Physics is built on measurement. Before solving any problem, you must understand the quantities we measure and the units we use. The international system of units is the SI system (Système International).
Fundamental Quantities and Units
| Fundamental quantity | SI unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Derived Quantities
A derived quantity is obtained by combining fundamental quantities. Examples:
- Speed = distance ÷ time → m/s (m s⁻¹)
- Volume = length × length × length → m³
- Density = mass ÷ volume → kg/m³
- Force = mass × acceleration → newton (N = kg m s⁻²)
Measuring Instruments
- Length: metre rule, vernier calipers (accurate to 0.1 mm or 0.01 cm), micrometer screw gauge (accurate to 0.01 mm).
- Mass: beam balance, lever balance.
- Time: stopwatch, clock.
- Volume of liquids: measuring cylinder, burette, pipette.
Remember: The vernier calipers and micrometer screw gauge measure small lengths more accurately than a metre rule. For WAEC, be able to identify these instruments and read their scales.
Accuracy and Precision
- Accuracy is how close a measurement is to the true value.
- Precision is how close repeated measurements are to each other.
- A measurement can be precise but inaccurate (e.g. a faulty instrument giving consistent but wrong readings).
Summary
There are seven fundamental SI quantities. All other quantities are derived from them. Learn the units and the correct instruments for measuring length, mass and time — they are the building blocks of every Physics calculation.
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