What is a unit? State the three qualities of a good unit, with an example of each.

1. 1 · Define a unit. A unit is the fixed standard used to measure a physical quantity, such as the metre for length.
2. 2 · Give the value rule. Value of a quantity = number × unit, so 5 m means 5 times 1 metre.
3. 3 · Relevant. The unit must suit the quantity: length in metres, never in kilograms.
4. 4 · Convenient. Its size must suit the job: distances between cities in kilometres, not centimetres.
5. 5 · Well defined. It must mean the same to everyone; 'my steps' change from person to person.
6. 6 · Link to daily life. Shop scales are checked and stamped, so a kilogram is the same in every shop.
7. 7 · Close. So a useful unit is relevant, convenient and well defined.

Scoring tip: Give one short example with each quality; the marks are for the three names plus examples.