Class 7 Science Chapter 8 NCERT Book - Measurement of Time and Motion

 Page 1


Prerna and her younger sister are watching a sports channel 
on television. Prerna enjoys running and she has been declared 
the fastest runner among the girls in her district for winning 
the 100 metre sprint at an interschool competition held at the 
district level. She is now training to compete at the state level. 
She dreams of representing India at the international level in 100 
metre sprint contests in future. 
While watching the rerun of sprints at the Olympic games held 
in the past, Prerna is always amazed that the measurement of 
the time taken for the race is so advanced that they could identify 
the winner even when two sprinters seemed to cross the ? nish 
line almost together. However, in her school, the sports teacher 
only used a special kind of watch called a ‘stopwatch’ for timing 
the school races. She had noticed her mother wearing a watch on 
her wrist and her sister looking at her mobile phone when she 
needed to check the time. Her uncle used a Braille watch and had 
recently acquired a talking watch that announced the time at the 
touch of a button. There was also a large clock on the wall near 
the school entrance. Her thoughts turned to people in the ancient 
past, who did not have the modern gadgets we have today and 
she wondered…
Measurement of 
Time and Motion
8
Chapter 8.indd   105 4/3/2025   4:54:31 PM
Page 2


Prerna and her younger sister are watching a sports channel 
on television. Prerna enjoys running and she has been declared 
the fastest runner among the girls in her district for winning 
the 100 metre sprint at an interschool competition held at the 
district level. She is now training to compete at the state level. 
She dreams of representing India at the international level in 100 
metre sprint contests in future. 
While watching the rerun of sprints at the Olympic games held 
in the past, Prerna is always amazed that the measurement of 
the time taken for the race is so advanced that they could identify 
the winner even when two sprinters seemed to cross the ? nish 
line almost together. However, in her school, the sports teacher 
only used a special kind of watch called a ‘stopwatch’ for timing 
the school races. She had noticed her mother wearing a watch on 
her wrist and her sister looking at her mobile phone when she 
needed to check the time. Her uncle used a Braille watch and had 
recently acquired a talking watch that announced the time at the 
touch of a button. There was also a large clock on the wall near 
the school entrance. Her thoughts turned to people in the ancient 
past, who did not have the modern gadgets we have today and 
she wondered…
Measurement of 
Time and Motion
8
Chapter 8.indd   105 4/3/2025   4:54:31 PM
Curiosity | Textbook of Science | Grade 7
106
8.1 Measurement of Time
Humans got interested in keeping track of time long ago. They 
started noticing that many events in nature repeat themselves 
after de? nite intervals of time. For example, the rising and setting 
of the Sun, the phases of the Moon and the changing seasons. 
They started using the cycles of these events for timekeeping. 
First, they devised calendars. A day was de? ned by the cycle of 
rising and setting of the Sun. Then began the quest to ? nd ways 
of knowing the time of day. 
How was time 
measured when there 
were no clocks and 
watches?
So, they made many devices which 
helped them to measure smaller intervals 
of time within a day. Some of these were 
sundials, water clocks, hourglasses, and 
candle clocks. 
In a sundial, time is determined with 
the changing position of the shadow of an 
object cast by the light of the Sun during 
the day (Fig. 8.1).
The water clocks used the ? ow of water 
out or into a vessel to measure time. In 
one type, water ? owed out from a vessel 
which had markings for time (Fig. 8.2a). In 
the other type, there would be a bowl, with 
a ? ne hole at the bottom, which was 
? oated on the surface of water (Fig. 8.2b). 
It gradually ? lled up in a ? xed time and 
? nally sank. Then, it was lifted up and 
? oated again. 
In an hourglass (Fig. 8.3), time was 
measured on the basis of the ? ow of sand 
from one bulb to another.
Candle clocks (Fig. 8.4) were candles 
with markings that indicated the passage 
of time when burned.
Fig. 8.1 A sundial
Fig. 8.2 A water clock (a) Water flowing out-type 
(b) Floating bowl-type
(a)
(b)
Fig. 8.4 A candle clock
Fig. 8.3 An hourglass
Chapter 8.indd   106 4/3/2025   4:54:32 PM
Page 3


Prerna and her younger sister are watching a sports channel 
on television. Prerna enjoys running and she has been declared 
the fastest runner among the girls in her district for winning 
the 100 metre sprint at an interschool competition held at the 
district level. She is now training to compete at the state level. 
She dreams of representing India at the international level in 100 
metre sprint contests in future. 
While watching the rerun of sprints at the Olympic games held 
in the past, Prerna is always amazed that the measurement of 
the time taken for the race is so advanced that they could identify 
the winner even when two sprinters seemed to cross the ? nish 
line almost together. However, in her school, the sports teacher 
only used a special kind of watch called a ‘stopwatch’ for timing 
the school races. She had noticed her mother wearing a watch on 
her wrist and her sister looking at her mobile phone when she 
needed to check the time. Her uncle used a Braille watch and had 
recently acquired a talking watch that announced the time at the 
touch of a button. There was also a large clock on the wall near 
the school entrance. Her thoughts turned to people in the ancient 
past, who did not have the modern gadgets we have today and 
she wondered…
Measurement of 
Time and Motion
8
Chapter 8.indd   105 4/3/2025   4:54:31 PM
Curiosity | Textbook of Science | Grade 7
106
8.1 Measurement of Time
Humans got interested in keeping track of time long ago. They 
started noticing that many events in nature repeat themselves 
after de? nite intervals of time. For example, the rising and setting 
of the Sun, the phases of the Moon and the changing seasons. 
They started using the cycles of these events for timekeeping. 
First, they devised calendars. A day was de? ned by the cycle of 
rising and setting of the Sun. Then began the quest to ? nd ways 
of knowing the time of day. 
How was time 
measured when there 
were no clocks and 
watches?
So, they made many devices which 
helped them to measure smaller intervals 
of time within a day. Some of these were 
sundials, water clocks, hourglasses, and 
candle clocks. 
In a sundial, time is determined with 
the changing position of the shadow of an 
object cast by the light of the Sun during 
the day (Fig. 8.1).
The water clocks used the ? ow of water 
out or into a vessel to measure time. In 
one type, water ? owed out from a vessel 
which had markings for time (Fig. 8.2a). In 
the other type, there would be a bowl, with 
a ? ne hole at the bottom, which was 
? oated on the surface of water (Fig. 8.2b). 
It gradually ? lled up in a ? xed time and 
? nally sank. Then, it was lifted up and 
? oated again. 
In an hourglass (Fig. 8.3), time was 
measured on the basis of the ? ow of sand 
from one bulb to another.
Candle clocks (Fig. 8.4) were candles 
with markings that indicated the passage 
of time when burned.
Fig. 8.1 A sundial
Fig. 8.2 A water clock (a) Water flowing out-type 
(b) Floating bowl-type
(a)
(b)
Fig. 8.4 A candle clock
Fig. 8.3 An hourglass
Chapter 8.indd   106 4/3/2025   4:54:32 PM
Measurement of Time and Motion
107
Should we make a simple water clock?
Activity 8.1: Let us construct
? Take a used transparent plastic  bottle (1/2 litre or larger) 
with its cap.
? Cut it into two, roughly in the middle as shown in Fig. 8.5a.
? Using a drawing pin, make a small hole in the cap of the 
bottle (Fig. 8.5b).
Fig. 8.5: Making a simple water clock
(b) (a) (c)
(d)
(a)
? Place the upper part of the bottle in an inverted position over 
the lower half (Fig. 8.5c).
? Fill the upper part of the bottle with water. You may add a 
few drops of ink or colour to make the water level easily 
visible (Fig. 8.5d). 
? The water will start dripping into the lower part of the bottle. 
Using a watch, mark the level of water after every one minute 
till all the water drips down.
Your water clock is ready. Can you now guess how to use it? 
Pour the water from the lower part back into the top part and 
watch the level of water dripping into the lower part. Every time 
it touches a mark made by you, one more minute has passed.
FASCINATING FACTS
The world’s largest stone sundial, 
the Samrat Yantra, was built 
around 300 years ago at the Jantar 
Mantar, in Jaipur, Rajasthan, 
a UNESCO World Heritage site 
that houses several astronomical 
instruments. With its imposing 
height of 27 metres, its shadow 
moves at about 1 millimetre per 
second and falls on a scale ? nely marked to measure time intervals as 
short as 2 seconds. Like any sundial, it measures local or ‘solar time’, 
requiring a correction to determine Indian Standard Time.
Chapter 8.indd   107 4/3/2025   4:54:33 PM
Page 4


Prerna and her younger sister are watching a sports channel 
on television. Prerna enjoys running and she has been declared 
the fastest runner among the girls in her district for winning 
the 100 metre sprint at an interschool competition held at the 
district level. She is now training to compete at the state level. 
She dreams of representing India at the international level in 100 
metre sprint contests in future. 
While watching the rerun of sprints at the Olympic games held 
in the past, Prerna is always amazed that the measurement of 
the time taken for the race is so advanced that they could identify 
the winner even when two sprinters seemed to cross the ? nish 
line almost together. However, in her school, the sports teacher 
only used a special kind of watch called a ‘stopwatch’ for timing 
the school races. She had noticed her mother wearing a watch on 
her wrist and her sister looking at her mobile phone when she 
needed to check the time. Her uncle used a Braille watch and had 
recently acquired a talking watch that announced the time at the 
touch of a button. There was also a large clock on the wall near 
the school entrance. Her thoughts turned to people in the ancient 
past, who did not have the modern gadgets we have today and 
she wondered…
Measurement of 
Time and Motion
8
Chapter 8.indd   105 4/3/2025   4:54:31 PM
Curiosity | Textbook of Science | Grade 7
106
8.1 Measurement of Time
Humans got interested in keeping track of time long ago. They 
started noticing that many events in nature repeat themselves 
after de? nite intervals of time. For example, the rising and setting 
of the Sun, the phases of the Moon and the changing seasons. 
They started using the cycles of these events for timekeeping. 
First, they devised calendars. A day was de? ned by the cycle of 
rising and setting of the Sun. Then began the quest to ? nd ways 
of knowing the time of day. 
How was time 
measured when there 
were no clocks and 
watches?
So, they made many devices which 
helped them to measure smaller intervals 
of time within a day. Some of these were 
sundials, water clocks, hourglasses, and 
candle clocks. 
In a sundial, time is determined with 
the changing position of the shadow of an 
object cast by the light of the Sun during 
the day (Fig. 8.1).
The water clocks used the ? ow of water 
out or into a vessel to measure time. In 
one type, water ? owed out from a vessel 
which had markings for time (Fig. 8.2a). In 
the other type, there would be a bowl, with 
a ? ne hole at the bottom, which was 
? oated on the surface of water (Fig. 8.2b). 
It gradually ? lled up in a ? xed time and 
? nally sank. Then, it was lifted up and 
? oated again. 
In an hourglass (Fig. 8.3), time was 
measured on the basis of the ? ow of sand 
from one bulb to another.
Candle clocks (Fig. 8.4) were candles 
with markings that indicated the passage 
of time when burned.
Fig. 8.1 A sundial
Fig. 8.2 A water clock (a) Water flowing out-type 
(b) Floating bowl-type
(a)
(b)
Fig. 8.4 A candle clock
Fig. 8.3 An hourglass
Chapter 8.indd   106 4/3/2025   4:54:32 PM
Measurement of Time and Motion
107
Should we make a simple water clock?
Activity 8.1: Let us construct
? Take a used transparent plastic  bottle (1/2 litre or larger) 
with its cap.
? Cut it into two, roughly in the middle as shown in Fig. 8.5a.
? Using a drawing pin, make a small hole in the cap of the 
bottle (Fig. 8.5b).
Fig. 8.5: Making a simple water clock
(b) (a) (c)
(d)
(a)
? Place the upper part of the bottle in an inverted position over 
the lower half (Fig. 8.5c).
? Fill the upper part of the bottle with water. You may add a 
few drops of ink or colour to make the water level easily 
visible (Fig. 8.5d). 
? The water will start dripping into the lower part of the bottle. 
Using a watch, mark the level of water after every one minute 
till all the water drips down.
Your water clock is ready. Can you now guess how to use it? 
Pour the water from the lower part back into the top part and 
watch the level of water dripping into the lower part. Every time 
it touches a mark made by you, one more minute has passed.
FASCINATING FACTS
The world’s largest stone sundial, 
the Samrat Yantra, was built 
around 300 years ago at the Jantar 
Mantar, in Jaipur, Rajasthan, 
a UNESCO World Heritage site 
that houses several astronomical 
instruments. With its imposing 
height of 27 metres, its shadow 
moves at about 1 millimetre per 
second and falls on a scale ? nely marked to measure time intervals as 
short as 2 seconds. Like any sundial, it measures local or ‘solar time’, 
requiring a correction to determine Indian Standard Time.
Chapter 8.indd   107 4/3/2025   4:54:33 PM
Curiosity | Textbook of Science | Grade 7
108
FASCINATING FACTS
In ancient India, time was measured using both shadows and water 
clocks. The earliest reference to shadow-based time measurement 
appears in the Arthasastra by Kautilya (was composed and revised 
between the second century BCE – third century CE). An accurate 
expression for time in terms of the shadow of a vertical stick was 
given by Varahamihira around 530 CE. The water clock with water 
? owing out was described in the Arthasastra, Sardulakarnavadana, 
and some other texts (early CE centuries). These clocks were not very 
accurate because as water levels dropped, the ? ow rate decreased. 
This led to the development of the sinking bowl water clock 
(Fig. 8.2b), or Ghatika-yantra, ? rst mentioned by Aryabhata, and then 
in several astronomical texts later. Time was measured constantly 
with Ghatika-yantra at Buddhist monasteries, royal palaces, town 
squares, and each time the bowl sank, it was announced by drums, 
conch shells, or striking a gong. Though the Ghatika-yantra was 
progressively replaced by pendulum clocks in the late nineteenth 
century, it continued to be used at the religious places for rituals.
, ? rst mentioned by Aryabhata, and then 
in several astronomical texts later. Time was measured constantly 
 at Buddhist monasteries, royal palaces, town 
squares, and each time the bowl sank, it was announced by drums, 
 was 
progressively replaced by pendulum clocks in the late nineteenth 
Fig. 8.6: Huygens’ 
Pendulum clock
Pendulum
As human civilisation advanced, and 
as people began to travel long distances, 
the measurement of time became very 
critical. This led to the development of 
increasingly better mechanical devices 
for the measurement of time, driven 
by weights, gears, and springs from the 
fourteenth century onwards. However, 
the invention of the pendulum clock 
in the seventeenth century marked 
a major breakthrough in mechanical 
timekeeping.
KNOW A SCIENTIST
The pendulum clock was invented in 1656 and patented in 1657 by 
Christiaan Huygens (1629 –1695). He was inspired by the investigations 
of pendulums by Galileo Galilei (1564 –1642). It is said that once while 
sitting in a church, Galileo’s attention was drawn to a lamp suspended 
from the ceiling, which was swinging back and forth. Using his pulse 
to measure time, Galileo found that the lamp took the same time for 
each swing. After experimenting with di? erent pendulums, Galileo 
concluded that the time taken to complete one oscillation was always 
the same for a pendulum of a given length. 
Christiaan Huygens (1629 –1695). He was inspired by the investigations 
of pendulums by Galileo Galilei (1564 –1642). It is said that once while 
sitting in a church, Galileo’s attention was drawn to a lamp suspended 
from the ceiling, which was swinging back and forth. Using his pulse 
to measure time, Galileo found that the lamp took the same time for 
each swing. After experimenting with di? erent pendulums, Galileo 
concluded that the time taken to complete one oscillation was always 
the same for a pendulum of a given length. 
Chapter 8.indd   108 4/3/2025   4:54:34 PM
Page 5


Prerna and her younger sister are watching a sports channel 
on television. Prerna enjoys running and she has been declared 
the fastest runner among the girls in her district for winning 
the 100 metre sprint at an interschool competition held at the 
district level. She is now training to compete at the state level. 
She dreams of representing India at the international level in 100 
metre sprint contests in future. 
While watching the rerun of sprints at the Olympic games held 
in the past, Prerna is always amazed that the measurement of 
the time taken for the race is so advanced that they could identify 
the winner even when two sprinters seemed to cross the ? nish 
line almost together. However, in her school, the sports teacher 
only used a special kind of watch called a ‘stopwatch’ for timing 
the school races. She had noticed her mother wearing a watch on 
her wrist and her sister looking at her mobile phone when she 
needed to check the time. Her uncle used a Braille watch and had 
recently acquired a talking watch that announced the time at the 
touch of a button. There was also a large clock on the wall near 
the school entrance. Her thoughts turned to people in the ancient 
past, who did not have the modern gadgets we have today and 
she wondered…
Measurement of 
Time and Motion
8
Chapter 8.indd   105 4/3/2025   4:54:31 PM
Curiosity | Textbook of Science | Grade 7
106
8.1 Measurement of Time
Humans got interested in keeping track of time long ago. They 
started noticing that many events in nature repeat themselves 
after de? nite intervals of time. For example, the rising and setting 
of the Sun, the phases of the Moon and the changing seasons. 
They started using the cycles of these events for timekeeping. 
First, they devised calendars. A day was de? ned by the cycle of 
rising and setting of the Sun. Then began the quest to ? nd ways 
of knowing the time of day. 
How was time 
measured when there 
were no clocks and 
watches?
So, they made many devices which 
helped them to measure smaller intervals 
of time within a day. Some of these were 
sundials, water clocks, hourglasses, and 
candle clocks. 
In a sundial, time is determined with 
the changing position of the shadow of an 
object cast by the light of the Sun during 
the day (Fig. 8.1).
The water clocks used the ? ow of water 
out or into a vessel to measure time. In 
one type, water ? owed out from a vessel 
which had markings for time (Fig. 8.2a). In 
the other type, there would be a bowl, with 
a ? ne hole at the bottom, which was 
? oated on the surface of water (Fig. 8.2b). 
It gradually ? lled up in a ? xed time and 
? nally sank. Then, it was lifted up and 
? oated again. 
In an hourglass (Fig. 8.3), time was 
measured on the basis of the ? ow of sand 
from one bulb to another.
Candle clocks (Fig. 8.4) were candles 
with markings that indicated the passage 
of time when burned.
Fig. 8.1 A sundial
Fig. 8.2 A water clock (a) Water flowing out-type 
(b) Floating bowl-type
(a)
(b)
Fig. 8.4 A candle clock
Fig. 8.3 An hourglass
Chapter 8.indd   106 4/3/2025   4:54:32 PM
Measurement of Time and Motion
107
Should we make a simple water clock?
Activity 8.1: Let us construct
? Take a used transparent plastic  bottle (1/2 litre or larger) 
with its cap.
? Cut it into two, roughly in the middle as shown in Fig. 8.5a.
? Using a drawing pin, make a small hole in the cap of the 
bottle (Fig. 8.5b).
Fig. 8.5: Making a simple water clock
(b) (a) (c)
(d)
(a)
? Place the upper part of the bottle in an inverted position over 
the lower half (Fig. 8.5c).
? Fill the upper part of the bottle with water. You may add a 
few drops of ink or colour to make the water level easily 
visible (Fig. 8.5d). 
? The water will start dripping into the lower part of the bottle. 
Using a watch, mark the level of water after every one minute 
till all the water drips down.
Your water clock is ready. Can you now guess how to use it? 
Pour the water from the lower part back into the top part and 
watch the level of water dripping into the lower part. Every time 
it touches a mark made by you, one more minute has passed.
FASCINATING FACTS
The world’s largest stone sundial, 
the Samrat Yantra, was built 
around 300 years ago at the Jantar 
Mantar, in Jaipur, Rajasthan, 
a UNESCO World Heritage site 
that houses several astronomical 
instruments. With its imposing 
height of 27 metres, its shadow 
moves at about 1 millimetre per 
second and falls on a scale ? nely marked to measure time intervals as 
short as 2 seconds. Like any sundial, it measures local or ‘solar time’, 
requiring a correction to determine Indian Standard Time.
Chapter 8.indd   107 4/3/2025   4:54:33 PM
Curiosity | Textbook of Science | Grade 7
108
FASCINATING FACTS
In ancient India, time was measured using both shadows and water 
clocks. The earliest reference to shadow-based time measurement 
appears in the Arthasastra by Kautilya (was composed and revised 
between the second century BCE – third century CE). An accurate 
expression for time in terms of the shadow of a vertical stick was 
given by Varahamihira around 530 CE. The water clock with water 
? owing out was described in the Arthasastra, Sardulakarnavadana, 
and some other texts (early CE centuries). These clocks were not very 
accurate because as water levels dropped, the ? ow rate decreased. 
This led to the development of the sinking bowl water clock 
(Fig. 8.2b), or Ghatika-yantra, ? rst mentioned by Aryabhata, and then 
in several astronomical texts later. Time was measured constantly 
with Ghatika-yantra at Buddhist monasteries, royal palaces, town 
squares, and each time the bowl sank, it was announced by drums, 
conch shells, or striking a gong. Though the Ghatika-yantra was 
progressively replaced by pendulum clocks in the late nineteenth 
century, it continued to be used at the religious places for rituals.
, ? rst mentioned by Aryabhata, and then 
in several astronomical texts later. Time was measured constantly 
 at Buddhist monasteries, royal palaces, town 
squares, and each time the bowl sank, it was announced by drums, 
 was 
progressively replaced by pendulum clocks in the late nineteenth 
Fig. 8.6: Huygens’ 
Pendulum clock
Pendulum
As human civilisation advanced, and 
as people began to travel long distances, 
the measurement of time became very 
critical. This led to the development of 
increasingly better mechanical devices 
for the measurement of time, driven 
by weights, gears, and springs from the 
fourteenth century onwards. However, 
the invention of the pendulum clock 
in the seventeenth century marked 
a major breakthrough in mechanical 
timekeeping.
KNOW A SCIENTIST
The pendulum clock was invented in 1656 and patented in 1657 by 
Christiaan Huygens (1629 –1695). He was inspired by the investigations 
of pendulums by Galileo Galilei (1564 –1642). It is said that once while 
sitting in a church, Galileo’s attention was drawn to a lamp suspended 
from the ceiling, which was swinging back and forth. Using his pulse 
to measure time, Galileo found that the lamp took the same time for 
each swing. After experimenting with di? erent pendulums, Galileo 
concluded that the time taken to complete one oscillation was always 
the same for a pendulum of a given length. 
Christiaan Huygens (1629 –1695). He was inspired by the investigations 
of pendulums by Galileo Galilei (1564 –1642). It is said that once while 
sitting in a church, Galileo’s attention was drawn to a lamp suspended 
from the ceiling, which was swinging back and forth. Using his pulse 
to measure time, Galileo found that the lamp took the same time for 
each swing. After experimenting with di? erent pendulums, Galileo 
concluded that the time taken to complete one oscillation was always 
the same for a pendulum of a given length. 
Chapter 8.indd   108 4/3/2025   4:54:34 PM
Measurement of Time and Motion
109
8.1.1 A simple pendulum
A simple pendulum 
consists of a small 
metallic ball (called the 
bob of the pendulum) 
suspended from a rigid 
support by a long thread 
as shown in Fig. 8.7a.
Fig. 8.7 A simple pendulum
(b)
O
A B
Extreme 
position
Extreme 
position
Mean 
position
(a)
Rigid 
support
Long 
thread
Bob 
The pendulum at rest is in its mean position. When the bob of 
the pendulum is moved slightly to one side and released, it starts 
oscillatory motion. Its motion is periodic in nature because it 
repeats its path after a ? xed interval of time. 
The pendulum is said to have completed one oscillation when 
its bob, starting from its mean position O, moves to extreme 
position A, changes direction and moves to another extreme 
position B, changes direction and comes back to O (Fig. 8.7b). The 
pendulum also completes one oscillation when its bob moves 
from one extreme position A to another extreme position B and 
comes back to A. The time taken by the pendulum to complete 
one oscillation is called its time period. Let us now set up a 
pendulum and measure its time period.
Activity 8.2: Let us experiment
? Collect a piece of string around 150 cm long, a heavy metal 
ball with hook/ a stone (bob), a stopwatch/ watch, and a ruler.
? Tie the bob at one end of the string. 
? Fix the other end of the string to a rigid support such that the 
length of the string in between support and bob is around 
100 cm. 
? Wait for the bob to come to rest. Your pendulum is now 
ready.
8.1.1 A simple pendulum
We did an activity in the chapter 
‘Measurement of Length and Motion’ 
in the Grade 6 Science textbook 
Curiosity, where we observed the 
oscillatory motion of an eraser hung 
with a thread. Is the pendulum 
similar to that?
Chapter 8.indd   109 4/3/2025   4:54:35 PM