Class 7 Science NCERT Notes – Chapter 8: Measurement of Time and Motion (PDF, MindMap, Q&A, Quizzes)

Chapter 8 (Science): Measurement of Time and Motion – CBSE Class 7 NCERT Science Detailed Study Notes.

1. The Evolution of Time Measurement

The need to measure time has been critical throughout human history. The methods have evolved from simple natural observations to highly precise atomic clocks.

1.1 Ancient Timekeeping Devices

Before the invention of modern clocks, various devices were created to measure smaller intervals of time within a day.

• Sundials: These instruments determine time based on the changing position of a shadow cast by an object in sunlight. A notable example is the Samrat Yantra at Jantar Mantar in Jaipur, Rajasthan. Built around 300 years ago, this 27-meter-high stone sundial is the world’s largest and can measure time intervals as short as 2 seconds.
• Water Clocks: These devices used the flow of water to measure time. There were two main types:
  • Flowing Out Type: Water flowed out from a vessel with time markings.
  • Sinking Bowl Type (Ghatika-yantra): A bowl with a small hole was floated on water. It would gradually fill and sink in a fixed time, after which it was lifted and floated again. In ancient India, the sinking time was standardized to 24 minutes, a unit called a ghati.
• Hourglasses: Time was measured by the consistent flow of sand from an upper bulb to a lower one.
• Candle Clocks: These were candles with markings along their length. As the candle burned down, the passage of time was indicated by the markings.

1.2 The Mechanical Revolution and the Pendulum

The development of mechanical devices marked a significant leap in timekeeping accuracy.

• Early Mechanical Clocks (14th Century Onwards): These clocks were driven by a complex system of weights, gears, and springs.
• The Pendulum Clock (17th Century): This was a major breakthrough in mechanical timekeeping.
  • Galileo Galilei (1564–1642): While observing a swinging lamp in a church, Galileo used his pulse to discover that a pendulum takes the same amount of time for each swing. He concluded that the time to complete one oscillation is always the same for a pendulum of a given length.
  • Christiaan Huygens (1629–1695): Inspired by Galileo’s work, Huygens invented the pendulum clock in 1656 and patented it in 1657.

1.3 Modern Timekeeping

Modern clocks operate on the same principle of using a periodically repeating process but with much greater precision.

• Quartz Clocks: Use tiny, rapid vibrations from a quartz crystal to measure time.
• Atomic Clocks: Use vibrations from specific atoms. These are extraordinarily precise, losing only about one second in millions of years, compared to Huygens’ early clocks which could lose 10 seconds per day.

2. The Physics of a Simple Pendulum

The consistent motion of a pendulum made it the basis for accurate time measurement for centuries.

• Oscillation: One complete back-and-forth swing of the pendulum’s bob. It can be measured from one extreme position, to the other, and back again (e.g., A to B to A).
• Time Period: The time taken for the pendulum to complete one oscillation.
• Key Properties:
  • The time period of a simple pendulum depends on its length. A longer pendulum has a longer time period.
  • The time period does not depend on the mass of the bob.
  • This constant time period for a given length is the property utilized in pendulum clocks.

3. Units of Time

Standardization of time units is essential for science and daily life.

• SI Unit: The standard international (SI) unit of time is the second, with the symbol s.
• Larger Units:
  • Minute (min): 60 s = 1 min
  • Hour (h): 60 min = 1 h
• Writing Conventions: Unit names and symbols (s, min, h) are written in lowercase. A full stop is not used after the symbol (unless at the end of a sentence), and a space is left between the number and the unit. Writing ‘sec’ for second or ‘hrs’ for hour is incorrect.

4. Understanding Motion

Motion is described by the distance an object travels over a period of time.

4.1 Speed

• Definition: Speed is the measure of how fast an object is moving. It is calculated as the total distance covered divided by the total time taken.
• Average Speed: Because objects rarely move at a perfectly constant speed, the calculated value is typically the average speed over the entire journey.
• Formulas:
  • Speed = Total distance covered / Total time taken
  • Total distance covered = Speed × Total time taken
  • Total time taken = Total distance covered / Speed

4.2 Types of Linear Motion

• Linear Motion: Motion that occurs along a straight line.
• Uniform Linear Motion: An object moving along a straight line at a constant speed. It covers equal distances in equal intervals of time. This is an idealization and rarely observed perfectly in real life.
• Non-uniform Linear Motion: An object moving along a straight line whose speed keeps changing. It covers unequal distances in equal intervals of time. A common example is a car moving in city traffic.

4.3 Instruments for Measuring Motion

Vehicles are equipped with instruments to measure speed and distance.

• Speedometer: An instrument that measures and displays a vehicle’s instantaneous speed, typically in kilometres per hour (km/h).
• Odometer: An instrument that measures and records the total distance travelled by the vehicle.

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Quiz and Q&A Section

Part 1: Short-Answer Questions

Answer each question in 2-3 sentences.

1. What was the fundamental principle behind a sundial’s operation?
2. Describe the two types of water clocks mentioned in the text.
3. Who was Galileo Galilei and what was his key observation regarding pendulums?
4. How did Christiaan Huygens contribute to the history of timekeeping?
5. Define one complete “oscillation” of a pendulum.
6. What is the “time period” of a pendulum?
7. What is the single most important factor that determines a simple pendulum’s time period?
8. What factor does not affect the time period of a simple pendulum?
9. Name two types of modern clocks and the principle they use.
10. What is the SI unit of time and its symbol?
11. State the correct way to write the symbols for minute and hour.
12. How is the “average speed” of an object calculated?
13. Why do we typically calculate average speed instead of constant speed for a journey?
14. What is the definition of “uniform linear motion”?
15. Provide an example of non-uniform linear motion from the text.
16. How does an object’s travel pattern differ between uniform and non-uniform motion in terms of distance and time?
17. What is a speedometer and what does it measure?
18. What is an odometer and what is its function?
19. Describe the Samrat Yantra and its significance.
20. What was the “Ghatika-yantra” and how was time announced when it was used?
21. What unit of time was measured by the Ghatika-yantra and how long was it?
22. How has the precision of timekeeping evolved from Huygens’ pendulum clocks to modern atomic clocks?
23. List two real-world applications where measuring tiny fractions of a second is important.
24. What is the formula for calculating the distance an object travels if its speed and time are known?
25. How can you determine if a train is in uniform or non-uniform motion by looking at a table of its positions over time?

Part 2: Multiple-Choice Questions

Select the best answer for each question.

1. Which ancient device measured time based on the changing position of a shadow? a) Water Clock b) Candle Clock c) Sundial d) Hourglass
2. The sinking bowl type of water clock was also known as the: a) Samrat Yantra b) Arthasastra c) Ghatika-yantra d) Pendulum Clock
3. Who first observed that a pendulum of a given length has a constant time period for each swing? a) Christiaan Huygens b) Galileo Galilei c) Kautilya d) Aryabhata
4. The pendulum clock was invented in the seventeenth century by: a) Varahamihira b) Galileo Galilei c) Aryabhata d) Christiaan Huygens
5. The time taken by a pendulum to complete one full back-and-forth swing is called its: a) Oscillation b) Frequency c) Time period d) Length
6. The time period of a simple pendulum depends on its: a) Mass b) Color c) Length d) Width
7. What is the SI unit of time? a) Minute (min) b) Hour (h) c) Second (s) d) Ghati
8. The correct symbol for an hour is: a) hr b) h c) hrs d) H
9. An object moving along a straight line at a constant speed is in: a) Non-uniform linear motion b) Average motion c) Oscillatory motion d) Uniform linear motion
10. A car speeding up and slowing down in city traffic is an example of: a) Uniform linear motion b) Non-uniform linear motion c) Constant speed d) Pendulum motion
11. What is the formula to calculate speed? a) Speed = Distance × Time b) Speed = Time / Distance c) Speed = Distance / Time d) Speed = Distance + Time
12. The instrument in a vehicle that measures the distance travelled is the: a) Speedometer b) Stopwatch c) Odometer d) Water clock
13. The instrument in a vehicle that displays the speed in km/h is the: a) Odometer b) Speedometer c) Pendulum d) Hourglass
14. According to the text, a “ghati” was a unit of time equal to: a) 60 seconds b) 60 minutes c) 24 seconds d) 24 minutes
15. Modern atomic clocks are so precise they lose only one second in: a) A hundred years b) A thousand years c) Millions of years d) A decade
16. If a bus moves at 50 km/h for 2 hours, how far does it travel? a) 25 km b) 52 km c) 100 km d) 200 km
17. Which of the following is an INCORRECT way to write a unit of time? a) 15 min b) 10 s c) 2 hrs d) 5 h
18. Motion along a straight line is called: a) Circular motion b) Linear motion c) Oscillatory motion d) Random motion
19. An object in uniform motion covers: a) Unequal distances in equal time intervals b) Equal distances in unequal time intervals c) Equal distances in equal time intervals d) Unequal distances in unequal time intervals
20. Which modern technology uses tiny vibrations from a quartz crystal to keep time? a) Atomic clocks b) Pendulum clocks c) Quartz clocks d) Sundials

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Answer Keys

Answer Key for Short-Answer Questions

1. A sundial operates by casting a shadow from an object onto a marked surface. As the sun moves across the sky during the day, the position of the shadow changes, indicating the passage of time.
2. One type of water clock involved water flowing out of a vessel that had time markings on its side. The other type, a sinking bowl (Ghatika-yantra), was a bowl with a hole that was floated on water and would sink after a fixed amount of time.
3. Galileo Galilei was a scientist who observed a swinging lamp in a church. He discovered that the lamp took the same amount of time to complete each swing, leading to the conclusion that a pendulum’s time period is constant for a given length.
4. Christiaan Huygens, inspired by Galileo’s work on pendulums, invented the pendulum clock in 1656. This marked a major breakthrough in mechanical timekeeping, significantly improving accuracy.
5. One complete oscillation is the full back-and-forth movement of the pendulum. It is defined as the bob moving from its central mean position to one extreme, then to the other extreme, and finally back to the central position.
6. The time period of a pendulum is the specific amount of time it takes to complete one full oscillation. For a pendulum of a given length, this period is constant.
7. The single most important factor determining a simple pendulum’s time period is its length. The text states that the time period depends on its length.
8. The mass of the bob does not affect the time period of a simple pendulum. Experiments show that bobs of different masses on a pendulum of a fixed length will have the same time period.
9. Two modern clocks are quartz clocks, which use vibrations from a quartz crystal, and atomic clocks, which use vibrations from specific atoms. Both rely on a periodically repeating process.
10. The SI (Standard International) unit of time is the second. Its official symbol is a lowercase ‘s’.
11. The correct symbol for a minute is ‘min’ and the correct symbol for an hour is ‘h’. Both are written in lowercase letters.
12. Average speed is calculated by taking the total distance an object has covered and dividing it by the total time it took to cover that distance.
13. We calculate average speed because objects in real life seldom move at a constant speed over long distances. They often speed up or slow down, so the average gives an overall measure of their speed for the journey.
14. Uniform linear motion is defined as the motion of an object moving along a straight line with a constant, unchanging speed.
15. An example of non-uniform linear motion is a train that starts slowly from a station, speeds up, and then slows down to stop at the next station.
16. In uniform motion, an object covers equal distances in equal intervals of time. In non-uniform motion, it covers unequal distances in the same equal intervals of time.
17. A speedometer is an instrument found in vehicles like cars and buses. It measures and displays the vehicle’s instantaneous speed, usually in kilometres per hour (km/h).
18. An odometer is an instrument fitted in vehicles. Its function is to measure and record the total distance travelled by the vehicle.
19. The Samrat Yantra, located in Jaipur, is the world’s largest stone sundial. It is significant for its size (27 metres high) and precision, as it can measure time intervals as short as 2 seconds.
20. The “Ghatika-yantra” was a sinking bowl water clock used in ancient India. When the bowl sank, it was announced publicly by the beating of drums, blowing of conch shells, or striking a gong.
21. The Ghatika-yantra measured a unit of time called a “ghati” or “ghatika.” This unit of time was equal to 24 minutes.
22. The precision has improved dramatically. Huygens’ early pendulum clocks could gain or lose about 10 seconds each day, whereas modern atomic clocks are so accurate they lose only one second in millions of years.
23. In sports, precise timing down to a millisecond is used to determine race winners. In medicine, ECG machines measure millisecond variations in heartbeats to diagnose health issues.
24. The formula is Distance = Speed × Time. You can find the total distance by multiplying the object’s speed by the total time it was travelling.
25. By examining the distance the train covers in equal time intervals (e.g., every 10 minutes), you can determine its motion type. If it covers the same distance in each interval, it’s in uniform motion; if the distances vary, it’s in non-uniform motion.

Answer Key for Multiple-Choice Questions

1. c) Sundial
2. c) Ghatika-yantra
3. b) Galileo Galilei
4. d) Christiaan Huygens
5. c) Time period
6. c) Length
7. c) Second (s)
8. b) h
9. d) Uniform linear motion
10. b) Non-uniform linear motion
11. c) Speed = Distance / Time
12. c) Odometer
13. b) Speedometer
14. d) 24 minutes
15. c) Millions of years
16. c) 100 km
17. c) 2 hrs
18. b) Linear motion
19. c) Equal distances in equal time intervals
20. c) Quartz clocks

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Essay Questions and Answers

1. Trace the evolution of timekeeping from ancient devices to the breakthrough of the pendulum clock. Discuss the principles behind at least three different ancient devices.
   • Answer: The evolution of timekeeping demonstrates humanity’s increasing need for precision. In ancient times, people relied on natural phenomena and simple mechanical devices. One of the earliest was the sundial, which used the changing position of the sun’s shadow to mark the passage of daylight hours. Another common device was the water clock, which operated on the principle of a controlled flow of water, either dripping out of a marked vessel or filling a floating bowl until it sank. A third device, the hourglass, measured time by the consistent flow of fine sand from one glass bulb to another. While innovative, these methods had limitations in accuracy and were dependent on specific conditions. The development of mechanical clocks using gears and springs from the fourteenth century onwards improved consistency, but the major breakthrough came in the seventeenth century with the invention of the pendulum clock by Christiaan Huygens, which was based on Galileo’s discovery of the pendulum’s constant time period and offered unprecedented accuracy.
2. Explain Galileo’s crucial observations about pendulums and how this knowledge enabled the invention of the modern clock. In your answer, clearly define “oscillation” and “time period.”
   • Answer: Galileo Galilei made a foundational discovery that revolutionized timekeeping. While observing a lamp swinging in a church, he noticed that each complete back-and-forth swing took the same amount of time, regardless of how wide the swing was. This complete swing is defined as one “oscillation.” The time it takes to complete one oscillation is known as the “time period.” Galileo’s crucial conclusion was that for a pendulum of a given length, the time period is constant. This principle of isochronism (equal time) was the key that had been missing for creating a truly accurate timekeeping device. Christiaan Huygens later used this knowledge to design the pendulum clock, where the constant, predictable swing of the pendulum was used to regulate the movement of the clock’s gears, marking a major leap in accuracy over previous mechanical clocks.
3. Compare and contrast uniform linear motion and non-uniform linear motion. Provide a real-world example for each and explain how a data table could be used to differentiate between the two.
   • Answer: Both uniform and non-uniform linear motion describe an object moving along a straight line. The key difference lies in the object’s speed. Uniform linear motion occurs when an object travels at a constant, unchanging speed; for example, a train moving on a straight track between two distant points without speeding up or slowing down. In contrast, non-uniform linear motion occurs when an object’s speed changes over time; a car moving through city traffic, constantly starting, stopping, and changing speed, is a perfect example. A data table showing an object’s position at regular time intervals can clearly differentiate them. For an object in uniform motion, the distance covered in each time interval would be identical. For non-uniform motion, the distances covered in the same time intervals would vary.
4. Describe the components and function of the ‘Ghatika-yantra’ as used in ancient India. How was its measurement standardized and communicated?
   • Answer: The Ghatika-yantra was a sophisticated water clock of the sinking bowl type used in ancient India. It consisted of a bowl with a very fine hole drilled in its bottom. This bowl was floated on the surface of a larger vessel of water. Water would slowly seep into the bowl through the hole until it became heavy enough to sink. The process was standardized by designing the bowl and hole in such a way that it took a fixed time of 24 minutes to fill and sink. This 24-minute interval became a standard unit of time called a “ghati.” The measurement was communicated publicly; each time the bowl sank, the event was announced by striking a gong, beating drums, or blowing conch shells, ensuring that people in the vicinity, such as at royal palaces or monasteries, were aware of the passage of time.
5. What is the relationship between the length of a simple pendulum and its time period? How does the mass of the bob factor into this relationship? Describe a simple experiment to verify these principles.
   • Answer: The relationship between a pendulum’s length and its time period is direct and fundamental: the time period depends on the length. A pendulum with a longer string will have a longer time period, meaning it will take more time to complete one swing. Conversely, a shorter pendulum will have a shorter time period and swing faster. The mass of the bob, however, has no effect on the time period. Two pendulums of the same length but with bobs of different masses will have the same time period. To verify this, one could set up a pendulum with a string of a fixed length (e.g., 100 cm) and measure the time for 10 oscillations, then calculate the time period. The experiment can be repeated with a different length to show the time period changes, and then repeated again with the original length but a different mass to show the time period remains the same.
6. Explain the importance of highly precise time measurement in modern society, citing specific examples from sports, medicine, and technology.
   • Answer: In modern society, the ability to measure tiny fractions of a second is crucial across many fields. In sports, events like sprints are often decided by one-hundredth or even one-thousandth of a second (a millisecond), making high-precision timing essential to determine a winner accurately. In medicine, devices like the Electrocardiogram (ECG) monitor millisecond-level variations in heartbeats, allowing doctors to detect and diagnose critical health issues. In technology, the speed of our digital world relies on even smaller units; smartphones and computers process signals in microseconds (one-millionth of a second), which enables them to operate at incredible speeds. These examples show that as our clocks become more accurate, they unlock new capabilities in science, health, and daily life.
7. What is the SI unit of time, and what are the established conventions for writing it and other common time units? Why is standardization important?
   • Answer: The SI (Système International d’Unités) unit of time is the second, and its symbol is ‘s’. There are specific conventions for writing time units to ensure clarity and avoid confusion in scientific and technical communication. The symbols for second (s), minute (min), and hour (h) should always be written in lowercase and without a full stop, unless at the end of a sentence. A space should always be left between the numerical value and the unit symbol (e.g., “15 min,” not “15min”). Standardization is crucial because it creates a universal language for measurement, allowing scientists, engineers, and professionals worldwide to collaborate and share data without ambiguity.
8. Describe the functions of a speedometer and an odometer. How do these two instruments work together to provide information about a vehicle’s journey?
   • Answer: A speedometer and an odometer are two essential instruments found on the dashboard of most vehicles. The speedometer’s function is to measure and display the vehicle’s instantaneous speed—how fast it is moving at any given moment—typically in kilometres per hour (km/h). The odometer’s function is to measure and record the total distance the vehicle has travelled over its lifetime. Together, they provide a complete picture of a journey. The speedometer tells the driver their current speed, which is important for safety and adhering to speed limits, while the odometer tracks the cumulative distance, which is useful for maintenance schedules, navigation, and calculating fuel efficiency.
9. Using the concepts of speed, distance, and time, explain how you would calculate the average speed of a train between two stations based on a railway timetable.
   • Answer: To calculate a train’s average speed between two stations using a timetable, you need to find three pieces of information: the departure time from the starting station, the arrival time at the next station, and the distance between the two stations. First, you calculate the total time taken for the journey by finding the difference between the arrival and departure times. Next, you find the total distance between the two stations, which is usually listed in the timetable. Finally, you apply the formula for average speed: Speed = Total distance covered / Total time taken. This calculation gives the average speed of the train for that specific segment of its journey, accounting for any periods of acceleration, deceleration, or constant travel.
10. Discuss the statement: “Uniform linear motion is an idealisation.” Why is this true, and why is the concept of “average speed” more practical for describing real-world motion?
    • Answer: The statement “Uniform linear motion is an idealisation” is true because objects in the real world seldom move at a perfectly constant speed for a prolonged duration. Uniform linear motion requires an object to travel in a straight line with an unchanging speed, which ignores factors like friction, air resistance, and the need to accelerate from a stop or decelerate to a halt. For instance, a train must start slowly, speed up, and then slow down before stopping at the next station; only for a part of its journey might it approach uniform motion. Therefore, the concept of “average speed,” calculated as total distance divided by total time, is much more practical. It provides a single, useful value that describes the overall pace of a journey, smoothing out all the variations in speed that occur along the way.

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Glossary of Key Terms

Term	Definition
Atomic Clock	A modern, highly precise clock that measures time using the vibrations from specific atoms.
Average Speed	The total distance covered by an object divided by the total time taken to cover it.
Candle Clock	An ancient timekeeping device consisting of a candle with markings that indicate the passage of time as it burns.
Ghatika-yantra	A sinking bowl type of water clock used in ancient India.
Hourglass	A device that measures time by the flow of sand from an upper bulb to a lower one.
Linear Motion	The motion of an object along a straight line.
Non-uniform Linear Motion	The motion of an object along a straight line where its speed keeps changing. It covers unequal distances in equal intervals of time.
Odometer	An instrument in a vehicle that measures the total distance travelled.
Oscillation	One complete back-and-forth swing of a pendulum.
Pendulum	A weight (bob) suspended from a pivot so that it can swing freely. Its motion is used in pendulum clocks.
Pendulum Clock	A clock that uses a swinging pendulum as its timekeeping element, invented by Christiaan Huygens.
Quartz Clock	A modern clock that measures time using the tiny and rapid vibrations from a quartz crystal.
Second (s)	The SI (Standard International) unit of time.
Speed	The measure of the distance covered by an object per unit of time.
Speedometer	An instrument in a vehicle that measures and displays its instantaneous speed.
Stopwatch	A special kind of watch used for timing races or events.
Sundial	An instrument that determines time based on the position of a shadow cast by the Sun.
Time Period	The time taken by a pendulum to complete one full oscillation.
Uniform Linear Motion	The motion of an object along a straight line with a constant, unchanging speed. It covers equal distances in equal intervals of time.
Water Clock	A device that measures time by the regulated flow of liquid either into or out of a vessel.