1. What is ‘Measurement of Time’?

Measurement of time is the process of determining the duration between two events. In daily life we use clocks and watches to measure time. In science, precise instruments such as stopwatches and digital timers are often used to measure short intervals. The SI unit of time is the second (s). Larger units — minutes and hours — are commonly used for everyday activities.

2. Instruments to measure time

• Clock: Shows time of day (hours, minutes, seconds).
• Watch: Portable clock worn on wrist.
• Stopwatch / Timer: Measures time intervals precisely (used in experiments and sports).
• Calendar: Used for long-term measurement — days, months, years.

3. Motion — Basic ideas

Motion means change in position of an object with time. If an object changes its location relative to a reference point, it is said to be in motion. Motion can be described in simple words (fast, slow) or quantified using numbers (distance, time, speed).

• Rest: An object that does not change its position with time is at rest.
• Uniform motion: When an object covers equal distances in equal intervals of time, it is said to be in uniform motion.
• Non-uniform motion: When an object covers unequal distances in equal intervals of time (speed changes).

4. Distance vs Displacement (simple qualitative idea)

Distance is the total path length travelled by an object. It is always a positive scalar quantity. Displacement is the straight-line distance from starting point to final point in a particular direction (a vector). In Class 7, focus on distance as the quantity used in speed calculations; displacement may be introduced qualitatively.

5. Speed — the key quantity

Speed tells us how fast an object is moving. It is defined as the distance travelled per unit time.

Formula: speed = distance / time (s = d / t). Example units: m/s (metre per second), km/h (kilometre per hour).

Average speed

When motion involves changing speed, we use average speed = total distance travelled / total time taken. This gives a single value that describes the overall motion.

Instantaneous speed (simple idea)

Instantaneous speed is how fast an object is moving at a particular moment (like speedometer reading). This is introduced conceptually — for Class 7, calculations focus on average speed.

6. Unit conversions — practical tips

Because distance is measured in metres or kilometres and time in seconds or hours, converting units is a common task.

• To convert km/h → m/s: multiply by 5/18. (Example: 36 km/h = 36 × 5/18 = 10 m/s.)
• To convert m/s → km/h: multiply by 18/5. (Example: 5 m/s = 5 × 18/5 = 18 km/h.)

7. Worked examples (stepwise)

8. Example with changing speeds — average speed

9. Common problem types and solving strategy

Many questions in this chapter ask you to compute speed, distance or time given the other two. Use the basic formula and rearrange as needed:

• speed = distance / time → s = d / t
• distance = speed × time → d = s × t
• time = distance / speed → t = d / s

Strategy: (1) Convert all quantities to standard units (metres, seconds) if necessary. (2) Substitute into formula. (3) Carry units through to check answer.

10. Tips for CBSE exams — what teachers expect

11. Simple experiments you can try at home

• Use a stopwatch to time a friend walking a measured distance (say 20 m), then calculate speed = distance / time.
• Measure how long it takes water to flow a set distance in a narrow channel — relate flow speed to time measurements.
• Compare speeds for walking and running over the same distance and discuss uniform vs non-uniform motion.

12. Common mistakes to avoid

• Forgetting to convert minutes or hours into seconds when using m/s units.
• Mixing km and m without converting — ensure distance and time use compatible units.
• Using instantaneous and average speed interchangeably — read question carefully.

13. Practice questions (short)

Answers (try before checking): Q1: 45 km/h = 12.5 m/s. Q2: 6 km. Q3: 10 m/s.

14. Quick revision checklist

• Understand instruments for time measurement (clock, watch, stopwatch).
• Know units of time and conversions (s, min, h).
• Be clear about distance vs displacement (qualitatively).
• Master the formula s = d / t and its rearrangements.
• Practice unit conversions between km/h and m/s (multiply by 5/18 or 18/5).
• Show working and units clearly in exam answers.

15. Final summary

This chapter builds foundational understanding about how we measure time and how motion is described using distance, time and speed. The emphasis is on applying the simple formula speed = distance / time and performing unit conversions correctly. These ideas are essential for future physics topics (velocity, acceleration) — so a solid grasp of measurement and speed will make later concepts much easier.