Measurement of Time and Motion – Long Answer Type Questions
Class 7
Science
Chapter 8
Measurement of Time and Motion — 30 Long Answer Questions & Answers
Concise, exam-focused long-answer Q&A strictly aligned with NCERT syllabus for CBSE Class 7. Use for study, tests and board exam preparation.
Content Bank — Key formulas & units
- Speed = Distance / Time (s = d / t)
- Distance = Speed × Time (d = s × t)
- Time = Distance / Speed (t = d / s)
- Unit conversions: 1 min = 60 s; 1 h = 3600 s; km/h → m/s multiply by 5/18.
1. Explain what is meant by 'measurement of time' and name instruments used for timekeeping.
Measurement of time refers to determining the duration between two events or the value of a clock reading at an instant. Instruments include analogue and digital clocks for time of day, watches for portability, stopwatches and timers for short intervals, and calendars for long periods (days, months, years). In science, stopwatches and digital timers are preferred for precision; the SI unit is the second (s).
2. Define motion with suitable examples and explain the difference between rest and motion.
Motion is the change in position of an object with respect to a reference point over time. For example, a car moving along a road or a planet orbiting the Sun are in motion. Rest refers to an object not changing its position—e.g., a parked bicycle. The key difference: in motion position changes with time; at rest it does not. Motion is described using distance, time and speed.
3. What is uniform motion? Give an example and contrast it with non-uniform motion.
Uniform motion occurs when an object covers equal distances in equal intervals of time—its speed remains constant. Example: a toy car moving at 2 m/s on a straight track (ideal case). Non-uniform motion means speed changes over time—for example, a car speeding up and slowing down in city traffic. Uniform motion has constant average and instantaneous speed; non-uniform motion's speeds vary and require average speed calculations over intervals.
4. Explain the terms 'distance' and 'displacement' qualitatively and mention how they differ.
Distance is the total path length travelled by an object and is a scalar (only magnitude). Displacement is the straight-line measure from initial to final position along a specific direction and is a vector. For example, walking 3 km east then 2 km west gives distance 5 km but displacement 1 km east. In Class 7 the emphasis is on distance for speed calculations; displacement is introduced qualitatively to show directional differences.
5. State and explain the formula for speed. Include units used.
Speed is defined as the distance travelled per unit time. Mathematically: speed = distance / time (s = d / t). If distance is in metres (m) and time in seconds (s), speed is m/s. If distance is in kilometres (km) and time in hours (h), speed is km/h. Units must be consistent when applying the formula; convert units where necessary (e.g., km/h to m/s by multiplying by 5/18).
6. A bus travels 150 km in 3 hours. Calculate its speed and explain the steps.
Use speed = distance / time. Distance = 150 km, time = 3 h. Speed = 150 / 3 = 50 km/h. Steps: (1) Identify given values, (2) apply formula, (3) compute result and state units. If required in m/s, convert: 50 × 5/18 ≈ 13.89 m/s. Always show working and units.
7. How would you convert 36 km/h to m/s? Explain why conversions are necessary.
To convert km/h to m/s multiply by 5/18 because 1 km = 1000 m and 1 h = 3600 s so 1 km/h = 1000/3600 = 5/18 m/s. Thus 36 × 5/18 = 10 m/s. Conversions are necessary to keep units consistent when applying formulas or comparing quantities measured in different units.
8. Describe how a stopwatch can be used in a simple experiment to find the speed of a walking person.
Measure a known straight distance (e.g., 20 m) and mark start and end points. Use a stopwatch to time how long the person takes to walk between marks. Record time t in seconds. Compute speed = distance/time (s = 20 / t m/s). Repeat trials and average times for accuracy. Ensure the walker maintains steady pace to approximate uniform motion. Show units and working.
9. Explain the idea of average speed when a body moves at varying speeds.
Average speed = total distance travelled divided by total time taken. For motion with varying speeds, average speed gives a single overall value. Example: if a car covers 60 km in 1 h and 120 km in 2 h, total distance 180 km, total time 3 h; average speed = 180/3 = 60 km/h. Average speed depends on total quantities, not on intermediate speed values.
10. A cyclist covers 15 km in 30 minutes. Find speed in km/h and m/s and show steps.
Time = 30 min = 0.5 h. Speed in km/h = distance/time = 15 / 0.5 = 30 km/h. Convert to m/s: 30 × 5/18 = 30 × 0.2778 ≈ 8.33 m/s. Steps: convert units, apply formula, convert result if needed. Include units in final answer.
11. What is instantaneous speed and how does it differ from average speed? (Simple explanation)
Instantaneous speed is the speed of an object at a particular instant (e.g., speedometer reading). Average speed is computed over a time interval as total distance divided by total time. Instantaneous speed may vary from moment to moment, while average smooths these variations to give an overall measure. Class 7 focuses mainly on average speed; instantaneous speed is introduced conceptually.
12. Explain why mixing units like km and metres without conversion leads to wrong answers, with an example.
Formulas require consistent units. For example, distance 2 km and time 30 s; directly computing speed as 2/30 gives km/s, not m/s or km/h. To get m/s, convert 2 km to 2000 m first: speed = 2000/30 ≈ 66.67 m/s. Mixing units changes numerical factors and yields incorrect magnitudes. Always convert to compatible units before calculation.
13. A car travels 40 m in 4 s, then 60 m in 6 s. Calculate its average speed over the whole journey.
Total distance = 40 + 60 = 100 m. Total time = 4 + 6 = 10 s. Average speed = 100/10 = 10 m/s. Steps: add distances and times, then divide for average. Note that instantaneous speeds in segments were 10 m/s and 10 m/s here, so motion is uniform in this example.
14. How many metres are there in 5.5 km? Show work.
1 km = 1000 m; therefore 5.5 km = 5.5 × 1000 = 5500 m. Conversion uses basic metric prefix rule; show multiplication and units.
15. Discuss the importance of clocks and calendars in everyday life and science.
Clocks allow scheduling, coordination and measurement of short time intervals (appointments, experiments). Calendars help plan long-term events (holidays, agriculture). In science, accurate time measurement is crucial for experiments, motion studies and data recording. Together they structure human activity and enable comparisons, repeatability and synchronization across systems. Precise timing underpins technology, transport, communication and scientific measurements.
16. A runner's average speed is 8 m/s. How long will she take to cover 400 m? Explain steps.
Use time = distance / speed. Time = 400 / 8 = 50 s. Steps: identify distance (400 m) and speed (8 m/s), apply formula t = d / s, compute and state units.
17. Why is the second chosen as the SI unit of time? (Brief reasoning)
The second was chosen historically and standardized based on Earth's rotation and later atomic transitions because it is a stable, reproducible interval suitable for scientific precision. Modern definition uses the frequency of radiation from caesium atoms, ensuring high accuracy. The second's small size makes it practical for measuring a wide range of intervals in experiments and daily use.
18. Explain with an example how distance and time measurements together describe motion fully.
Distance tells how far an object moved; time tells how long it took. For example, if a bus covers 120 km in 2 hours, combined information gives speed 60 km/h, indicating how quickly the bus moved. Distance without time does not indicate rate; time without distance does not show how much ground was covered. Both are necessary to compute speed and understand motion quantitatively.
19. A person travels 3 km north, then 4 km north. What is the total distance and what is the displacement? (Simple explanation)
Total distance = 3 + 4 = 7 km. Displacement = straight-line distance from start to end (both in same direction north), so 7 km north. If directions had differed, displacement would differ from distance. This example shows when distance and displacement are equal.
20. How would you find the average speed of a vehicle that stops for 20 minutes during a journey? Include stepwise plan.
Plan: (1) Compute total distance travelled. (2) Compute total time including stop duration (driving time + 20 min). (3) Convert time to hours or seconds consistently. (4) Average speed = total distance / total time. Stopping increases total time and reduces average speed; include stop duration in total time for correct average.
21. What are common mistakes students make when solving time and speed problems and how to avoid them?
Common mistakes: mixing units (km with m or h with s), forgetting unit conversion, not including rest times in average speed, and incorrect rearrangement of formulas. Avoid by writing units beside values, convert units before calculation, list knowns and unknowns, and perform dimensional checks of results. Practice sample problems to build correct habits.
22. A car travels at 72 km/h. Express this speed in m/s and show the calculation.
Multiply by 5/18: 72 × 5/18 = 72 × 0.2778 = 20 m/s. Show steps: 72 km/h = 72 × (1000/3600) m/s = 72 × 5/18 m/s = 20 m/s. Include units in final answer.
23. How can a graph of distance vs time help describe motion qualitatively? (Short explanation)
A distance-time graph shows how distance changes with time. A straight line with constant slope indicates uniform motion (constant speed). A curved line shows changing speed (non-uniform motion). Horizontal segments indicate rest (distance not changing). Although detailed graph plotting is usually taught later, Class 7 students benefit from qualitative interpretation.
24. A vehicle travels 90 km at 30 km/h and 60 km at 60 km/h. Compute total time and overall average speed.
Time for 90 km = 90/30 = 3 h. Time for 60 km = 60/60 = 1 h. Total distance = 150 km. Total time = 4 h. Average speed = 150/4 = 37.5 km/h. Show stepwise computation and units.
25. Why is it important to repeat timing experiments and take the average? Explain briefly.
Repeating reduces random error and increases reliability. Human reaction time, slight pace changes, and measurement inaccuracies can affect single-trial results. Averaging multiple trials smooths out anomalies and gives a more accurate estimate of true time or speed. This practice improves precision and is standard in experimental work.
26. A swimming pool lap is 50 m. A swimmer completes it in 40 s. Calculate her speed and suggest how to improve timing accuracy.
Speed = distance / time = 50 / 40 = 1.25 m/s. To improve timing accuracy: use a digital stopwatch, have multiple trials, ensure consistent start/stop method (e.g., electronic touchpads), and reduce distractions. Report average time from several runs.
27. Explain qualitatively how time measurement is necessary for understanding other physical quantities later (like velocity and acceleration).
Time measurement is fundamental: velocity refines speed by including direction (requires time), and acceleration measures change of velocity per unit time. Precise timing allows calculation of rates of change, which are central to dynamics in higher classes. Without accurate time measurement we cannot quantify how quickly motion changes. Thus mastering time and speed is foundational for future physics topics.
28. Describe a classroom activity to demonstrate uniform and non-uniform motion.
Mark equal segments on the floor (e.g., 1 m intervals). Have a student walk at constant pace across segments while another records time per segment with a stopwatch—equal times indicate uniform motion. Then ask the student to vary pace; recorded times will differ showing non-uniform motion. Discuss results and compute average speeds. This hands-on activity reinforces definitions.
29. A bus travels 240 km in 4 hours including a 20-minute break. What is its average speed? Show steps.
Total time = 4 hours (including break) — if 4 h already includes break, use 4 h directly. Average speed = distance/time = 240/4 = 60 km/h. If break was extra, add 20 min = 1/3 h to driving time accordingly. Always clarify whether time given includes stops before computing.
30. Summarise the chapter in your own words highlighting why these concepts matter for further study in physics.
This chapter introduces time measurement and the description of motion using distance and speed. Students learn instruments for timekeeping, unit conversions, and how to compute speed, distance and time. Mastery of these concepts allows quantitative study of motion and prepares students for velocity, acceleration and dynamics in higher classes. Accurate measurement and clear units form the backbone of experimental science.