The big picture
A clock counts a repeat. Speed is distance divided by time.
| Idea | Remember | Example |
|---|---|---|
| Repeats | A clock counts a repeat with equal gaps. | The Sun, a pendulum |
| Units of time | 60 s = 1 min. 60 min = 1 h. | 2 h = 7200 s |
| Pendulum | Time period depends on length, not on bob mass. | 10 oscillations ÷ 10 |
| Speed | Speed = distance ÷ time. | 600 m in 75 s = 8 m/s |
| Units of speed | 1 m/s = 3.6 km/h. | 90 km/h = 25 m/s |
| Uniform motion | Equal distances in equal times. | 30, 30, 30, 30 m |
| Average speed | Total distance ÷ total time. | 150 km in 3 h = 50 km/h |
| Old clocks | A sundial uses a moving shadow, so it needs sunlight. A water clock uses flowing water. An hourglass uses falling sand. A candle clock uses a burning candle. | Samrat Yantra, Jaipur |
| Water clock marks | A draining jar loses water fast when it is full. It loses water slowly when it is nearly empty. So in a straight jar, marks for equal minutes are spaced unevenly. | Marks come closer near the bottom |
| How exact | A good clock gains or loses very little time. A quartz clock counts the vibrations of a crystal. An atomic clock drifts by about one second in millions of years. | Pendulum: 10 s a day. Quartz: up to 15 s a month. |
| Smallest tick | A clock cannot show a gap smaller than its smallest tick. To tell close times apart, use a smaller tick. | 12.46 s against 12.49 s |
| Timetable | Time taken is the arrival time minus the leaving time. Speed is the distance divided by that time. This is the average speed. | 09:00 to 10:30, 90 km = 60 km/h |
Now read Chapter 8, Measurement of Time and Motion, in NCERT. You have timed a pendulum and compared speeds. The chapter will feel familiar.