Physical quantities and units
A physical quantity is something we can measure, such as length, mass or time. Every measurement has a number and a unit. Scientists use the SI (International System) units so that everyone understands each other.
| Quantity | SI unit | Symbol | Instrument |
|---|---|---|---|
| Length | metre | m | metre rule, tape, vernier calipers |
| Mass | kilogram | kg | beam balance |
| Time | second | s | stopwatch, clock |
| Temperature | kelvin | K | thermometer |
Other quantities are made from these. Area is in m², volume in m³ (or litres), density in kg/m³ and speed in m/s.
Length, volume and mass
- density (kg/m³)
- mass (kg)
- volume (m³)
A stone is lowered into a measuring cylinder. The water level rises from 40 cm³ to 55 cm³. The stone has a mass of 42 g. Find its volume and density.
- Volume = rise in water level.
- Density.
Answer: Volume 15 cm³, density 2.8 g/cm³ (2 800 kg/m³).
Accuracy and precision
A measurement is only as good as the instrument. A ruler marked in millimetres can measure to the nearest 1 mm. Vernier calipers read to 0.1 mm or better, so they are more precise. Try it below.
The true length of a bolt is shown. Choose how many decimal places your instrument can read. More decimal places means a smaller error.
Key terms
- Physical quantity
- Something that can be measured.
- SI units
- The international standard system of units.
- Density
- Mass per unit volume.
- Precision
- How finely an instrument can measure.
Summary
- Measurements have a number and a unit; SI units are standard.
- Base units: metre, kilogram, second, kelvin.
- Density = mass ÷ volume.
- A finer instrument gives a smaller error; repeat and average readings.