NCERT Solutions for Class 7th Science Chapter 8 Measurement of Time and Motion
Updated on 2026-09-19
About this chapter
People first kept time by watching events in nature that repeat after definite intervals — the rising and setting of the Sun, the phases of the Moon, the changing seasons. From these they made calendars, and then devices for smaller intervals within a day: sundials , water clocks , hourglasses and candle clocks . India has a rich record of timekeeping. Shadow-based measurement appears in Kautilya's Arthasastra , and Varahamihira gave an accurate expression around 530 CE. The sinking-bowl water clock or Ghatika-yantra , first mentioned by Aryabhata, took 24 minutes to fill and sink — so a day was divided into 60 ghatis . The Samrat Yantra at Jantar Mantar, Jaipur, is 27 m tall and can measure intervals as short as 2 seconds . A simple pendulum is a small metallic ball (the bob ) hung from a
- .1 Measurement of Time
- .1.1 A simple pendulum
- .1.2 SI unit of time
- .2 Slow or Fast · 8.3 Speed
- .3 Speed
- .4 Uniform and Non-uniform Linear Motion
- Chapter exercises
- Interdisciplinary projects
- Interdisciplinary task
Quick revision
| Term | What it means | Example / detail from the chapter | Point to remember |
|---|---|---|---|
| Sundial | A device in which time is read from the changing position of the shadow of an object cast by sunlight | The Samrat Yantra at Jantar Mantar, Jaipur — 27 m tall, shadow moves about 1 mm per second | It shows local or 'solar time', so a correction is needed to get Indian Standard Time |
| Water clock | A clock that uses the flow of water out of, or into, a vessel to measure time | The out-flow type (Fig. 8.2a) and the floating-bowl type or Ghatika-yantra (Fig. 8.2b) | The out-flow type slows down as the water level drops — that is why the sinking bowl was developed |
| Ghatika (ghati) | The time unit measured by one sinking of the bowl of the Ghatika-yantra | The hole was made so that the bowl took 24 minutes to fill and sink | 24 h ÷ 24 min = 60, so a day was divided into 60 equal ghatis |
| Simple pendulum | A small metallic ball (bob) suspended from a rigid support by a long thread | Fig. 8.7a — rigid support, long thread, bob | At rest the bob hangs at the mean position O |
| Oscillation | One complete to-and-fro journey of the bob: O → A → B → O (or A → B → A) | Fig. 8.7b, with A and B the two extreme positions | Going only from O to A and back to O is half an oscillation |
| Time period | The time taken by a pendulum to complete one oscillation | Measured in Activity 8.2 as (time for 10 oscillations) ÷ 10 | For a given length it is constant at a place; it does not depend on the bob's mass |
| Periodic motion | Motion that repeats its path after a fixed interval of time | The oscillatory motion of a pendulum | Every clock — old or modern — is built on some process that repeats |
| Second (s) | The SI unit of time | 60 s = 1 min; 60 min = 1 h | Write '5 s', not '5 sec.'; symbols stay lower case and singular |
| Speed | The distance covered by an object in unit time | Speed = total distance covered ÷ total time taken | SI unit m/s; also km/h. 1 m/s = 3.6 km/h |
| Average speed | Total distance covered divided by the total time taken for the whole journey | A car covering 60 km, 70 km and 50 km in three successive hours has an average speed of 60 km/h | It is not the average of the separate speeds |
| Uniform linear motion | Motion along a straight line with a constant (unchanging) speed | Train X in Table 8.3 covers 20 km in every 10-minute interval | Equal distances in equal intervals of time — a straight-line distance–time graph |
| Non-uniform linear motion | Motion along a straight line in which the speed keeps changing | Train Y in Table 8.3 covers 20, 15, 15, 25, 20 and 25 km in successive 10-minute intervals | Unequal distances in equal intervals of time — a bent distance–time graph |
| Speedometer / Odometer | Instruments fitted in vehicles: the speedometer displays the speed in km/h, the odometer measures the distance travelled in kilometre | Seen on scooters, motorbikes, cars and buses | Speedometer → speed; odometer → distance |
Exercises
- .1 Measurement of Time — In-text Question Page 1068
- .1 Measurement of Time — Activity 8.1: Let us construct Page 1078
- .1.1 A simple pendulum — In-text Question Page 1098
- .1.1 A simple pendulum — Activity 8.2: Let us experiment Page 109 & 1108
- .1.1 A simple pendulum — Think Like a Scientist! Page 1108
- .1.2 SI unit of time — Activity 8.3: Let us identify Page 1128
- .2 Slow or Fast · 8.3 Speed — In-text Questions Page 112 & 1138
- .3 Speed — Activity 8.4: Let us calculate Page 114 & 1158
- .4 Uniform and Non-uniform Linear Motion — In-text Questions Page 1168
- .4 Uniform and Non-uniform Linear Motion — In-text Question Page 1178
- Chapter exercises — Let Us Enhance Our Learning Page 118 & 119
- Interdisciplinary projects — Exploratory Projects Page 119 & 120
- Interdisciplinary task — Holistic Lens Page 120