Units
Comprehensive notes, formulas, and practice questions for Units.
Units
Units
What you'll learn
- Why scientists use standard (SI) units so measurements are comparable worldwide.
- The SI units for length (metre), mass (kilogram), and time (second).
- How to convert between mm, cm, m, and km without place-value errors.
- Why informal units like hand-span fail as scientific measures.
Key concepts
Level 1 — Standard vs non-standard
Verbal: A standard unit is an agreed, fixed reference everyone uses (e.g. the metre).
Visual: 1 m = 100 cm = 1000 mm. A 15 cm ruler has 150 mm divisions.
| Unit | Symbol | Typical use |
|---|---|---|
| millimetre | mm | Thickness of card, small insects |
| centimetre | cm | Book length, body height (also m) |
| metre | m | Room length, cricket pitch (20.12 m) |
| kilometre | km | Distance between towns |
Level 2 — Conversions and fairness
Symbolic: km → m: multiply by 1000. cm → m: divide by 100.
Why hand-span fails: Different people get different counts for the same desk — not reproducible.
Mass vs weight (Class 6): Use kg for mass of objects; "weight" in daily speech often means mass here.
Worked example
Convert 3 km 250 m to metres
Step 1 — 3 km = 3 × 1000 = 3000 m
Step 2 — Add 250 m → 3000 + 250 = 3250 m
Step 3 — Check: 3.25 km × 1000 = 3250 m ✓
Common mistakes
| Mistake | Why it happens | Fix |
|---|---|---|
| 1 km = 100 m | Confusion with cm–m (×100) | 1 km = 1000 m |
| Writing 120 without cm | Forgetting unit carries meaning | Always attach symbol: 120 cm |
| Any ruler = SI unit | Not checking calibration | Use metre scale marked in cm/mm |
| Mass = weight always | Everyday language overlap | At Class 6, kg measures mass |
Quick check
- Convert 450 cm to metres and 2.5 km to metres.
- Why did ancient traders need standard weights?
- Name the SI unit of length.
Open the Practice tab for graded questions on Units.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Worked example
- Common mistakes
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