Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Why Do We Measure?
Measurement helps us compare, communicate, and understand the world around us. Saying "the table is long" is not useful — saying "the table is 1.5 metres long" is precise.
History of Measurement
Early humans used body parts for measurement:
- Hand span (width of spread hand)
- Cubit (elbow to fingertip)
- Foot (length of foot)
Problem: These vary from person to person. So standard units were developed.
Standard Units of Length
The SI (Systéme International) unit of length is the metre (m).
| Unit | Relation to metre | Used for |
|---|---|---|
| Kilometre (km) | 1 km = 1000 m | Long distances (roads, cities) |
| Metre (m) | Base unit | Height, room length |
| Centimetre (cm) | 100 cm = 1 m | Everyday objects |
| Millimetre (mm) | 10 mm = 1 cm | Small objects (thickness of book) |
Using a Ruler (Scale)
Rules for correct measurement: 1. Place the scale flat along the object. 2. Zero mark of the scale at one end of the object. 3. Eye should be directly above the mark to avoid parallax error. 4. Read from the nearer edge of the mark.
If the zero end of the scale is worn/broken, start from the 1 cm mark. Subtract 1 cm from the final reading.
Measuring Curved Lines
A ruler cannot measure a curved line directly.
Method 1 — String:
- Lay a string along the curve, following every bend.
- Mark where the string ends.
- Straighten the string and measure with a ruler.
Method 2 — Divider:
- Use a divider to walk along the curve in small steps.
- Count the steps and multiply by the length of each step.
Types of Motion
1. Rectilinear Motion (Linear Motion)
Movement in a straight line.
Examples: A car on a straight road, a ball rolling on the floor, a falling stone, marching soldiers.
2. Circular Motion
Movement along a circular path.
Examples: Hands of a clock, blades of a fan, a stone tied to a string and swung in a circle, the Earth revolving around the Sun.
3. Periodic Motion (Oscillatory Motion)
Motion that repeats after equal intervals of time.
Examples: Pendulum of a clock, a child on a swing, strings of a guitar, heartbeat.
Objects Can Have More Than One Type of Motion
- The wheel of a moving bicycle:
- Circular — the wheel spins.
- Rectilinear — the bicycle moves forward.
- Earth:
- Circular — revolves around the Sun.
- Circular — rotates on its own axis.
Key Vocabulary
| Term | Meaning |
|---|---|
| Measurement | Comparing an unknown quantity with a known standard |
| SI unit | International standard unit system |
| Metre | SI unit of length |
| Rectilinear motion | Motion in a straight line |
| Circular motion | Motion along a circular path |
| Periodic motion | Motion that repeats after equal time |
| Parallax | Error caused by looking at scale from an angle |
Exam Tips
- SI unit of length is metre — not centimetre or kilometre.
- Learn all unit conversions: 1 km = 1000 m; 1 m = 100 cm; 1 cm = 10 mm.
- Types of motion with examples: 2 examples per type minimum.
- Measuring a curved line — the string method is the standard answer.
- Fan/clock hands = circular motion. Pendulum = periodic motion. Don't swap.