Measurement of Time and Motion – Class 7 Science Chapter 8 Notes & NCERT PDF

Introduction

Prerna won the 100 metre sprint among girls at the district-level interschool competition and dreams of representing India. Watching Olympic reruns, she is amazed that timing is so precise that a winner can be found even when two sprinters seem to finish together. Her school uses a stopwatch; her mother wears a wristwatch; her sister checks a mobile phone; her uncle uses a Braille watch and a talking watch. She wonders: how was time measured when there were no clocks and watches?

Key Concepts

8.1 Measurement of time

People noticed events that repeat at definite intervals — sunrise and sunset, phases of the Moon, changing seasons — and used them to make calendars. To measure shorter intervals within a day they made:

The Samrat Yantra at Jantar Mantar, Jaipur, built about 300 years ago, is the world's largest stone sundial — 27 metres high; its shadow moves about 1 millimetre per second and can measure intervals as short as 2 seconds. Like any sundial, it shows local solar time, which needs correction to get Indian Standard Time.

In ancient India, the Arthasastra by Kautilya first referred to shadow-based time; Varahamihira (around 530 CE) gave an expression for time from a vertical stick's shadow. The sinking-bowl water clock, the Ghatika-yantra, was first mentioned by Aryabhata. Its bowl took 24 minutes to fill and sink — one ghatika (ghati), so a 24-hour day had 60 ghatis. Each sinking was announced with drums, conch shells or a gong.

Activity 8.1 builds a simple water clock: cut a plastic bottle in two, make a hole in the cap with a drawing pin, invert the top over the bottom, fill with coloured water and mark the level every minute.

Pendulums

Mechanical clocks driven by weights, gears and springs developed from the 14th century. The pendulum clock, invented in 1656 and patented in 1657 by Christiaan Huygens, was a breakthrough. He was inspired by Galileo Galilei, who, watching a swinging church lamp and using his pulse, found that each swing took the same time.

Simple pendulum: a small metal ball (the bob) hung from a rigid support by a long thread.

Time period: the time taken by a pendulum to complete one oscillation.

Activity 8.2: with a string length of about 100 cm, time 10 oscillations and divide by 10. For example, if 10 oscillations take 20 s, the time period is 20 ÷ 10 = 2 s. Repeated readings give almost the same value.

The time period of a simple pendulum depends on its length but not on the mass of the bob. Pendulums of the same length have the same time period at a given place.

All clocks use some process that repeats continuously. Modern quartz clocks use vibrations of a quartz crystal and atomic clocks use vibrations of atoms. Huygens' clocks could gain or lose 10 seconds a day; atomic clocks lose only one second in millions of years.

8.1.2 SI unit of time

Today, sports timers measure hundredths or thousandths of a second (a millisecond); ECG machines detect millisecond variations in heartbeats; computers work in microseconds.

8.2 and 8.3 Slow or fast — speed

In a race over the same distance, the runner who covers more distance in the same time is faster.

Speed = Total distance covered ÷ Total time taken

8.4 Uniform and non-uniform linear motion

Motion along a straight line is linear motion. A train leaving station A speeds up, runs at constant speed from B to C, then slows to stop at D.

Worked Examples (from the chapter)

Activities in the Chapter

Key Terms

Real-life Connections

Common Misconceptions

Keep the Curiosity Alive (exercise hints)