ABSTRACT
The quantities you will come across most often are length, mass, amount of substance, time and temperature. Measurements of these consist of a number and a unit. The number expresses the ratio of the measured quantity to a fixed standard, and the unit is the name of the standard. For example, if a piece of wood is 3 metres long it is 3 times the length of the standard used to define 1 metre. It is vitally important that you use units correctly and especially that you write down the correct unit after calculating a numerical answer. (Some measurements do not have units. This may be because they are ratios of two measurements with the same units, or because they are a particular kind of mathematical transformation of a measurement called a logarithm. Logarithms are explained in Chapter 6.)
A large number of different units could be used. Distance could be measured in metres, inches, hands, feet, furlongs, miles, or light-years! The accepted convention for use in science is the ‘SI’ system in which there are ‘base’ units and ‘derived’ units each of which has a specified abbreviation or symbol. The base units for length, mass, amount of substance, time and temperature are metre (m), kilogram (kg), mole (mol), second (s) and kelvin (K) respectively. Derived units are made up from combinations of base units; for example, the unit of energy, the joule, is metres squared times kilograms divided by seconds squared.
