Measurements and units

What the syllabus demands

• —Use rulers, measuring cylinders, balances and clocks to measure length, volume, mass and time
• —Take repeat readings and average — including timing multiple oscillations or intervals
• —Use micrometer screw gauges and digital calipers for small distances (Extended)
• —Know the SI units of every quantity on the syllabus and convert between prefixes (km, cm, mm, ms)

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

SI base units

The metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for current and kelvin (K) for temperature — every other unit on the syllabus is built from these.

Scalar quantity

A quantity that has magnitude (size) only — e.g. speed, distance, mass, energy, time.

Vector quantity

A quantity that has both magnitude and direction — e.g. velocity, displacement, force, acceleration, momentum.

The equations

More equations to practise: the IGCSE formula sheet.

Where the marks die

Common mistakes to check

1. 01

   Measuring the period of a pendulum with one swing. Time 20 oscillations and divide by 20 — one swing makes your reaction time a huge fraction of the reading.

2. 02

   Forgetting to convert units before substituting: 250 g must become 0.25 kg, 40 cm must become 0.40 m. Examiners set numbers deliberately to punish this.

3. 03

   Reading a measuring cylinder from the top of the meniscus, or with your eye above the level. Read the bottom of the meniscus, at eye level.

4. 04

   Writing an answer with no unit. A correct number with a missing or wrong unit routinely loses the final mark.

One worked example, done properly