s
Carbon dioxide (0°C) 1325 m/s
1533 m/s
Water (0°C) 3650 m/s
5100 m/s
2. Liquid Alcohol (25°C) 5200 m/s
Turpentine (25°C) 3560 m/s
5500 m/s
Sea-water (25°C)

Brick (0°C)

Aluminium (25°C)

3. Solid Steel (0°C)

Copper (25°C)

Glass (25°C)

Speed of sound in gaseous medium

The speed of sound in a gaseous medium is the least. It is because the molecules
are loosely packed. The speed of sound in a gaseous medium is given by

V=

Where, V = speed of sound in gas

P = pressure of air

r = density of air

There are several factors that affect the speed of sound in gas. Some of them are
described below.

1. Density (r): The speed of sound is inversely proportional to the square root
of the density of any gas. It means, the higher the density of gas, the lower
will be the speed of sound and vice-versa i.e.
Speed of sound ∝

78 | Sound

2. Temperature: The speed of sound is directly proportional to the square
root of temperature. It is because at a higher temperature, density will be
minimum resulting a greater speed.
Speed of sound ∝ Temperature

3. Humidity: The amount of water vapour present in the air is called humidity.
In humid air, the density is little so that the speed of sound is high and vice-
versa. So, the higher the humidity, the higher will be the speed of sound
and so on. Due to this reason, sound is clearer during rainy days than hotter
days.
Density of dry air > Density of moist air
i.e. Speed of sound in dry air < Speed of sound in moist air

4. Direction of wind or wind velocity: If the direction of a moving wind is in the
same direction of propagation of sound wave, the speed of the sound will
be higher and if the direction of the wind is opposite to the direction of the
propagation of wave, the velocity of sound will be minimum.

Reflection of sound

The bouncing back of sound waves after striking the reflecting surface is called
the reflection of sound. It is reflected by hard surface like walls, wooden boards,
bricks, etc. It also follows the laws of reflection of light. Due to the reflection of sound,
there is an echo and reverberation.

ACTIVITY

Objectives
To demonstrate that the reflection of sound
wave follows the laws of reflection of light.

Materials required
Table, wooden board, two tubes, tickling watch.

Procedure
1. Arrange two tubes in front of any hard
surface (say glass) and put a wooden
board perpendicular over a table.
2. Put a tickling watch in one end of the first tube and hear from the end of another tube.
3. Measure the inclination of the tube.

Observation and calculation
You will hear the sound in the second tube and the angles of inclination of both tubes are equal.

Conclusion
Hence, the reflection of sound follows the laws of reflection of light.

New Creative Science, Class 9 | 79

Echo

The repetition of sound after its reflection is called an echo. For the echo to be
heard, the distance between the source and reflecting surface must be beyond 17 m.
For example, if we shout near the hills, a repeated sound can be heard. This is an echo.
In the given figure below, we can see sound wave being reflected from bat and ship
through transmitter.

Transmitter

Examples of echo

Reverberation

Due to the reflection of sound, the original sound and reflected sound get mixed
up. This is called reverberation. Therefore, reverberation is the prolongation of sound
formed by the overlapping the original sound and the reflected sound. For the
reverberation, the distance between the source of sound and reflection surface should
be less than 17 m. To reduce the reverberation in halls, auditoriums, etc., the walls
and ceilings are covered with sound absorbing materials.

Differences between echo and reverberation

S.N. Echo S.N Reverberation

1. An echo is the repetition of a sound 1. Reverberation is the prolongation

wave. of a sound wave.

2. The distance between the source of 2. The distance between the source of

the sound and the reflector must be the sound and the reflector must

more than 17 m. be less than 17 m.

80 | Sound

Refraction of sound

The phenomenon of bending of sound waves as it passes from one medium
to another (rarer to denser or denser to rarer is called the refraction of sound. The
refraction of the sound also follows the laws of the refraction of light. Due to the
refraction of sound, it is heard farther at night than during the day.

During the day, the earth’s surface is hot and the air molecules nearer to the
earth’s surface become rare. It means the lower surface of the air is rarer and the
upper surface is denser. When the sound wave propagates from the rarer to the
denser medium, it bends towards the normal as shown in the figure (A).

At night, the land cools faster and the air molecules nearer to the earth’s surface
become dense. It means the lower part of the air is denser whereas the upper layer
of the air is rarer. During the propagation of the sound wave from a denser to a rarer
medium, it bends away from the normal. Due to this fact, the sound is clearer and
more distinct at night than during day.

Pitch of sound

The sharpness or shrillness of any sound is the pitch of sound. It is the character
of sound that helps us to distinguish and differentiate sharp or shrill sounds from
dull or flat sounds. It is the physical sensation but not physical quantity. It depends
upon the frequency of sound. Higher the frequency, the higher will be the pitch i.e.
shriller the sound and vice-versa. For example, the sound of a child is sharper than
the sound produced by an old man.

Loudness of sound

The character of sound that helps us to distinguish loud sounds from faint sounds
is loudness of sound. It depends upon the intensity of sound. It is the sensation which
cannot be measured. Higher the intensity of the sound, the louder is the sound and
the lower the intensity of the sound, the fainter the sound is.

New Creative Science, Class 9 | 81

Intensity of sound

The rate of the flow of sound energy from per unit area of the sounding surface
is called intensity.

Mathematically,

I = E t
A×

Where, I = Intensity of sound

E = Energy

A = area through which energy is propagated
t = time taken

Since the sound is transmitted in all the directions
A = 4pr2 (where ‘r’ is the distance between source and listener)

\ I = E t
4pr2 ×

As ‘E’ and ‘t’ are constant

I ∝ 1
r2

Hence, the intensity of sound is inversely proportional to the square of distance

between the source and the listener.

It is also directly proportional to the square of amplitude of any wave i.e. I ∝ a2

Similarly, the density of medium is also directly proportional to the intensity of
sound i.e. the loudness of sound. The intensity of sound is also directly proportional
to the size of the source. The bigger the source, the larger will be the intensity and
vice-versa.

It is measured in watt per square metre (w/m2) or decibel (dB). The intensity of
different sounds are:

S.N. Sound Intensity (dB)

1. Threshold of hearing 0

2. Chirping of birds 10

3. Rustling of leaves 20

4. Whispering 10

5. Conversation 50

6. Telephone ringing 80

7. Running motorcycle 90

8. Thunder 100

9. Threshold of pain 130

10. Flying jet plane 140

82 | Sound

Quality of sound or timbre

It is the character of sound that helps us to distinguish the sound of same
frequencies and the same pitch. For example, you can distinguish your two friends
from their sounds although the sounds produced by both of them have an equal pitch

and loudness. It is because of the quality or the timbre of the sound.

Differences between pitch and intensity.

S.N. Pitch S.N. Intensity

1. It is the sharpness or shrillness 1. It is the rate of the flow of sound

of sound. energy from per unit area of the

sounding surface.

2. It is qualitative measure of 2. It is quantitative measure of sound.
sound.

3. It depends upon the frequency 3. It depends upon the amplitude of

of sound waves. sound waves.

4. It is unitless. 4. Its unit is decibel (dB).

MEMORY TIPS

According to the WHO, the threshold limit of hearing during the day should be 55
dB and at night, it should be 45 dB. The sound above 80 dB causes discomfort and
above 140 dB may cause permanent loss of hearing

Musical sound and noise

The sound in which the wave propagates with definite periodicity and amplitude
is called a musical sound. A musical sound produces a pleasing effect to our ears. The
sound in which the wave propagates with indefinite and irregular periodicity and
amplitude is called noise. It causes irritating effects to our ears.

Differences between musical sound and noise

S.N. Musical sound S.N. Noise

1. The sound wave having definite 1. The sound wave having indefinite

and regular periodicity and and irregular periodicity and

amplitude is called a musical amplitude is called noise.

sound.

New Creative Science, Class 9 | 83

2. It causes a pleasing effect to our 2. It causes irritating and annoying

ears. effects to our ears.

3. Its wave has a regular shape. 3. Its wave has an irregular shape.

Sound pollution

The mixing of different sounds of different frequencies is called sound pollution.
It is the disturbance produced by different undesired and unwanted sounds in the
environment. Noise is also called sound pollution.

Causes of sound pollution

Some of the causes of sound pollution are as follows:
(i) Over crowding and over population.
(ii) Non-systematic and unscientific establishment of factories and industries.
(iii) Vehicles giving horn haphazardly.
(iv) Use of loudspeakers.

Effects of noise pollution

There are several effects of noise pollution. Some of them are as follows:
(i) It creates difficulty in conversation.
(ii) It irritates and annoy in simple matters.
(iii) It causes high blood pressure.
(iv) It may bring permanent deafness.
(v) The concentration of people on any work is disturbed.

Preventive measures of sound pollution

(i) Industries and factories should be systematically and scientifically
established far away from residential areas.

(ii) Old vehicles and machines should be replaced by new ones.
(iii) No horn zones and green belts should be established.
(iv) Television, radio, music system, etc. should be heard in a low volume.
(v) People should not talk loudly.

ANSWER WRITING SKILLS

1. The peed of sound is more in solids than in liquids. Why?
Ü In solids, molecules are closely packed. So, sound energy quickly propagates

from one molecule to another whereas in liquids, the molecules are loosely
packed. So, the wave of sound takes some time to propagate from one molecule
to another. Thus, the speed of sound in solids is more than the speed of sound in
liquids.

84 | Sound

2. During thundering, the sound of thunder is heard later than lightening. Why?
Ü The speed of sound in the air is 332 m/s whereas the speed of light is 3 × 108 m/s.

Due to the higher speed of light, lightening is observed at first during thundering.
It means light travels farther than sounds in one second.

3. During nights, sound is more distinct. Why?
Ü The lower layer of the air is denser than the upper layer at night. So, the sound

wave propagating from a denser to a rarer medium bends away from the normal
whereas during the day, sound waves propagate by bending towards the normal
as it travels from a rarer to a denser medium. Due to such refraction of sound, the
sound is heard more distinct at night than during the day.

4. The speed of sound in dry air is less than the speed of sound in moist air.
Why?

Ü The density of dry air is greater than the density of humid air and we know that,

Speed of sound ∝

Due to this reason, the speed of sound is more in humid air than in dry air.

5. The sound produced by loudspeakers is louder than smaller speakers. Why?
Ü The loudness of sound is directly proportional to the intensity of sound whereas

the intensity of sound is directly proportional to the size of the source of sound.
Since, loudspeakers are bigger in size than smaller ones, sound produced by
loudspeakers is louder.

SUMMARY

” The sound is the form of energy produced by the vibration of molecules of any
body.

” Mechanical waves need a medium for propagation whereas electromagnetic
waves do not.

” The wave in which the vibrating particles propagate in the direction
perpendicular to the motion of the wave is called the transverse wave.

” The wave in which the vibrating particles propagate in the direction of wave
motion is called the longitudinal wave.

” The sound waves below 20 Hz are called infrasonic.
” The sound waves ranging between 20 Hz to 20 KHz are called audible sounds.
” The sound waves beyond 20 KHz are called ultrasonic.
” For a transverse wave, the distance between the two consecutive crests and

troughs is called wavelength. For a longitudinal wave, the distance between
two consecutive compressions or rarefactions is called a wavelength.
” The number of complete waves made in one second is called frequency.
” The speed of sound is more in solid and the less in gas.

New Creative Science, Class 9 | 85

” The speed of sound in gas is affected by density, temperature, humidity and the
direction of the wind.

” The sharpness or shrillness of sound is called a pitch.
” The intensity of sound is the rate of the flow of energy from per unit area to

the sounding body.
” Timbre helps us to differentiate the sounds of the same pitch and intensity.
” Noise is called sound pollution.
” A musical sound is pleasant to hear whereas noise is unpleasant.
” Noise pollution causes permanent deafness, high blood pressure, irritation,

difficult in conversation, etc.

EXERCISE

1. What is sound? How is it produced?
2. What is ultrasound? Name two animals that can produce and hear such sounds.
3. What do you mean by mechanical and electromagnetic waves? Give examples.
4. What is wave equation? Write the relation between the velocity of sound, wavelength

and frequency.
5. What is an echo? Write the conditions necessary for an echo.
6. What are audible waves? Mention its sources.
7. What is infrasonic sound? Mention its sources.
8. What is frequency? How can sound wave having different frequencies have the same

speed in the same medium?
9. Can you predict a dark room is empty or not with the help of sound? How?

10. Give reasons:
(a) An echo is heard near the hills.
(b) Sound is clearer at nights than during day.
(c) The sound of a girl is sharper than that of a boy.
(d) A sound wave is called a mechanical wave.
(e) The sound is heard fainter as we go away from the source of the sound.
(f) Ultrasound is useful to us.

(g) Walls and ceilings of auditoriums are covered with sound absorbing materials.
(h) In space, we cannot hear each other.

11. Write the differences between:
(a) Transverse wave and longitudinal waves
(b) Echo and reverberation
(c) Musical sound and noise
(d) Ultra sound and infra sound
(e) Mechanical and electromagnetic waves.
(f) Light waves and sound waves

86 | Sound

(g) Pitch and the intensity of sound

12. What is sound pollution? Mention its causes.
13. What are the effects of sound pollution? How can you remove sound pollution?

14. Study the table given, where three mediums: solid, Medium Speed of sound
liquid and air are given and answer the following 5200 m/s
questions. A 332 m/s
B 1450 m/s
(a) Which is air, which is solid and which is liquid?

(b) The speed of sound in medium ‘A’ is high. C

Why?

15. The velocity of sound in three different gas mediums Medium Velocity (m/s)

is given. Answer the following questions.

(a) Which gas medium has higher density and which A 5000
has the least density? B 1000
C 800
(b) If the sound waves have the same frequency, in
which medium wavelength is longer and in which
is it shorter?

Numerical problems

16. The time period of a sound wave is 0.08 seconds. Calculate its frequency.

17. The wavelength of the sound wave of 15 kHz, frequency is 0.0022 m. What is its speed?

18. The frequency and velocity of a sound wave is 0.1 kHz and 330 m/s respectively. Find
its wavelength and time period.

19. Calculate the frequency and the time period of sound wave of 35 m wave length
propagating at a speed of 3500 m/s.

20. A person hears an echo after 2 seconds as he shouts 400 m away from the cliff of a hill.
Calculate the speed of the sound.

21. After 20 seconds of lightening, we hear a thundering storm. How far has the thunderstorm
occurred? [Speed of sound in air is 332 m/s]

22. Calculate the depth of the sea if the echo is heard after 6 seconds. [the speed of the sound
in water is 1500 m/s]

23. An echo is heard after 0.1 second. Calculate the distance between the source of sound
and the reflector if the speed of the sound is 350 m/s.

A

B GLOSSARY
C

Pitch : the sharpness of sound
Trough : the depressed point in a transverse wave peak
Crest : the peak point in a transverse wave peak
Compression : the region of more density



New Creative Science, Class 9 | 87

UNIT

6 LIGHT

About the Scientist Introduction

Galileo-Galilei During the daytime, it is easy and clear to see the
(1564-1642) objects whereas it is quite difficult at night. Why is it like
this? Can you guess? It is because of light. We use electric
Galileo Galilei was born on bulbs in the dark. We use a torchlight to move in a dark
15 February 1564 in Pisa, area. We switch on the headlight of vehicles to drive
Italy. He developed a series vehicles when it is dark. All these activities are done to
of telescopes where optical provide the sensation of vision to our eyes.
performance was much
better than that of other Light is a form of energy that provides us the sensation
telescopes available during of vision to our eyes. It means we cannot see the objects
those days. without light. The bodies at a higher temperature emit
light. The speed of light in the air is 3 × 108 m/s, in water,
it is 2.2 × 108m/s and in glass it is 2 × 108m/s. Light always
propagates in a straight line. This property of light is the
called rectilinear propagation of light. Light rays undergo
different phenomena such as reflection, refraction and
dispersion. In this chapter, we will deal with the refraction
and the dispersion of lights.

Refraction of light

As light passes through a medium to another, its
direction of propagation is changed. This is called the
refraction of light. Therefore, the phenomenon of the
bending of light rays as they pass through one medium
to another is called the refraction of light. It is observed as
the light passes through the optically rarer to the optically
denser medium or the optically denser medium to optically
rarer medium.

cold air
observer

tree hot air
road

image of tree

88 | Light

The above figure shows the refraction of light in a glass slab. In the figure above,
AO = Incident ray
OO1 = Refracted ray
O1B = Emergent ray
ON = Normal
∠i = Angle of incident ray/incidence
∠r = Angle of refracted ray/ refraction

MEMORY TIPS

The distance between an emergent ray and the original direction of an incident ray
is called a lateral displacement.

Cause of refraction of light

The refraction of light is caused due to difference in the speed of light in different
media regarded as an optically denser and optically rarer medium. The medium that
has higher density is an called optically denser medium and the medium that has low
density is called a rarer medium.

As the light passes

through a rarer to denser

medium, its velocity Refracted ray

decreases and the light

bends towards the normal.

For example, the light Refracted ray

passing through air (3 × 108

m/s) to glass (2 × 108 m/s) bends towards the normal.

As the light passes through a denser to rarer medium, its velocity increases and
it bends away from the normal. For example, as the light passes from water (2.2 × 108
m/s) to air (3 ×108 m/s), it bends away from the normal.

MEMORY TIPS

The main cause of the refraction of light is the difference in the velocity of light in
different mediums.

Laws of the refraction of light

There are four laws of refraction of light. They are as follows.
1. The incident ray, refracted ray and normal (at the point of incidence) all lie

on the same plane.

New Creative Science, Class 9 | 89

2. A ray of light passing through a rarer to a denser medium bends towards
the normal and the ray of light passing through a denser to a rarer medium
bends away from the normal.

3. The incident ray passing through normal does not refract.

4. The ratio of sine of an angle of incidence to the sine of the angle of refraction

is constant. This is also called Snell’s law.
\ sin i
sin r = m (constant)

This constant is called the refractive index of the medium.

ACTIVITY

Objectives
To demonstrate the laws of the refraction of light.

Materials required
A glass slab, pins, chart paper, wooden board, etc.

Procedure

1. Take a white sheet of chart paper and fix it to
a wooden board.

2. Draw the outline of a glass slab and also
incident rays with different angles of
incidence (say 10°, 20°, 30°, 40°, 50°)

3. Insert two pins on the incident ray and find the location of pins after refraction and join the
holes to obtain the refracted ray.

4. Repeat this process for the entire incident rays and measure the angle of refraction.

5. Calculate the refractive index for each case.

Observation and calculation
sin i
After calculating sin r for all the cases, you will get the same value.

Conclusion
Hence, the ratio of the sine of the angle of incidence to the sine of the angle of the refraction is constant.

Refractive index

According to Snell’s law, the ratio of the sine of the angle of incidence to the
sine of the angle of the refraction is called the refractive index. If ‘i’ is the angle of the
incidence and ‘r’ is the angle of refraction, the refractive index (m) is given by

m= sin i
sin r

According to the wave theory of light, it is the ratio of the velocity of light in one

medium to the velocity of light in another medium i.e.

90 | Light

velocity of light in medium 1
1m2 = velocity of light in medium 2

In general case, light travels from a vacuum to any medium. This results in
absolute refractive index i.e.

Absolute refractive index (m) = velocity of light in vacuum (c)
velocity of light in medium (v)

\ m = c
v

It is unitless quantity and shows the extent by which velocity is reduced as it
passes from a vacuum to any medium. For example, the refractive index of water is
1.33. It means the velocity of light in water decreases by 1.33 times than in the air.

The refractive indexes of some mediums are as follows:

S.N. Medium (from air to) Refractive Index
1. Air 1
2. Ice
3. Water 1.31
4. Alcohol 1.33
5. Paraffin 1.36
6. Glycerin 1.44
7. Turpentine 1.47
8. Ordinary glass 1.47
9. Ruby 1.5
10. Diamond 1.76
2.42

MEMORY TIPS
1m2 represents the refractive index of medium 2 with respect to medium 1.

| NUMERICALS |

1. The angle of incidence and refraction are 45° and 28° respectively as the light
passes from air to a glass slab. Calculate the refractive index of glass.

Solution: Given, angle of incidence (∠ i) = 45°

Angle of retraction (∠ r) = 28° sin i
sin r
\ Refractive index of glass (mg) = = sin 45° = 1.5
sin 28°

New Creative Science, Class 9 | 91

2. Calculate the refractive index of a glass slab (vg = 2 × 108 m/s) as the light enters
from water to the glass slab. (vw = 2.2 × 108 m/s).
Solution: Given, velocity of light in water (Vw) = 2.2 × 108 m/s

Velocity of light in glass (Vg) = 2 × 108 m/s

Refractive index of glass (wmg) = ?

We have,wmg = Vw = 2.2 × 108 = 2.2 = 1.1
Vg 2 × 108 2

Real depth and apparent depth

Have you ever seen a pond with clear water and have you ever tried to catch the
stones found inside the water? What have you experienced? If you have done so, it
is quite difficult to locate the exact position of the stones because of the refraction of
light. This causes real and apparent depth.

The actual position of an object from the water surface is called the actual depth
and the depth at which the body is seen or appeared is called the apparent depth. It
can be illustrated in the diagram below:

A

B

In the diagram above, A represents the apparent depth whereas B represents the
real depth. Since A is always less than B, so the apparent depth is always less than
the real depth. Due to real and apparent depth, the pond appears shallower than it
really is.

The ratio of the real depth to the apparent depth is also called the refractive

index. It was experimentally proved i.e.

Refractive index (m) = Real depth
Apparent depth

92 | Light

| NUMERICALS |

The real depth of a coin placed in a beaker is 5 m and it appears at the depth of
3.75 m. Calculate m.

Solution: Given, real depth = 5 m Real depth
Apparent depth = 3.75 m = Apparent depth

\ Refractive index of water

= 5 = 1.33
3.75

Practical examples of refraction of light

1. A pond appears shallower than it really is.

When the ray of light passes from the water
(denser) to the air (rarer) medium, it bends away
from the normal. As a result, the real depth
appears to be virtual causing apparent depth
just little above the real one. Due to this, a pond
appears shallower than it really is.

2. A pencil appears bent immersed in water.

As the ray of light passes from the water (denser)
to the air (rarer) medium, it bends away from the
normal. As the ray of light reaches our eyes, we
see the virtual image of the pencil’s part inside
the water. This causes the pencil to be bent.

3. Stars twinkle at night.
The light rays coming from stars pass through different layers of the atmosphere
having different densities and different refractive indexes. Due to the variable
refractive index of the layers of air, the refraction of light takes place. So, stars
twinkle at night.

Critical angle and total internal reflection

As light passes from the denser medium to the rarer medium, the angle of
incidence is formed in the denser medium and the angle of refraction is formed in the
rarer medium. If the angle of incidence in the denser medium is increased to a certain
extent the angle of the refraction will be equal to 90°, the angle of incidence is called
the critical angle as shown in the figure below.

New Creative Science, Class 9 | 93

The critical angle is the angle of incidence in the denser medium for which the
angle of refraction in a rarer medium is 90°. It is represented by ‘C’.

Angle of incidence (i) = C

Angle of refraction (r) = 90°

Then, refractive index (gma) = sin i
sin r

or, amg = sin 90°
sin C

or, mg = 1 C
sin

\ C = sin–1

Critical angles for some mediums are:

S.N. Medium Critical angle
1. Ice 50°
2. Water 49°
3. Alcohol 48°
4. Paraffin 44°
5. Turpentine 43°
6. Glycerin 43°
7. Glass 42°
8. Quartz 35°
9. Diamond 24°

As the angle of incidence in the denser medium increases more than in critical
angles, the rays of light undergo the reflection phenomenon instead of refraction as
shown in the figure (iii). This is called the total internal reflection.

The phenomenon of the turning back of light in the same denser medium when
the angle of incidence is greater than the critical angle in the denser medium is called
the total internal reflection. It obeys the laws of reflection. The following conditions
are necessary for the total internal reflection:

94 | Light

(i) The ray of light must pass from a denser medium to a rarer medium.
(ii) The angle of incidence in the denser medium must be greater than in the

critical angle.

| NUMERICALS |

1. The critical angle for water and glass is 49° and 42° respectively. Calculate
their refractive index.

Solution: Given, critical angle for water (Cw) = 49°

Critical angle for glass (Cg) = 42°

Now,

(i) Refractive index of water (mw) = 1
sin Cw

= 1 = 1.33
sin 49°

(ii) Refractive index of glass (mg) = 1
sin Cg

= 1 = 1.5
sin 42°

Totally reflecting prism

An isosceles right-angled glass prism is called
a totally reflecting prism as shown in the diagram
below.

In the above figure, ∠A = 90°, ∠B = ∠C = 45°. A
ray of light PQ strikes the surface AC at Q and reflects
with the angle of reflection = 45°. The reflected ray
QR again strikes the surface AB with the angle of
incidence 45° and reflects through SR with the angle
of reflection equal to 45°. In this case, the light is deviated through 180° like plane
a mirror and this prism produces a single image. For this reason, totally reflecting
prisms are used instead of plane mirrors.

Mirage

The optical illusion due to the total internal reflection observed on hot days
in tarred roads or deserts is called a mirage. During hotter days, the lower layer
of the air is rarer than the upper layer. As the light rays pass from a denser to a
rarer medium, the angle of incidence is greater than the critical angle causing a total
internal reflection. As a result, an observer sees the inverted image like the image of

New Creative Science, Class 9 | 95

any object in water.

cold air
observer

tree hot air
road

image of tree
For the formation of a mirage, the following conditions are required:
(a) The day must be hot so that different layers of air with different density are

formed.
(b) The object must be taller so that its inverted image can be noticed.

Shinning of air bubbles inside water

As the ray of light strikes the air bubbles inside the water with the angle of
incidence greater than a critical angle, the light does not pass through the air bubble.
Instead, it is reflected due to the total internal reflection. This causes the shining of air
bubble inside the water.

Sparkling of diamond

The critical angle of diamond is 24. The edges of the diamond are cut in such
a way that the angle of incidence is always greater than 24 (i.e. critical angle). As
the angle of incidence is greater than the critical angle, the incident rays striking the
diamond reflect due to the total internal reflection. This makes the diamond spark
brightly..

Sparkling of diamond

96 | Light