Contents
1. Measurement
1
2. Moon
17
3. Sound
36
4. Heat
55
5. Transfer of Heat
71
6. Light and Shadows
83
7. Reflecon of Light
100
8. Reflecon of Light by Spherical Mirrors
112
9. Electricity
123
Assessment Sheet 1
143
Assessment Sheet 2
144
Important definions
in Vocabulary
In-text quesons
in Stop and Reflect
Interesng projects
in Lab Acvity
Thought-provoking
quesons in Curious Mind
Features of Viva Physics
ICSE edion
Assessment sheets at the
end of each book
A wide variety of exercises at the
end of each chapter
Extra informaon relevant to the
concept in Enlighten Your Mind
1
Measurement
Learning Objectives
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•
•
•
•
•
•
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Mass and its measurement
Weight and its measurement
Density
Determination of density of solids
Relative density
Determination of relative density of a liquid
Applications of relative density
Variation of density of liquids and gases with temperature
In the previous class, we have learnt about the role of measurement in our day-to-day acvies and
the importance of taking correct measurements. In this chapter, we will learn about the measurement
of some more physical quanes like mass and weight and understand the concept of density.
MASS
Mass is defined as the amount of maer present in a body. All objects around us are maer and
hence have mass. We already know that mass is a fundamental quanty. This means that mass of a
body is a constant quanty and does not change from place to place.
Units of Mass
The SI unit of mass is kilogram (kg). Some other units used for expressing mass are milligram (mg),
gram (g), tonne, quintal, etc.
Relaonship between various units of mass
1 g = 1000 mg
1 kg = 1000 g
1 quintal = 100 kg
1 tonne = 10 quintal
Enlighten Your Mind
To measure the mass of atoms and molecules, we use a
unit called dalton (Da) which is also known as uniied
atomic mass unit.
1 Da = 1.66 × 10–27 kg
Measurement of Mass
Mass of a body can be measured by instruments like beam balance, physical balance, weighing scale,
etc. These devices compare the mass of the body with known and standard masses called weights.
Let’s study a beam balance in detail.
1
Beam balance
Most of you must have seen a simple beam balance at a grocery shop. A
beam balance consists of a horizontal metallic beam with a support and
a pointer at its centre. The beam can move freely about the support.
From the ends of the beam, two similar pans are suspended such that
they are equidistant from the centre of the beam.
Principle: When both the pans are empty or loaded with equal masses,
a state of equilibrium (balance) is achieved. In such a state, the beam is
horizontal and the pointer points vercally up.
horizontal
beam
pointer
pan
Fig. 1.1: Beam balance
Working: To find the mass of an object, it is placed on one pan and
standard weights are kept on the other pan of the beam balance. The
weights are adjusted ll the beam is horizontal and the pointer points
vercally up. The sum of the masses of the standard weights gives the
mass of the object.
Physical balance
A physical balance is used for the measurement of small masses with
more accuracy. That is why it is used in laboratories for scienfic work.
It works on the same principle as beam balance and is provided with a
box of standard weights having masses from 1 mg to 100 g.
Fig. 1.2: Physical balance
WEIGHT
We know that the earth aracts all bodies towards itself by gravitaonal force or gravity. The force
of aracon depends on the mass of the body. The more is the mass, the greater is the aracon
between the body and the earth.
The weight of a body is the force with which the earth aracts the body towards its centre. So
the weight of a body is the measure of gravitaonal pull on the body and hence it varies with the
gravitaonal pull.
The weight of a body is determined as:
W=m×g
where, W = the weight of the object
Enlighten Your Mind
The gravitational pull on the moon is one-sixth of its
value on the earth. This means the weight of a body
on the moon is one-sixth of its weight on the earth.
m = the mass of the object
g = the value of gravitaonal pull at that place (= 9.8 m s–2 or 10 m s–2).
Units of Weight
Since weight is a force, it has the same units as force. The SI unit of weight is newton (N). Other units
used for measuring weight are dyne, kilogram-force, gram-force, etc.
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Relaonship between various units of weight
1 gf = 980 dynes
1 kgf = 9.8 N
1 N = 105 dynes
Measurement of Weight
Weight of a body is measured with the help of a spring balance. Let’s study a spring balance in detail.
Spring balance
A spring balance consists of a coiled spring which is enclosed in a metallic
case and fixed to a support at one end. At the other end of the spring
is a pointer and a long rod that ends in a hook. The body to be weighed
is suspended from this hook. The metallic case has a vercal slit along
its length through which posion of the pointer can be seen. There are
graduaons on the metallic case which start from the point where the
pointer lies when no weight is suspended from the hook and increase in
the downward direcon as 0 gf, 100 gf, 200 gf, …, 1 kgf and so on.
Principle: When a load is suspended from the hook of the spring balance,
the spring elongates due to the weight of the body. The distance through
which it is stretched is directly proporonal to the weight of the body.
The heavier the body, the more is the elongaon of the spring.
Working: Before using the spring balance, make sure that the pointer
reads zero and no load is suspended from the balance. Now suspend
the body whose weight is to be measured from the hook of the spring
balance. This results in the elongaon of the spring. The pointer posion
on the scale gives the weight of the body.
Note: In case of a beam balance, the gravitaonal force is equal on both
the pans. Hence the effect of gravity is nullified there. So a beam balance
gives the measure of the mass of the body only and not its weight.
metallic
case
pointer
hook
Fig. 1.3: Spring balance
Enlighten Your Mind
There is a law known as
Hooke’s law of elasticity
which states that the
extension of a spring is
directly proportional to
the load applied to it.
The law was named in
the honour of the British
physicist Robert Hooke
who irst stated it in the
year 1660.
Difference between mass and weight
Mass
Weight
1. The mass of a body is the quantity of matter
contained in it.
1. Weight of a body is the force with which the earth
attracts it towards its centre.
2. The SI unit of mass is kilogram.
2. The SI unit of weight is newton.
3. It is a constant quantity and does not change
with position/location of the body.
3. Its value changes from place to place as it depends
on the gravitational pull at that place.
4. It is measured with a beam balance.
4. It is measured with a spring balance.
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Solved Numericals
1. What is the weight of a body having mass 20 kg?
Sol: Mass of the body = 20 kg
Gravitaonal pull = 9.8 m s–2
As we know, W = m × g
= 20 × 9.8 kg m s–2
= 196 N
Or we can simply represent the weight of the body as 20 kgf also.
2. Find the mass of a ball whose weight is 49 N on the earth. (g = 9.8 m s–2)
Sol: Weight of the ball = 49 N
Gravitaonal pull = 9.8 m s–2
As
W=m×g
Therefore,
m=
W
g
49 N
= 5 kg
9.8 m s–2
The mass of the ball is 5 kg.
=
Stop and Reflect
State whether the following statements are true or false and correct the false statements.
1. 1 tonne = 100 quintal
2. 1 N = 105 dyne
3. A physical balance measures the weight of a body.
4. Mass and weight are two different terms.
5. The SI unit of weight is kgf.
DENSITY
Take a look at the two boxes shown alongside. Each box
has the same volume. If each of the red balls has the
same mass, which box do you think weighs more? Why?
The box that has more balls has more mass per unit
of volume. Hence, it will be denser and heavier. This
property of maer which relates the mass and volume
of a body is called density.
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A
B
In this example if we consider the red balls as molecules of a substance we can see that there are
more molecules in box A as compared to box B. Or we can say that for the same volume, molecules
are more ghtly packed in box A, hence it is denser.
Therefore density of a substance can be defined as the mass per unit volume.
It is represented as:
Density (D) =
Mass (m)
Volume (V)
For example, a 1 cm3 block of wood weighs lesser than 1 cm3 block of iron. This is because the atoms
in iron are more densely packed as compared to the atoms in wooden block. In other words, the
density of iron is more than that of wood. The more ghtly packed the molecules are in a substance,
the denser that substance is.
Depending upon the density, substances may be classified as heavy or light. A heavy body has more
mass per unit volume than a lighter body, i.e., it has higher density.
Activity 1
Demonstrating that equal volumes of different substances have different masses
Requirement: 2 identical glasses, 50 mL water, 50 mL vegetable oil, beam balance
Procedure:
• Pour water in one glass and vegetable oil in the other glass.
• Keep these glasses in the 2 pans of the beam balance.
What do you see?
You will notice that the glass containing water is heavier, although
both the glasses have the same volume of liquids. This is because
water has higher density as compared to oil. This proves that equal
volumes of different substances have different masses.
Activity 2
Comparing the densities of 3 substances
Requirement: Beam balance, 3 identical cubes of wood, aluminium and copper having same volume
Procedure:
• Place the cubes of wood and aluminium in the 2 pans of the beam balance.
• Observe which of the cubes is heavier.
• Now replace the lighter cube with copper cube and observe the pans again.
What do you observe?
Draw conclusions on the basis of this activity and arrange wood, copper and aluminium in increasing
order of their densities.
Units of Density
The density of a material is the mass in its unit volume. Since the SI unit of mass is kilogram (kg) and
that of volume is cubic metre (m3), therefore the SI unit of density is kg m–3. The CGS unit of density
is g cm–3. The density of a material is denoted by the symbol D or ρ (pronounced as rho).
5
The densies of some common substances are given in the following table:
Densies of common substances
Substance
Density (kg m–3)
Air (293 K)
1.207
Water (freshwater)
1000
Ice
920
Aluminium
2700
Copper
8900
Iron
7870
Lead
11350
Gold
19300
Methanol
791.80
Ethanol
789.0
Conversion of kg m–3 to g cm–3 and vice-versa
1 kg = 1000 g
1 m = 100 cm
Therefore,
1 m–1 = 10–2 cm–1
1 m–3 = 10–6 cm–3
Thus
1 kg m–3 = 1000 g × 10–6 cm–3
Enlighten Your Mind
The density of water is approximately equal
to 1 g cm–3 and is dependent on temperature.
At 4 °C, pure water reaches its maximum
density. When it is cooled further, it expands
to become less dense.
= 10–3 g cm–3
So to convert 1 kg m–3 into 1 g cm–3, we divide it by 1000. Similarly, for converng 1 g cm–3 to 1 kg m–3
we mulply it by 1000.
Determination of Density of a Solid
To determine the density of a solid, we need to measure its mass and volume. The mass (m) of a
solid can be detemined using a physical balance. To measure the volume (V) of a solid, we proceed
in either of the two ways depending upon the shape of the solid.
1. The volume of regular solids such as sphere, cube, cylinder, etc. can be calculated using the
known formulae.
2. The volume of irregular solids such as stones, pebbles, etc. can be determined by water
displacement method.
m
Finally, the density of the solid is calculated using the formula D = .
V
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Activity 3
Determining the density of an irregular solid (stone)
Requirement: Measuring cylinder, physical balance, stone, thread/string, water
Procedure:
• First determine the mass of the stone using the physical balance. Let the mass be m.
• Take a measuring cylinder and pour some water in it. Note the
initial water level in the cylinder as V1.
• Tie the stone with a string and immerse it fully in water so that
it does not touch the base of the cylinder. Due to this the level
of the water in the cylinder rises. Note the inal water level in
the cylinder as V2.
The difference in the level of water in the cylinder (V2 – V1) gives the
volume of the stone.
m
Therefore the density of the stone, D =
V
m
=
(V2 – V1)
V2
V1
Substituting all the values, the density of the stone or any such irregular solid can be determined.
Solved Numericals
1. A block of mass 1500 kg has volume 3 m3. Calculate its density in:
(a) kg m–3 (b) g cm–3
Sol: Mass of the block = 1500 kg
Volume of the block = 3 m3
We know density, D =
m
V
(a) So density of the block in kg m–3 =
1500 kg
= 500 kg m–3
3 m3
(b) To convert density from kg m–3 to g cm–3, we divide the result in kg m–3 by 1000.
Thus, 500 kg m–3 =
500
= 0.5 g cm–3
1000
2. The density of aluminium is 2700 kg m–3. What is the mass of a block of aluminium which
occupies a volume of 5 m3?
Density of the block = 2700 kg m–3
Sol:
Volume of the block = 5 m3
m
V
As
D=
So,
m=D×V
= 2700 × 5 kg = 13500 kg
Therefore, the mass of the aluminium block is 13500 kg.
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