CLASS 7 SCIENCE · CHAPTER 8 · STUDY NOTES
Measurement of Time and Motion
Count complete swings, keep units consistent and solve speed questions one clear step at a time.
01 / EXPLORE & UNDERSTAND
Repeated events help us measure time
A clock turns a repeatable process into a count of time.
1. Begin with natural cycles
Day and night, the Moon’s phases and seasons helped people organise calendars. To measure smaller intervals, people developed clocks. Time of day tells when something happens; an interval tells how long it lasts.
2. Choose a suitable clock
A stopwatch measures elapsed time. A wall clock gives the time of day. Talking and Braille watches make time accessible in different ways. Measuring a close race needs finer intervals than planning a school day.
Your turn
Question
Is “the lesson starts at 9:00” a duration?
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02 / EXPLORE & UNDERSTAND
Shadows, water, sand and candles
Each old clock needs a process that can be calibrated.
A moving shadow marks time
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1. Follow what changes
A sundial uses a changing shadow. A water clock uses flowing water or a bowl that fills and sinks. An hourglass measures a set interval as sand falls. A candle clock uses marks along a burning candle. These are different mechanisms, not equally accurate clocks.
2. Notice the limitations
A sundial needs sunlight and normally shows local solar time. In an outflow water clock, falling water level can change the flow rate. Therefore equal time intervals need not give equally spaced height marks.
A little deeper: Clocks also have a history
NCERT describes India’s Ghatika-yantra and Jaipur’s Samrat Yantra. It also traces mechanical clocks and Huygens’ pendulum clock. These examples show different ways people improved repeatability; a historical device’s local time is not automatically Indian Standard Time.
Your turn
Question
Why should you mark a homemade water clock using a working watch rather than equal ruler gaps?
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03 / EXPLORE & UNDERSTAND
One complete swing returns in the same direction
Going from one side to the other is only half an oscillation.
Identify a complete oscillation
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1. Locate the parts
A simple pendulum has a bob suspended by a light thread from a fixed support. The middle resting position is the mean position. The turning points on the two sides are extreme positions. For a small swing, the motion repeats approximately regularly.
2. Count a full journey
Start at the left extreme: left → right → left is one oscillation. Starting at the centre also works, but return to the centre moving in the same direction. Passing through the centre once is not necessarily a full cycle.
Your turn
Question
A bob goes left to right and stops there. How much of an oscillation is that?
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04 / EXPLORE & UNDERSTAND
Time several swings, then divide
A longer timing interval reduces the relative effect of reaction delay.
1. Use a clear count
Release a small swing gently, without a push. Begin timing at a chosen position and count complete cycles. Repeat the measurement. Start and finish must refer to the same stage of the cycle.
2. Work through an example
Suppose 12 complete oscillations take 24 s. Time per oscillation = 24 s ÷ 12 = 2 s. This is an illustrative reading, not a result that every pendulum must give.
Your turn
Question
Ten complete oscillations take 18 s. Find the period.
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05 / EXPLORE & UNDERSTAND
A longer pendulum swings more slowly
Change one factor at a time when comparing pendulums.
1. Compare lengths fairly
At the same place, a longer simple pendulum has a longer period for small swings. Keep the release angle small and similar. Length is measured from the suspension point to the bob’s centre, not just the visible thread.
2. Do not confuse mass with length
Changing only the bob’s mass does not change the ideal small-swing period. In real trials, timing errors, friction and air resistance cause small differences. Compare repeated readings instead of treating a tiny difference as proof.
A little deeper: Why “small swings” matters
The near-constant period is an approximation for small angles. Very large release angles take slightly longer. A clock must also replace energy lost to friction so that its pendulum keeps swinging.
Your turn
Question
Two pendulums have equal lengths at the same place but different bob masses. What do you expect?
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06 / EXPLORE & UNDERSTAND
Keep the time units consistent
Seconds, minutes and hours are related by factors of 60.
1. Convert before calculating
1 min = 60 s and 1 h = 3,600 s. For example, 2 min 30 s = 120 s + 30 s = 150 s. Do not write 2.30 minutes for 2 minutes 30 seconds: 0.30 min is only 18 s.
2. Read the instrument’s scale
Write 20 s, with a space and no plural on the symbol. A stopwatch displaying hundredths can show finer intervals than a clock with only second marks, but the person’s reaction time still affects manual measurements.
Your turn
Question
Convert 3 min 15 s into seconds.
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07 / EXPLORE & UNDERSTAND
Compare distance per unit time
Neither distance alone nor time alone tells the whole story.
1. Compare like with like
For equal distances, the shorter time means the higher average speed. For equal times, the longer distance means the higher average speed. Someone ahead in a race has covered more distance so far, but could be slowing down at that moment.
2. Calculate the rate
A toy travels 18 m in 6 s. Average speed = 18 ÷ 6 = 3 m/s. This means an average of 3 metres each second; it does not prove that every second was identical.
Your turn
Question
A travels 24 m in 8 s; B travels 20 m in 5 s. Who has the higher average speed?
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08 / EXPLORE & UNDERSTAND
Match distance and time units
m/s needs metres and seconds; km/h needs kilometres and hours.
1. Convert the inputs
A cyclist covers 1.8 km in 6 min. Convert 1.8 km to 1,800 m and 6 min to 360 s. Then 1,800 ÷ 360 = 5 m/s. Dividing 1.8 by 6 directly would give km/min, not m/s.
2. Check using another unit
6 min = 0.1 h, so the same journey gives 1.8 ÷ 0.1 = 18 km/h. Therefore 5 m/s = 18 km/h. To convert m/s to km/h, multiply by 3.6; reverse by dividing by 3.6.
Your turn
Question
Convert 8 m/s into km/h.
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09 / EXPLORE & UNDERSTAND
Use speed to find distance or time
First decide what is given and what you need.
1. Find a distance
A model vehicle moves steadily at 2 m/s for 7 s. It covers 2 × 7 = 14 m. The seconds cancel between m/s and s. The answer must be a length, not m/s.
2. Find a time
At a steady 3 m/s, travelling 21 m takes 21 ÷ 3 = 7 s. Time = distance ÷ speed. The formula needs a non-zero speed; an object that stays still cannot cover a positive distance.
Your turn
Question
A steady 4 m/s motion lasts 9 s. Find the distance.
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10 / EXPLORE & UNDERSTAND
Include the whole journey, including stops
Average speed is a total-to-total calculation.
Speed now versus distance so far
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1. Add before dividing
A walker covers 120 m in 60 s, rests for 30 s, then covers 90 m in 30 s. Total distance = 210 m. Total time = 60 + 30 + 30 = 120 s. Average speed = 210 ÷ 120 = 1.75 m/s.
2. Read the correct instrument
A speedometer shows the speed at that moment. An odometer records accumulated distance. Subtract starting from ending odometer readings to get journey distance; divide by elapsed time for average speed.
Your turn
Question
Why is averaging just the two moving speeds wrong in the walking example?
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11 / EXPLORE & UNDERSTAND
Equal time gaps reveal the motion
Compare distances travelled during each interval, not just final totals.
Same finish, different journeys
Move through equal one-second intervals. Each dot records a position.
A · equal gaps
0 m at 0 s
B · growing gaps
0 m at 0 s
Both finish at 6 m after 3 s: average speed = 2 m/s. Only A has equal gaps at these observations.
1. Inspect equal intervals
At 0, 1, 2 and 3 s, one object is at 0, 2, 4 and 6 m along a straight track. It covers 2 m in each second, consistent with uniform motion. Assume it does not speed up and slow down between the observations.
2. Contrast changing gaps
Another object is at 0, 1, 3 and 6 m at those same times. Its interval distances are 1, 2 and 3 m: the motion is non-uniform. Both reach 6 m in 3 s, so both still have the same overall average speed of 2 m/s.
A little deeper: A model, not a perfect description of traffic
Uniform motion is a useful idealisation. A real vehicle often starts, slows, turns and stops. Short observations can suggest a pattern, but they do not prove that speed stayed constant at every instant.
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Question
Can equal overall average speeds hide different motion patterns?
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Sign inNew here? Sign up freeCHAPTER RECAP
Take these ideas with you
- A full oscillation returns to the same position and direction.
- Period = total time ÷ number of complete oscillations.
- Use seconds (s), minutes (min) and hours (h) consistently.
- Average speed = total distance ÷ total elapsed time.
- Distance = speed × time; time = distance ÷ speed.
- Uniform motion covers equal distances in equal times along a straight line.
Use your understanding in a new situation
Question
A toy moves 30 m in 10 s, then waits 5 s. Find its average speed over all 15 s.
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Sign inNew here? Sign up freeQuestion
Why does timing 15 oscillations usually give a better period estimate than timing one?
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Sign inNew here? Sign up freeNCERT · Chapter source PDF, pages 12, 13 · Original Learnijoy explanations and practice.