NCERT Class 7 Science Curiosity Chapter 08 Measurement of Time and Motion PDF Download

Chapter 8: Measurement of Time and Motion

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 finish 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...

[Figure: A group of runners on a track with a teacher holding a stopwatch, See in your textbook]

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 definite 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 defined by the cycle of rising and setting of the Sun. Then began the quest to find ways of knowing the time of day.

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.

[Figure 8.1: A sundial, See in your textbook]

The water clocks used the flow of water out or into a vessel to measure time. In one type, water flowed out from a vessel which had markings for time. In the other type, there would be a bowl, with a fine hole at the bottom, which was floated on the surface of water. It gradually filled up in a fixed time and finally sank. Then, it was lifted up and floated again.

[Figure 8.2: A water clock (a) Water flowing out-type (b) Floating bowl-type, See in your textbook]

In an hourglass, time was measured on the basis of the flow of sand from one bulb to another.

[Figure 8.3: An hourglass, See in your textbook]

Candle clocks were candles with markings that indicated the passage of time when burned.

[Figure 8.4: A candle clock, See in your textbook]

Teacher's Note

Ancient civilizations developed water clocks, sundials, and hourglasses because they needed to measure time without electricity. The key idea is that all these devices relied on natural, steady processes \u2014 the Sun's shadow, water flowing, sand falling. When you see these devices in later topics, remember they work because the process is constant and predictable.

Activity 8.1: Let us construct a simple water clock

• 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 the figures.
• Using a drawing pin, make a small hole in the cap of the bottle.
• Place the upper part of the bottle in an inverted position over the lower half.
• 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.
• 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.

[Figure 8.5: Making a simple water clock (a) (b) (c) (d), See in your textbook]

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 finely 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.

Teacher's Note

The Samrat Yantra is a real example showing how shadow-based timekeeping can be extremely accurate: a 27-metre tall instrument can measure 2-second intervals. This tells you that even without modern electronics, if you understand the physics (the Sun's constant motion) and build carefully (precise angles and scale), you can achieve precision.

Key Points

• Ancient people used natural cycles like the rising and setting of the Sun to keep track of time and created the calendar and clocks.
• Early time-measuring devices used steady natural processes: sundials used the Sun's shadow, water clocks used flowing water, hourglasses used falling sand, and candle clocks used burning candles.
• A simple water clock can be made by allowing water to drip from a marked container at a constant rate, making marks on the receiving vessel at regular time intervals.
• The Samrat Yantra in Jaipur shows that shadow-based timekeeping can be very accurate when built with precision, measuring time intervals as short as 2 seconds.

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