Ch. 2: Physical Quantities and Measurement

# Level 2 — Full Concepts ## Definitions - **Physical quantity:** physical property measured with numbers - **Measure:** a reference standard for comparison - **Measurement:** the process of measuring ## Ancient body units (history) 4 digits = 1 palm · 3 palms = 1 span · 2 spans = 1 cubit · 4 cubits = 1 stature Romans: 1 uncia ≈ thumb width · 12 uncia = 1 foot · 3 feet = 1 yard ## Need for standard units 1. Measurement = **magnitude + unit** 2. Without unit, number is meaningless 3. Unit must stay accurate (not change like wet grain) 4. Unit must be understood by all ## Metric system - Simple, logical, almost worldwide - Developed in France (1795) - Easy because units change by **10s** - **SI** = International System of Units ### Length ladder 10 mm = 1 cm · 10 cm = 1 dm · 10 dm = 1 m · … · **1 km = 1000 m** ### Seven fundamental SI units | Unit | Symbol | Quantity | |------|--------|----------| | metre | m | length | | kilogram | kg | mass | | second | s | time | | ampere | A | electric current | | kelvin | K | temperature | | mole | mol | amount of substance | | candela | cd | luminous intensity | ## Length ideas - **Length:** horizontal size - **Height:** vertical size - **Width/breadth:** shorter side - **Distance:** space between two points ### Measuring carefully - Align object with **0 mark** - Eye directly above the reading (no slant) - Don’t use worn ends - **Direct:** ruler against object - **Indirect:** stack cards / wrap wire and divide ## Mass vs weight - **Mass:** amount of matter — **constant** · SI unit **kg** - **Weight:** force of gravity — **changes** (less on Moon) - Chain: tonne ↔ kg ↔ g ↔ mg (× or ÷ **1000**)