Motion and Time - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter connects the concepts of motion and time, introducing speed, types of motion, and time measurement devices.
1. Why this chapter matters
Motion is all around us -- cars moving, birds flying, the Earth revolving around the Sun. Understanding how to measure speed and time is fundamental to physics. In CBSE exams, this chapter contributes 6-8 marks with a focus on calculations and graph interpretation.
2. Types of motion
Uniform motion
An object moves with uniform motion if it covers equal distances in equal intervals of time, regardless of how small the time interval is.
Example: A car moving at a constant speed of 40 km/h on a straight road.
Non-uniform motion
An object moves with non-uniform motion if it covers unequal distances in equal intervals of time.
Example: A car moving through city traffic, stopping and starting.
Other types of motion
- Rectilinear motion: Motion along a straight line.
- Circular motion: Motion along a circular path (e.g., clock hands, merry-go-round).
- Periodic motion: Motion that repeats after a fixed interval of time (e.g., pendulum, heartbeats).
- Oscillatory motion: To-and-fro motion (e.g., swing, pendulum).
3. Speed
Speed is the distance covered by an object in unit time.
Speed = Distance / Time.
Units of speed
- km/h (kilometres per hour) for vehicles.
- m/s (metres per second) in scientific calculations.
Conversion between km/h and m/s
To convert km/h to m/s: multiply by 5/18. To convert m/s to km/h: multiply by 18/5.
Example: 72 km/h = 72 x (5/18) = 20 m/s.
4. Measurement of time
Historical methods
Early humans used natural phenomena: sunrise, sunset, seasons, phases of the moon.
Sundial
A sundial uses the position of the Sun's shadow to tell time. The length and direction of the shadow changes during the day.
Modern devices
- Quartz clock: Uses vibrations of a quartz crystal.
- Atomic clock: Uses vibrations of atoms (most accurate; error of 1 second in millions of years).
5. The simple pendulum
A simple pendulum consists of a small metal ball (bob) suspended from a fixed point by a light, inextensible thread.
Key terms
- Oscillation: One complete to-and-fro motion of the bob.
- Time period: The time taken for one complete oscillation.
- Amplitude: The maximum displacement of the bob from its mean position.
Factors affecting time period
The time period depends on:
- Length of the pendulum (longer length = longer time period).
- It does NOT depend on the mass of the bob or amplitude (for small amplitudes).
Formula
Time period = Total time taken / Number of oscillations.
6. Speed-time-distance table
| Quantity | Formula | Unit |
|---|---|---|
| Speed | Distance / Time | m/s or km/h |
| Distance | Speed x Time | m or km |
| Time | Distance / Speed | s or h |
7. Worked examples
Example 1: A car covers 180 km in 3 hours. Find its speed.
Speed = Distance / Time = 180 km / 3 h = 60 km/h. Convert to m/s: 60 x (5/18) = 300/18 = 16.67 m/s.
Example 2: A train moves at 72 km/h for 30 minutes. How far does it travel?
Speed = 72 km/h = 20 m/s. Time = 30 minutes = 0.5 h. Distance = Speed x Time = 72 x 0.5 = 36 km.
Example 3: A pendulum takes 20 seconds for 10 oscillations. Find its time period.
Time period = Total time / Number of oscillations = 20/10 = 2 seconds.
Example 4: A cyclist covers 200 m in 40 seconds. Find speed in m/s and km/h.
Speed = 200/40 = 5 m/s. In km/h: 5 x (18/5) = 18 km/h.
8. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Forgetting unit conversion in speed | Always check if distance and time are in consistent units |
| Confusing time period with frequency | Time period = time per oscillation; frequency = oscillations per second |
| Thinking pendulum mass affects time period | Time period depends on length, not mass of bob |
| Adding speeds incorrectly in non-uniform motion | Average speed = total distance / total time |
| Using km/h for speed and minutes for time | Convert time to hours when using km/h |
9. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Speed calculation from distance and time | 2-3 marks | 1 question |
| Unit conversion (km/h to m/s or reverse) | 2 marks | 1 question |
| Simple pendulum time period | 2-3 marks | 1 question |
| Uniform vs non-uniform motion | 2 marks | 1 question |
| Graph-based speed-distance questions | 3 marks | Occasional |
10. Self-test
- A bus travels 240 km in 4 hours. Find its speed in km/h and m/s.
- Convert 90 km/h to m/s.
- A pendulum makes 15 oscillations in 30 seconds. Find its time period.
- A girl runs 300 m in 60 seconds. Find her speed in m/s.
- Classify the following as uniform or non-uniform motion: (a) A train entering a station (b) The Earth revolving around the Sun (c) A car moving at constant speed
- How long will a cyclist moving at 15 km/h take to cover 45 km?
11. Answer key
- Speed = 240/4 = 60 km/h. In m/s: 60 x (5/18) = 300/18 = 16.67 m/s.
- 90 x (5/18) = 450/18 = 25 m/s.
- Time period = 30/15 = 2 seconds.
- Speed = 300/60 = 5 m/s.
- (a) Non-uniform motion. (b) Uniform motion (approximately). (c) Uniform motion.
- Time = Distance/Speed = 45/15 = 3 hours.
12. Quick revision
- Speed = Distance / Time.
- Uniform motion: equal distances in equal time intervals.
- Non-uniform motion: unequal distances in equal time intervals.
- Time period of pendulum depends on length, not mass.
- 1 km/h = 5/18 m/s.
- Distance graph: steeper slope = higher speed.
- Use consistent units in all calculations.
