NCERT Textbook: Motion & Measurement of Distances | Science & Technology for UPSC CSE PDF Download

 Page 1


74 SCIENCE
7
Motion and
Measurement of Distances
T
here was a general discussion
among the children in Paheli and
Boojho's class about the places
they had visited during the summer
vacations. Someone had gone to their
native village by a train, then a bus, and
finally a bullock cart. One student had
travelled by an aeroplane. Another spent
many days of his holidays going on
fishing trips in his uncle's boat.
The teacher then asked them to read
newspaper articles that mentioned
about small wheeled vehicles that moved
on the soil of Mars and conducted
experiments. These vehicles were taken
by spacecraft all the way to Mars!
Meanwhile, Paheli had been reading
stories about ancient India and wanted
to know how people travelled from one
place to another in earlier times.
7.1 STORY OF TRANSPORT
Long ago people did not have any means
of transport. They used to move only on
foot and carry goods on their back. Later
on they began to use animals for
transportation.
For transport through water, routes,
boats were used from ancient times. To
begin with, boats were simple logs of
wood in which a hollow cavity could be
made. Later, people learnt to put
together different pieces of wood and
give shapes to the boats. These shapes
imitated the shapes of the animals living
in water. Recall our discussions of this
streamlined shape of fish in Chapters
5 and 6.
Invention of the wheel made a great
change in modes of transport. The
design of the wheel was improved over
thousands of years.  Animals were used
to pull carts that moved on wheels.
Until the beginning of the 19th
century, people still depended on
animals, boats and ships to transport
them from place to place. The invention
of steam engine led to the development
of new means of transport. Railroads
were made for steam engine driven
carriages and wagons. Later came
Fig. 7.1 Some means of transportation
Rationalised 2023-24
Page 2


74 SCIENCE
7
Motion and
Measurement of Distances
T
here was a general discussion
among the children in Paheli and
Boojho's class about the places
they had visited during the summer
vacations. Someone had gone to their
native village by a train, then a bus, and
finally a bullock cart. One student had
travelled by an aeroplane. Another spent
many days of his holidays going on
fishing trips in his uncle's boat.
The teacher then asked them to read
newspaper articles that mentioned
about small wheeled vehicles that moved
on the soil of Mars and conducted
experiments. These vehicles were taken
by spacecraft all the way to Mars!
Meanwhile, Paheli had been reading
stories about ancient India and wanted
to know how people travelled from one
place to another in earlier times.
7.1 STORY OF TRANSPORT
Long ago people did not have any means
of transport. They used to move only on
foot and carry goods on their back. Later
on they began to use animals for
transportation.
For transport through water, routes,
boats were used from ancient times. To
begin with, boats were simple logs of
wood in which a hollow cavity could be
made. Later, people learnt to put
together different pieces of wood and
give shapes to the boats. These shapes
imitated the shapes of the animals living
in water. Recall our discussions of this
streamlined shape of fish in Chapters
5 and 6.
Invention of the wheel made a great
change in modes of transport. The
design of the wheel was improved over
thousands of years.  Animals were used
to pull carts that moved on wheels.
Until the beginning of the 19th
century, people still depended on
animals, boats and ships to transport
them from place to place. The invention
of steam engine led to the development
of new means of transport. Railroads
were made for steam engine driven
carriages and wagons. Later came
Fig. 7.1 Some means of transportation
Rationalised 2023-24
75 MOTION AND MEASUREMENT OF DISTANCES
automobiles such as motor cars, trucks
and buses. Motorised boats and ships were
used as means of transport on water. The
early years of 1900 saw the development
of aeroplanes. These were later improved
to carry passengers and goods.  Electric
trains, monorail, supersonic aeroplanes
and spacecraft are some of the
contributions of the 20th century.
Fig. 7.1 shows some of the different
modes of transport. Place them in the
correct order — from the earliest modes
of transport to the most recent.
Are there any of the early modes of
transport that are not in use today?
7.2 HOW WIDE IS THIS DESK?
How did people know how far they have
travelled?
How will you know whether you can
walk all the way to your school or whether
you will need to take a bus or a rickshaw
to reach your school?  When you need to
purchase something, is it possible for you
to walk to the market? How will you know
the answers to these questions?
It is often important to know how far
a place is, so that we can have an idea
how we are going to reach that place —
walk, take a bus or a train, a ship, an
aeroplane or even a spacecraft!
Sometimes, there are objects whose
length or width we need to know.
In Paheli and Boojho's  classroom,
there are large desks which are to be
shared by two students. Paheli and
Boojho share one desk, but, frequently
end up objecting that the other is using
a larger share of the desk.
On the teacher's suggestion, they
decided to measure the length of the
desk, make a mark exactly in the middle
of it and draw a line to separate the two
halves of the desk.
Both Paheli and Boojho are very fond
of playing gilli danda with their friends.
Boojho  brought a set of gilli and danda
with him.
Here is how they tried to measure
the length of the desk using the danda
and the gilli (Fig. 7.2).
The desk seems to be having a
length equal to two danda lengths and
two lengths of the gilli. Drawing a line
in the middle of the desk leaves each
of them happy with a half of the desk
equal to a danda and a gilli in length.
After a few days, the marked line gets
wiped out. Boojho now has a new set
of gilli and danda as he lost his old
one. Here is how, the length of the
desk seems to measure using the gilli
and danda (Fig. 7.3).
Fig. 7.3 Measuring the length of the desk with a
different set of gilli and danda
Fig. 7.2 Measuring the length of a desk with
gilli and danda
Rationalised 2023-24
Page 3


74 SCIENCE
7
Motion and
Measurement of Distances
T
here was a general discussion
among the children in Paheli and
Boojho's class about the places
they had visited during the summer
vacations. Someone had gone to their
native village by a train, then a bus, and
finally a bullock cart. One student had
travelled by an aeroplane. Another spent
many days of his holidays going on
fishing trips in his uncle's boat.
The teacher then asked them to read
newspaper articles that mentioned
about small wheeled vehicles that moved
on the soil of Mars and conducted
experiments. These vehicles were taken
by spacecraft all the way to Mars!
Meanwhile, Paheli had been reading
stories about ancient India and wanted
to know how people travelled from one
place to another in earlier times.
7.1 STORY OF TRANSPORT
Long ago people did not have any means
of transport. They used to move only on
foot and carry goods on their back. Later
on they began to use animals for
transportation.
For transport through water, routes,
boats were used from ancient times. To
begin with, boats were simple logs of
wood in which a hollow cavity could be
made. Later, people learnt to put
together different pieces of wood and
give shapes to the boats. These shapes
imitated the shapes of the animals living
in water. Recall our discussions of this
streamlined shape of fish in Chapters
5 and 6.
Invention of the wheel made a great
change in modes of transport. The
design of the wheel was improved over
thousands of years.  Animals were used
to pull carts that moved on wheels.
Until the beginning of the 19th
century, people still depended on
animals, boats and ships to transport
them from place to place. The invention
of steam engine led to the development
of new means of transport. Railroads
were made for steam engine driven
carriages and wagons. Later came
Fig. 7.1 Some means of transportation
Rationalised 2023-24
75 MOTION AND MEASUREMENT OF DISTANCES
automobiles such as motor cars, trucks
and buses. Motorised boats and ships were
used as means of transport on water. The
early years of 1900 saw the development
of aeroplanes. These were later improved
to carry passengers and goods.  Electric
trains, monorail, supersonic aeroplanes
and spacecraft are some of the
contributions of the 20th century.
Fig. 7.1 shows some of the different
modes of transport. Place them in the
correct order — from the earliest modes
of transport to the most recent.
Are there any of the early modes of
transport that are not in use today?
7.2 HOW WIDE IS THIS DESK?
How did people know how far they have
travelled?
How will you know whether you can
walk all the way to your school or whether
you will need to take a bus or a rickshaw
to reach your school?  When you need to
purchase something, is it possible for you
to walk to the market? How will you know
the answers to these questions?
It is often important to know how far
a place is, so that we can have an idea
how we are going to reach that place —
walk, take a bus or a train, a ship, an
aeroplane or even a spacecraft!
Sometimes, there are objects whose
length or width we need to know.
In Paheli and Boojho's  classroom,
there are large desks which are to be
shared by two students. Paheli and
Boojho share one desk, but, frequently
end up objecting that the other is using
a larger share of the desk.
On the teacher's suggestion, they
decided to measure the length of the
desk, make a mark exactly in the middle
of it and draw a line to separate the two
halves of the desk.
Both Paheli and Boojho are very fond
of playing gilli danda with their friends.
Boojho  brought a set of gilli and danda
with him.
Here is how they tried to measure
the length of the desk using the danda
and the gilli (Fig. 7.2).
The desk seems to be having a
length equal to two danda lengths and
two lengths of the gilli. Drawing a line
in the middle of the desk leaves each
of them happy with a half of the desk
equal to a danda and a gilli in length.
After a few days, the marked line gets
wiped out. Boojho now has a new set
of gilli and danda as he lost his old
one. Here is how, the length of the
desk seems to measure using the gilli
and danda (Fig. 7.3).
Fig. 7.3 Measuring the length of the desk with a
different set of gilli and danda
Fig. 7.2 Measuring the length of a desk with
gilli and danda
Rationalised 2023-24
76 SCIENCE
Hello! Now, when measured with the
new set of gilli and danda, the desk
length seems to be about two danda
lengths, one gilli length with a small
length still left out. This is less than one
gilli length. Now what?
What would you suggest Paheli and
Boojho do, to measure the length of the
whole desk? Can they use a cricket
wicket and bails to measure the length
or do you think that this might create
the similar problem?
One thing they could do is to take a
small length of string and mark two points
on it. This will be a string length. They
can measure the width of the desk in
string lengths (Fig. 7.4). How can they
use the string to measure distances less
than the length of a string? They can fold
the string and mark it into 
1
2
,  
1
4
 and 
1
8
'string lengths'. Now, perhaps Paheli and
Boojho can measure the exact length of
the desk using the string.
You would say that they should use
the scale in their geometry box and solve
their problem? Yes, Of course!
Boojho has been reading about the
way people used to measure distances
before such standard scales were made
and he has been trying to follow different
methods of measuring distances.
There are so many occasions when we
come across a need to measure lengths
and distances. The tailor needs to measure
the length of the cloth to know if it is
enough to stitch a kurta. A carpenter
needs to measure the height and width
of a cupboard to know how much wood
he would need to make its door. The
farmer needs to know the length and
breadth or the area of his land to know
how much seed he can sow and how
much water would be needed for his crops.
Suppose, you are asked how tall you
are? You want to tell the length of a
straight line from the top of your head
to the heel of your feet.
How long is this room?
How wide is this desk?
How far is it from Delhi to Lucknow?
How far away is the Moon from the
Earth?
All these questions have one thing
in common. They all concern distance
between two places.  The two places may
be close enough, like the two ends of a
table or they may be far apart, like
Jammu and Kanyakumari.
Let us do a few measurements to see
what exactly we need to do, when we
measure distances or lengths.
7.3 SOME MEASUREMENTS
Activity 1
Work in groups and each of you do this
activity one by one. Using your foot as a
Fig. 7.4 Measuring the length of the desk with
string lengths
Rationalised 2023-24
Page 4


74 SCIENCE
7
Motion and
Measurement of Distances
T
here was a general discussion
among the children in Paheli and
Boojho's class about the places
they had visited during the summer
vacations. Someone had gone to their
native village by a train, then a bus, and
finally a bullock cart. One student had
travelled by an aeroplane. Another spent
many days of his holidays going on
fishing trips in his uncle's boat.
The teacher then asked them to read
newspaper articles that mentioned
about small wheeled vehicles that moved
on the soil of Mars and conducted
experiments. These vehicles were taken
by spacecraft all the way to Mars!
Meanwhile, Paheli had been reading
stories about ancient India and wanted
to know how people travelled from one
place to another in earlier times.
7.1 STORY OF TRANSPORT
Long ago people did not have any means
of transport. They used to move only on
foot and carry goods on their back. Later
on they began to use animals for
transportation.
For transport through water, routes,
boats were used from ancient times. To
begin with, boats were simple logs of
wood in which a hollow cavity could be
made. Later, people learnt to put
together different pieces of wood and
give shapes to the boats. These shapes
imitated the shapes of the animals living
in water. Recall our discussions of this
streamlined shape of fish in Chapters
5 and 6.
Invention of the wheel made a great
change in modes of transport. The
design of the wheel was improved over
thousands of years.  Animals were used
to pull carts that moved on wheels.
Until the beginning of the 19th
century, people still depended on
animals, boats and ships to transport
them from place to place. The invention
of steam engine led to the development
of new means of transport. Railroads
were made for steam engine driven
carriages and wagons. Later came
Fig. 7.1 Some means of transportation
Rationalised 2023-24
75 MOTION AND MEASUREMENT OF DISTANCES
automobiles such as motor cars, trucks
and buses. Motorised boats and ships were
used as means of transport on water. The
early years of 1900 saw the development
of aeroplanes. These were later improved
to carry passengers and goods.  Electric
trains, monorail, supersonic aeroplanes
and spacecraft are some of the
contributions of the 20th century.
Fig. 7.1 shows some of the different
modes of transport. Place them in the
correct order — from the earliest modes
of transport to the most recent.
Are there any of the early modes of
transport that are not in use today?
7.2 HOW WIDE IS THIS DESK?
How did people know how far they have
travelled?
How will you know whether you can
walk all the way to your school or whether
you will need to take a bus or a rickshaw
to reach your school?  When you need to
purchase something, is it possible for you
to walk to the market? How will you know
the answers to these questions?
It is often important to know how far
a place is, so that we can have an idea
how we are going to reach that place —
walk, take a bus or a train, a ship, an
aeroplane or even a spacecraft!
Sometimes, there are objects whose
length or width we need to know.
In Paheli and Boojho's  classroom,
there are large desks which are to be
shared by two students. Paheli and
Boojho share one desk, but, frequently
end up objecting that the other is using
a larger share of the desk.
On the teacher's suggestion, they
decided to measure the length of the
desk, make a mark exactly in the middle
of it and draw a line to separate the two
halves of the desk.
Both Paheli and Boojho are very fond
of playing gilli danda with their friends.
Boojho  brought a set of gilli and danda
with him.
Here is how they tried to measure
the length of the desk using the danda
and the gilli (Fig. 7.2).
The desk seems to be having a
length equal to two danda lengths and
two lengths of the gilli. Drawing a line
in the middle of the desk leaves each
of them happy with a half of the desk
equal to a danda and a gilli in length.
After a few days, the marked line gets
wiped out. Boojho now has a new set
of gilli and danda as he lost his old
one. Here is how, the length of the
desk seems to measure using the gilli
and danda (Fig. 7.3).
Fig. 7.3 Measuring the length of the desk with a
different set of gilli and danda
Fig. 7.2 Measuring the length of a desk with
gilli and danda
Rationalised 2023-24
76 SCIENCE
Hello! Now, when measured with the
new set of gilli and danda, the desk
length seems to be about two danda
lengths, one gilli length with a small
length still left out. This is less than one
gilli length. Now what?
What would you suggest Paheli and
Boojho do, to measure the length of the
whole desk? Can they use a cricket
wicket and bails to measure the length
or do you think that this might create
the similar problem?
One thing they could do is to take a
small length of string and mark two points
on it. This will be a string length. They
can measure the width of the desk in
string lengths (Fig. 7.4). How can they
use the string to measure distances less
than the length of a string? They can fold
the string and mark it into 
1
2
,  
1
4
 and 
1
8
'string lengths'. Now, perhaps Paheli and
Boojho can measure the exact length of
the desk using the string.
You would say that they should use
the scale in their geometry box and solve
their problem? Yes, Of course!
Boojho has been reading about the
way people used to measure distances
before such standard scales were made
and he has been trying to follow different
methods of measuring distances.
There are so many occasions when we
come across a need to measure lengths
and distances. The tailor needs to measure
the length of the cloth to know if it is
enough to stitch a kurta. A carpenter
needs to measure the height and width
of a cupboard to know how much wood
he would need to make its door. The
farmer needs to know the length and
breadth or the area of his land to know
how much seed he can sow and how
much water would be needed for his crops.
Suppose, you are asked how tall you
are? You want to tell the length of a
straight line from the top of your head
to the heel of your feet.
How long is this room?
How wide is this desk?
How far is it from Delhi to Lucknow?
How far away is the Moon from the
Earth?
All these questions have one thing
in common. They all concern distance
between two places.  The two places may
be close enough, like the two ends of a
table or they may be far apart, like
Jammu and Kanyakumari.
Let us do a few measurements to see
what exactly we need to do, when we
measure distances or lengths.
7.3 SOME MEASUREMENTS
Activity 1
Work in groups and each of you do this
activity one by one. Using your foot as a
Fig. 7.4 Measuring the length of the desk with
string lengths
Rationalised 2023-24
77 MOTION AND MEASUREMENT OF DISTANCES
unit of length, measure the length and
breadth of the classroom. It is possible
that while measuring these you may find
some part remains to be measured as it
is smaller than your foot. Use a string
to measure the length of a part of your
foot as you did before. Record your
observations in Table 7.1.
Table 7.1 Measuring length and
breadth of classroom
f o e m a N
t n e d u t s
e h t f o h t g n e L
m o o r s s a l c
e h t f o h t d i W
m o o r s s a l c
Activity 2
Work in a group and each of you use
your handspan as a unit to measure
the width of a table or a desk in the
classroom (Fig. 7.5).
with some known quantity. This known
fixed quantity is called a unit. The result
of a measurement is expressed in two
parts. One part is a number.  The other
part is the unit of the measurement. For
example, if in Activity 1, the length of
the room is found to be 12 lengths of
your foot, then 12 is the number and
'foot length' is the unit selected for the
measurement.
Now, study all the measurements
recorded in Table 7.1 and 7.2. Are all
the measurements for the room using
everybody's foot, equal? Are everybody's
measurement, by handspan, of the
width of the table equal? Perhaps the
results could be different as the length
of your handspan and that of your
friends may not be the same. Similarly,
the length of the foot may be slightly
different for all the students. Therefore,
when you tell your measurement using
your handspan or length of foot  as a
unit to others, they will not be able to
understand how big the actual length
is, unless they know the length of your
handspan or foot.
We see therefore, that some standard
units of measurement are needed, that
do not change from person to person.
Fig. 7.5 Measuring the width of a table with a
handspan
Here too, you may find that you
need string lengths equal to your
handspan and then fractions of
this string length to make the
measurement. Record all observations
in Table 7.2.
We see that, measurement means the
comparison of an unknown quantity
Table 7.2 Measuring width of a table
e h t d e r u s a e m o h W
? e l b a t e h t f o h t d i w
f o r e b m u N
s n a p s d n a h
Rationalised 2023-24
Page 5


74 SCIENCE
7
Motion and
Measurement of Distances
T
here was a general discussion
among the children in Paheli and
Boojho's class about the places
they had visited during the summer
vacations. Someone had gone to their
native village by a train, then a bus, and
finally a bullock cart. One student had
travelled by an aeroplane. Another spent
many days of his holidays going on
fishing trips in his uncle's boat.
The teacher then asked them to read
newspaper articles that mentioned
about small wheeled vehicles that moved
on the soil of Mars and conducted
experiments. These vehicles were taken
by spacecraft all the way to Mars!
Meanwhile, Paheli had been reading
stories about ancient India and wanted
to know how people travelled from one
place to another in earlier times.
7.1 STORY OF TRANSPORT
Long ago people did not have any means
of transport. They used to move only on
foot and carry goods on their back. Later
on they began to use animals for
transportation.
For transport through water, routes,
boats were used from ancient times. To
begin with, boats were simple logs of
wood in which a hollow cavity could be
made. Later, people learnt to put
together different pieces of wood and
give shapes to the boats. These shapes
imitated the shapes of the animals living
in water. Recall our discussions of this
streamlined shape of fish in Chapters
5 and 6.
Invention of the wheel made a great
change in modes of transport. The
design of the wheel was improved over
thousands of years.  Animals were used
to pull carts that moved on wheels.
Until the beginning of the 19th
century, people still depended on
animals, boats and ships to transport
them from place to place. The invention
of steam engine led to the development
of new means of transport. Railroads
were made for steam engine driven
carriages and wagons. Later came
Fig. 7.1 Some means of transportation
Rationalised 2023-24
75 MOTION AND MEASUREMENT OF DISTANCES
automobiles such as motor cars, trucks
and buses. Motorised boats and ships were
used as means of transport on water. The
early years of 1900 saw the development
of aeroplanes. These were later improved
to carry passengers and goods.  Electric
trains, monorail, supersonic aeroplanes
and spacecraft are some of the
contributions of the 20th century.
Fig. 7.1 shows some of the different
modes of transport. Place them in the
correct order — from the earliest modes
of transport to the most recent.
Are there any of the early modes of
transport that are not in use today?
7.2 HOW WIDE IS THIS DESK?
How did people know how far they have
travelled?
How will you know whether you can
walk all the way to your school or whether
you will need to take a bus or a rickshaw
to reach your school?  When you need to
purchase something, is it possible for you
to walk to the market? How will you know
the answers to these questions?
It is often important to know how far
a place is, so that we can have an idea
how we are going to reach that place —
walk, take a bus or a train, a ship, an
aeroplane or even a spacecraft!
Sometimes, there are objects whose
length or width we need to know.
In Paheli and Boojho's  classroom,
there are large desks which are to be
shared by two students. Paheli and
Boojho share one desk, but, frequently
end up objecting that the other is using
a larger share of the desk.
On the teacher's suggestion, they
decided to measure the length of the
desk, make a mark exactly in the middle
of it and draw a line to separate the two
halves of the desk.
Both Paheli and Boojho are very fond
of playing gilli danda with their friends.
Boojho  brought a set of gilli and danda
with him.
Here is how they tried to measure
the length of the desk using the danda
and the gilli (Fig. 7.2).
The desk seems to be having a
length equal to two danda lengths and
two lengths of the gilli. Drawing a line
in the middle of the desk leaves each
of them happy with a half of the desk
equal to a danda and a gilli in length.
After a few days, the marked line gets
wiped out. Boojho now has a new set
of gilli and danda as he lost his old
one. Here is how, the length of the
desk seems to measure using the gilli
and danda (Fig. 7.3).
Fig. 7.3 Measuring the length of the desk with a
different set of gilli and danda
Fig. 7.2 Measuring the length of a desk with
gilli and danda
Rationalised 2023-24
76 SCIENCE
Hello! Now, when measured with the
new set of gilli and danda, the desk
length seems to be about two danda
lengths, one gilli length with a small
length still left out. This is less than one
gilli length. Now what?
What would you suggest Paheli and
Boojho do, to measure the length of the
whole desk? Can they use a cricket
wicket and bails to measure the length
or do you think that this might create
the similar problem?
One thing they could do is to take a
small length of string and mark two points
on it. This will be a string length. They
can measure the width of the desk in
string lengths (Fig. 7.4). How can they
use the string to measure distances less
than the length of a string? They can fold
the string and mark it into 
1
2
,  
1
4
 and 
1
8
'string lengths'. Now, perhaps Paheli and
Boojho can measure the exact length of
the desk using the string.
You would say that they should use
the scale in their geometry box and solve
their problem? Yes, Of course!
Boojho has been reading about the
way people used to measure distances
before such standard scales were made
and he has been trying to follow different
methods of measuring distances.
There are so many occasions when we
come across a need to measure lengths
and distances. The tailor needs to measure
the length of the cloth to know if it is
enough to stitch a kurta. A carpenter
needs to measure the height and width
of a cupboard to know how much wood
he would need to make its door. The
farmer needs to know the length and
breadth or the area of his land to know
how much seed he can sow and how
much water would be needed for his crops.
Suppose, you are asked how tall you
are? You want to tell the length of a
straight line from the top of your head
to the heel of your feet.
How long is this room?
How wide is this desk?
How far is it from Delhi to Lucknow?
How far away is the Moon from the
Earth?
All these questions have one thing
in common. They all concern distance
between two places.  The two places may
be close enough, like the two ends of a
table or they may be far apart, like
Jammu and Kanyakumari.
Let us do a few measurements to see
what exactly we need to do, when we
measure distances or lengths.
7.3 SOME MEASUREMENTS
Activity 1
Work in groups and each of you do this
activity one by one. Using your foot as a
Fig. 7.4 Measuring the length of the desk with
string lengths
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77 MOTION AND MEASUREMENT OF DISTANCES
unit of length, measure the length and
breadth of the classroom. It is possible
that while measuring these you may find
some part remains to be measured as it
is smaller than your foot. Use a string
to measure the length of a part of your
foot as you did before. Record your
observations in Table 7.1.
Table 7.1 Measuring length and
breadth of classroom
f o e m a N
t n e d u t s
e h t f o h t g n e L
m o o r s s a l c
e h t f o h t d i W
m o o r s s a l c
Activity 2
Work in a group and each of you use
your handspan as a unit to measure
the width of a table or a desk in the
classroom (Fig. 7.5).
with some known quantity. This known
fixed quantity is called a unit. The result
of a measurement is expressed in two
parts. One part is a number.  The other
part is the unit of the measurement. For
example, if in Activity 1, the length of
the room is found to be 12 lengths of
your foot, then 12 is the number and
'foot length' is the unit selected for the
measurement.
Now, study all the measurements
recorded in Table 7.1 and 7.2. Are all
the measurements for the room using
everybody's foot, equal? Are everybody's
measurement, by handspan, of the
width of the table equal? Perhaps the
results could be different as the length
of your handspan and that of your
friends may not be the same. Similarly,
the length of the foot may be slightly
different for all the students. Therefore,
when you tell your measurement using
your handspan or length of foot  as a
unit to others, they will not be able to
understand how big the actual length
is, unless they know the length of your
handspan or foot.
We see therefore, that some standard
units of measurement are needed, that
do not change from person to person.
Fig. 7.5 Measuring the width of a table with a
handspan
Here too, you may find that you
need string lengths equal to your
handspan and then fractions of
this string length to make the
measurement. Record all observations
in Table 7.2.
We see that, measurement means the
comparison of an unknown quantity
Table 7.2 Measuring width of a table
e h t d e r u s a e m o h W
? e l b a t e h t f o h t d i w
f o r e b m u N
s n a p s d n a h
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78 SCIENCE
7.4 STANDARD UNITS OF
MEASUREMENTS
In ancient times, the length of a foot,
the width of a finger, and the distance
of a step were commonly used as
different units of measurements.
The people of the Indus valley
civilisation must have used very good
measurements of length because we see
evidence in excavations of perfectly
geometrical constructions.
A cubit as the length from the elbow
to the finger tips was used in ancient
Egypt and was also accepted as a unit
of length in other parts of the world.
People also used the "foot" as a unit
of length in different parts of the world.
The length of the foot used varied
slightly from region to region.
People measured a “yard” of cloth by
the distance between the end of the
outstretched arm and their chin. The
Romans measured with their pace
or steps.
In ancient India, small length
measurements used were an angul
(finger) or a mutthi (fist). Even today, we
can see flower sellers using their forearm
as a unit of length for garlands in many
towns of India. Many such body parts
continue to be in use as unit of length,
when convenient.
However, everyone's body parts could
be of slightly different sizes. This must
have caused confusion in measurement.
In 1790, the French created a standard
unit of measurement called the
metric system.
For the sake of uniformity, scientists
all over the world have accepted a set of
standard units of measurement. The
system of units now used is known as
the International System of Units (SI
units). The SI unit of length is a metre.
A metre scale is shown in Fig. 7.6.  Also
shown is the 15 cm scale in your
geometry box.
Each metre (m) is divided into 100
equal divisions, called centimetre (cm).
Each centimetre has ten equal divisions,
called millimetre (mm). Thus,
1 m  = 100 cm
1 cm = 10 mm
For measuring large distances, metre
is not a convenient unit. We define a
larger unit of length. It is called
kilometre (km).
1 km = 1000 m
Now, we can repeat all our
measurement activities using a
standard scale and measure in SI units.
Before we do that, we do need to know
the correct way of measuring lengths
and distances.
7.5 CORRECT MEASUREMENT OF
LENGTH
In our daily life we use various types of
measuring devices. We  use a metre scale
Fig.7.6 A metre scale and a 15 cm scale
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