Motion and Measurement of Length: Class 6 Study Guide
Measuring lets scientists share exact results. Motion is all around us.
To measure anything we compare it with a standard unit. The international system (SI) uses the metre (m) for length. Smaller and larger units are made with prefixes: 1 m = 100 cm = 1000 mm and 1 km = 1000 m. Long ago, people used the hand span or foot, but these varied from person to person, which is why standard units were introduced.
Motion is a change of position with time. A car on a straight road shows rectilinear (straight-line) motion. A child on a merry-go-round shows circular motion. A swing or pendulum shows periodic motion, repeating at regular intervals.
How to measure length correctly
Keep the scale close to the object. Start from the 0 mark (or note the starting mark if the end is worn). Place your eye directly above the reading to avoid parallax error. To measure a curved line such as a coastline on a map, lay a thread along it and then measure the thread against a ruler.
Ruler, tape or thread.
With one end.
Eye above the mark.
For example, 14.3 cm.
Types of motion
| Type | Description | Example |
|---|---|---|
| Straight line | moves along a line | a falling coin, a train on a straight track |
| Circular | moves in a circle | a wheel, the tip of a clock hand |
| Periodic (oscillation) | repeats at regular intervals | a pendulum, a swing, a heartbeat |
| Rotation | spins on its own axis | a top, Earth |
Speed and units
Scientists describe how fast something moves by its speed: distance divided by time. A cyclist who covers 30 km in 2 hours has an average speed of 15 km per hour. In the SI system, speed is in metres per second (m/s). 36 km/h is 10 m/s (36 × 1000 m ÷ 3600 s).
Worked example and common mistakes
Example: A pencil starts at the 2.0 cm mark and ends at the 11.5 cm mark: its length is 11.5 − 2.0 = 9.5 cm. Common mistakes: forgetting the starting mark, mixing units (adding cm and m) and reading from the side. Repeat measurements and take an average for accuracy.
Check your understanding: model answers
1. 250 cm. 2. A pendulum swinging. 3. 40 km per hour.
✨ Key Takeaways
- The metre is the SI unit of length.
- Line up the zero and read straight on.
- Motion can be straight, circular or periodic.
- Speed = distance ÷ time.
💬 Questions for Discussion & Reflection
- Convert 2.5 m to centimetres.
- Name one example of periodic motion.
- A bus travels 120 km in 3 hours. Find its average speed.
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