Measurement of Time

Class 7 Science · Chapter 8: Measurement of Time and Motion · NCERT Curiosity section 8.1

Learn

Meher has won her school's 100 m race and wants to run for her state. At the stadium, electronic timers split hundredths of a second, but at school the coach uses a stopwatch. "How did people measure time before any clocks?" she wonders on the bus home, glancing at her phone, her grandmother's wristwatch and her uncle's talking watch.

People noticed that nature repeats: sunrise and sunset, the Moon's phases, the seasons. These gave days and calendars. To divide a day, they built sundials (a moving shadow), water clocks (water flowing out of, or into, a vessel), hourglasses (falling sand) and candle clocks (marked candles). The giant Samrat Yantra at Jaipur's Jantar Mantar, 27 m tall, can read time to about 2 seconds!

In ancient India, the Ghatika-yantra was a bowl with a tiny hole floating on water; it filled and sank in 24 minutes (one ghati), so a day had 60 ghatis. Each sinking was announced with a drum or gong. Later came gears and springs, and in 1656 the pendulum clock of Huygens, inspired by Galileo, who timed a swinging lamp with his pulse.

A simple pendulum is a small bob hanging from a rigid support by a thread. From the mean position O it swings to one extreme A, across to the other extreme B, and back to O: one oscillation. The time for one oscillation is the time period. Time 10 oscillations and divide by 10. For a given length the period stays the same; it depends on length, not the bob's mass.

All clocks use something that repeats at equal intervals: quartz watches use tiny rapid vibrations of a crystal, and atomic clocks use atoms and lose only about one second in millions of years. The SI unit of time is the second (s); 60 s = 1 min and 60 min = 1 h. Write "15 min" or "2 h", never "15 mins." or "2 hrs".

Explore: what sets a pendulum's period?

1. Time each pendulum and sort it.

Explore: the story of clocks.

1. Put the steps in order.

Remember

• Early clocks used shadows, water, sand and candles; the Ghatika-yantra measured ghatis of 24 minutes.
• A pendulum's time period depends on its length, not its bob; the SI unit of time is the second.

Quick check

A pendulum makes 10 oscillations in 15 s. What is its time period?

1. 1.5 s
2. 15 s
3. 10 s
4. 150 s

Think about it

A swing in the park has the same time period whether a small child or an adult sits on it. Why?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.