NCERT Curiosity · Class 7 · Science
Measurement of Time and Motion
Prerna's school uses a simple stopwatch, but Olympic races need ultra-precise timing. How did people measure time before clocks existed at all?
Key idea
Ancient Ways to Measure Time
1
Sundial
Tells time from the position of a shadow.
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2
Water clock
Measures time by water flowing out, or a bowl sinking.
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3
Hourglass
Measures time by sand flowing between two bulbs.
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4
Candle clock
Measures time by how far a candle has burned down.
Key idea
The Pendulum Clock, and Beyond
1
The 17th-century leap
Christiaan Huygens invented the pendulum clock, inspired by Galileo's studies of swinging pendulums — a major breakthrough in mechanical timekeeping.
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2
Modern precision
Quartz clocks use vibrations from a quartz crystal; atomic clocks use vibrations of specific atoms — both are far more precise than any pendulum.
Key idea
The Simple Pendulum
Time period. One oscillation is a full swing: mean → A → mean → B → mean. The time this takes is the time period — it depends only on the pendulum's length, not the bob's mass.
Key idea
The SI Unit of Time
The SI unit of time is the second (s). 60 seconds make a minute; 60 minutes make an hour.
Why precision mattersMilliseconds and microseconds matter in sports timing, medical devices like ECGs, music recording, and computing.
Key idea
Speed
Speed tells us how fast or slow something is moving.
The SI unit of speed is metre/second (m/s); it's also commonly expressed in kilometre/hour (km/h).
Key idea
Average Speed
1
Why "average"?
Objects rarely move at exactly one speed the whole way — the speed we calculate for a full journey is usually an average.
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2
Measuring instruments
A speedometer shows a vehicle's speed; an odometer shows the total distance it has travelled.
Key idea
Uniform and Non-uniform Linear Motion
1
Uniform linear motion
Constant speed along a straight line — equal distances covered in equal time intervals.
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2
Non-uniform linear motion
Changing speed along a straight line — unequal distances covered in equal time intervals.
Key idea
Is This Motion Uniform?
If distance increases by the same amount in every equal time interval, the motion is uniform.
Time (s) | 0 | 10 | 20 | 30 | 40
Distance (m)0 — 8 — 16 — 24 — 32
Chapter · Key terms to remember
Key Terms
Time period
Time for one full oscillation of a pendulum.
Oscillatory motion
Motion repeating back and forth.
Second (s)
The SI unit of time.
Chapter · Key terms to remember
More Key Terms
Speed
Distance covered per unit time.
Average speed
Total distance ÷ total time.
Speedometer / Odometer
Measures current speed / total distance.
Chapter · Key terms to remember
More Key Terms
Uniform linear motion
Constant speed, equal distances each equal time.
Non-uniform linear motion
Changing speed, unequal distances each equal time.
Questions for your notebook
Write these down, then discuss
1A car travels 150 metres in 10 seconds. Calculate its speed, and express your answer in km/h.
2Runner 1 covers 400 m in 50 s. Runner 2 covers the same distance in 45 s. Who is faster, and by how much?
3A train travels at 25 m/s and covers 360 km. How much time does it take?
Project as-is — students copy the questions, then the class discusses answers together.