8. Measurement of Time and Motion​

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Measurement of Time and Motion

Sundial • Water Clock • Pendulum • Time Period • SI Units • Speed • Uniform & Non-uniform Motion

🧩20 Most-Important Words (Meanings in Hindi)

WordSimple Meaning (in Hindi)
Sundialसूरज की छाया से समय बताने वाला यंत्र
Water clockपानी के बहाव/भराव से समय नापने वाला घड़ी जैसा यंत्र
Hourglassरेत के बहने से समय बताने वाला यंत्र
Candle clockमोमबत्ती के जलने व निशानों से समय बताने का तरीका
Simple pendulumधागे से टंगा छोटा गोला जो आगे-पीछे झूलता है
Bobलोलक (पेंडुलम) में नीचे लगा धातु का छोटा गोला
Oscillationएक सिरे से दूसरे सिरे तक जाकर वापस आने की एक पूरी झूला गति
Time periodएक पूरी दोलन (झूला) में लगने वाला समय
Stopwatchदौड़ या घटना का सटीक समय नापने की घड़ी
Quartz clockक्वार्ट्ज क्रिस्टल की कंपन से समय बताने वाली घड़ी
Atomic clockपरमाणु के सूक्ष्म कंपन से समय बताने वाली अत्यंत सटीक घड़ी
SI unit (second)समय की मानक इकाई — सेकंड (s)
Speedएक इकाई समय में तय की गई दूरी
Distanceकितना रास्ता तय किया गया
Uniform motionबराबर-बराबर समय में बराबर दूरी तय करना
Non-uniform motionबराबर समय में अलग-अलग दूरी तय करना
Average speedकुल दूरी ÷ कुल समय से निकली हुई गति
Speedometerवाहन की क्षणिक गति दिखाने वाला यंत्र
Odometerवाहन द्वारा तय कुल दूरी बताने वाला यंत्र
Calendarदिन-महीना-साल की गणना करने की तालिका
🗣️ Instruction followed: Only the meanings above are in Hindi; the rest of the notes are in English.

📚Detailed Notes

8.1 Measurement of Time

Natural cycles for timekeeping
  • Day–Night (Sunrise–Sunset), phases of the Moon, seasons were the earliest “clocks”.
  • Calendars came from repeating natural events; then humans needed devices for smaller intervals within a day.
Early devices
  • Sundial — reads time from the shadow’s position.
  • Water clocks — (a) outflow vessel with time markings; (b) floating bowl with a tiny hole that sinks in a fixed time.
  • Hourglass — sand flows from one bulb to another.
  • Candle clock — marked candle indicates time as it burns.
Make & Learn (Water Clock): Use a plastic bottle, make a tiny hole in the cap, invert the top on the bottom half, fill with coloured water, and mark levels every minute as it drips. When reused, each mark = 1 minute.
India Connect: Samrat Yantra (Jantar Mantar, Jaipur) is the world’s largest stone sundial (~27 m). Ancient texts also describe Ghatika-yantra (sinking bowl water clock).

From Pendulum to Modern Clocks

  • Pendulum clock (Huygens, inspired by Galileo): precise, based on a swinging pendulum’s constant time period.
  • Simple pendulum: a bob on a string. One oscillation = O→A→B→O (or A→B→A). Time period = time for one oscillation.
Activity (Time Period)
  1. Make a 100 cm pendulum; pull slightly and release (no push).
  2. Measure time for 10 oscillations; repeat and average.
  3. Time period = (time for 10 oscillations) ÷ 10.
Observation: For the same length and place, the time period stays nearly constant.
Explore like Galileo
  • Vary length → time period changes (longer → slower).
  • Vary mass → time period does not change appreciably.
  • Modern timekeeping: Quartz clocks (crystal vibrations) & Atomic clocks (atomic vibrations) — extremely accurate.

8.1.2 SI Unit of Time

  • SI unit: second (s). Larger: minute (min), hour (h).
  • 60 s = 1 min; 60 min = 1 h. Write a space between number & unit (e.g., 15 s, 2 h).

8.2 “Slow” or “Fast” — What does it mean?

  • If, in the same time, one runner covers more distance, they’re faster.
  • Thus, comparing distance in a unit time helps us judge who is fast/slow.

8.3 Speed

Speed = (Total Distance Covered) ÷ (Total Time Taken)
  • SI unit: m/s. Common unit: km/h.
  • Conversions: 1 m/s = 3.6 km/h, 1 km/h = 5/18 m/s.
Example: A 3.6 km trip in 15 min → speed?
Speed = (3.6×1000 m) ÷ (15×60 s) = 4 m/s.
Distance & Time from Speed
Distance = Speed × Time
Time = Distance ÷ Speed
Example: Bus @ 50 km/h for 2 h → 100 km.
Average speed: In real life, speed changes; we usually compute average speed using total distance ÷ total time.

8.4 Uniform & Non-uniform Linear Motion

  • Linear motion: motion along a straight line (e.g., a train between two stations on a straight track).
  • Uniform linear motion: equal distances in equal times (speed is constant).
  • Non-uniform linear motion: unequal distances in equal times (speed changes).
  • Vehicles show speed on a speedometer; distance via odometer.
Careful with units! Always convert distance/time to the same system before calculating (e.g., km with h, or m with s).

In a Nutshell

  • Pendulum’s time period is constant for a given length at a place; used in timekeeping.
  • SI unit of time: second (s).
  • Speed = distance ÷ time; units m/s or km/h (know conversions).
  • Uniform motion → constant speed; Non-uniform → changing speed.
  • Early timekeepers: sundial, water clock, hourglass, candle clock; modern: quartz & atomic clocks.

🧭Quick Reference

Time Units
60 s = 1 min 60 min = 1 h
Speed Units
m/s km/h
1 m/s = 3.6 km/h 1 km/h = 5/18 m/s
Key Formulas
v = d / t
d = v × t
t = d / v

Practice Set — Measurement of Time and Motion

One-word answers • Fill in the blanks • True/False • Very short answers • Short answers

1) One-Word Answer Questions (10)

  1. SI unit of time? Second (s)
  2. Instrument that shows the speed of a vehicle? Speedometer
  3. Instrument that measures total distance travelled by a vehicle? Odometer
  4. Time-keeping device that uses the Sun’s shadow? Sundial
  5. Time-keeping device using flowing/collecting water? Water clock
  6. Device that measures time by sand flow? Hourglass
  7. One complete to-and-fro swing of a pendulum is called? Oscillation
  8. Time taken for one oscillation is called? Time period
  9. Motion with equal distances in equal time intervals? Uniform motion
  10. SI unit of speed? m/s (metre per second)

2) Fill in the Blanks (10)

  1. Speed = ______ ÷ ______. distance; time
  2. Distance = Speed × ______. time
  3. Time = ______ ÷ Speed. distance
  4. 1 m/s = ______ km/h. 3.6
  5. 18 km/h = ______ m/s. 5
  6. The pendulum bob swings about its ______ position. mean
  7. The factor of a pendulum that affects its time period is its ______. length
  8. A clock based on atomic vibrations is called an ______ clock. atomic
  9. Vehicles display instantaneous speed using a ______. speedometer
  10. In non-uniform motion, equal time intervals correspond to ______ distances. unequal

3) True/False — with Explanation (10)

  1. False. The time period of a simple pendulum increases if the bob is heavier. Time period depends on length (and gravity), not on the bob’s mass.
  2. True. 1 m/s equals 3.6 km/h. Because 1 m/s = (1×3600) m per hour = 3600 m/h = 3.6 km/h.
  3. False. Sundials can measure time at night. They need sunlight to cast a shadow; night reading isn’t feasible.
  4. True. Average speed = total distance ÷ total time. This is used when speed varies during the journey.
  5. True. In uniform linear motion, the speed remains constant. Equal distances are covered in equal time intervals.
  6. False. An odometer shows instantaneous speed. A speedometer shows instantaneous speed; odometer shows total distance.
  7. True. Water clocks and hourglasses both rely on steady flow. Water/sand must flow at a roughly constant rate to track time.
  8. False. The SI unit of time is minute (min). The SI unit is second (s).
  9. False. If a runner covers more distance in the same time, they are slower. More distance in the same time means higher speed → faster.
  10. False. Atomic clocks are less accurate than old pendulum clocks. Atomic clocks are far more precise (lose ~1 s in millions of years).

4) Very Short Answer Questions (2–3 lines each) — (10)

  1. Define the time period of a simple pendulum.
    The time period is the time taken for one complete oscillation (to-and-fro swing). It can be measured by timing several oscillations and dividing by the count.
  2. Why do we use average speed in daily travel?
    Because our speed keeps changing due to traffic, signals, and stops. Average speed (total distance ÷ total time) summarizes the entire trip.
  3. How does a floating-bowl water clock work?
    A bowl with a tiny hole is floated on water. It fills at a steady rate and sinks after a fixed time, allowing time intervals to be counted.
  4. Differentiate speedometer and odometer.
    A speedometer shows the vehicle’s speed at that instant (e.g., 40 km/h). An odometer shows total distance covered (e.g., 58,120 km).
  5. What is uniform linear motion? Give one example.
    Motion along a straight line with constant speed. Example: a train cruising steadily on a straight track between stations.
  6. How does pendulum length affect time period?
    A longer length gives a larger time period (slower swing); a shorter length gives a smaller time period (faster swing).
  7. Convert 54 km/h to m/s.
    54 × (5/18) = 15 m/s.
  8. A 400 m race is completed in 50 s. Find speed.
    Speed = 400/50 = 8 m/s = 8 × 3.6 = 28.8 km/h.
  9. What is an oscillation?
    One full to-and-fro motion of a pendulum about its mean position (e.g., left extreme → right extreme → back to left).
  10. Why measure time in milliseconds in sports?
    Finishes can be extremely close; millisecond precision fairly distinguishes winners and records.

5) Short Answer Questions (3–4 lines each) — (10)

  1. Describe any two ancient time-keeping devices.
    Sundial: Uses the Sun’s shadow movement across a dial. Hourglass: Measures a fixed interval as sand flows from one bulb to another. Both rely on steady, repeating natural/physical processes.
  2. How do you measure the time period of a pendulum experimentally?
    Set up a pendulum of known length, displace slightly and release. Measure time for, say, 20 oscillations with a stopwatch. Time period = total time ÷ number of oscillations; repeat and average.
  3. A train covers 60 km in 1 h and 90 km in 1.5 h. Find average speed.
    Total distance = 150 km; total time = 2.5 h ⇒ average speed = 150/2.5 = 60 km/h. (Speeds may vary, so we use total distance ÷ total time.)
  4. Why is uniform motion an idealization? Give examples.
    Real speeds fluctuate due to roads, traffic, and stops. City driving is non-uniform; highway cruising approximates uniform motion but still changes with overtakes/gradients.
  5. A cyclist travels 3.6 km in 15 min. Compute speed in m/s.
    3.6 km = 3600 m; 15 min = 900 s ⇒ speed = 3600/900 = 4 m/s. (Same as 14.4 km/h.)
  6. What makes atomic clocks more accurate than mechanical clocks?
    They use the extremely stable natural frequency of atomic transitions, which hardly drift with temperature or wear, achieving accuracy of ~1 s in millions of years.
  7. A car moves 120 km in 2 h, halts 0.5 h, then 60 km in 1 h. Average speed?
    Total distance = 180 km; total time = 2 + 0.5 + 1 = 3.5 h ⇒ average speed = 180/3.5 = ≈ 51.43 km/h.
  8. Explain why the time period is “constant at a place” for a given pendulum length.
    For small swings, T depends mainly on length and local gravity (nearly fixed at a place). Minor variations arise from large amplitude, air resistance, or timing errors.
  9. Show how to convert between m/s and km/h.
    1 km = 1000 m; 1 h = 3600 s ⇒ 1 m/s = (1×3600)/1000 = 3.6 km/h. Conversely, 1 km/h = 1000/3600 = 5/18 m/s.
  10. Runner A does 100 m in 12 s; Runner B does 200 m in 24 s. Who is faster?
    A: 100/12 = 8.33 m/s; B: 200/24 = 8.33 m/s. Same speed; it’s a tie on speed (though distances differ).

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