When we measure some kinds of units, we face a problem: the long string of digits needed to write out the measured data. For example, let's say a private car travels over 100,000,000 metres. It is difficult for car users to understand how far the car really has gone. Scientists have found ways to reduce the number of digits in measurement units by adding prefixes before base units.

100,000,000 metres = 100,000 kilometres

A prefix can be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten. Figure 1.2 sets out the standard prefixes for the SI units.

We use length and mass as examples:

1 metre (1 m) = 10 dm = 100 cm = 1000 mm

1 kilogram (1 kg) = 1000 grams (g) = 100000 mg

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