The thinnest hand on this clock is the second hand. One second is the about the time it takes for you to say "one one thousand".

The picture on the left is a coloured electron micrograph of a Salmonella typhimurium bacterium that is undergoing cell division. You've probably heard of Salmonella: it's a nasty tummy bug that you can catch by eating contaminated food. The bacterium is impossible to see with the naked eye: this electron micrograph was taken using an electron microscope at a magnification of \(15,000 \times\). Each half of the bacterium measures \(0.000002\) m. I don't know about you, but I find it a bit tedious to have to write down all those zeroes all the time, and I can't really visualise what they mean. The metric system has found a way around this, by introducing the prefix micro, which corresponds to \(10^{-6}\). So, we could, more conveniently, write this length as \(2\) micrometres, or using the symbol \(2 \mu \text{m}\).

While it might be OK to measure the distance that you walk on a track around a park in metres, it becomes very cumbersome to measure distances travelled on roads, such as the road from Sydney to Perth, in metres. Instead we use a larger unit called the kilometre. 1 kilometre is equal to 1,000 metres, and the prefix "kilo" means one thousand. The distance by road from Sydney to Perth is approximately 3,929.6 kilometres.

We can build area out of length. The area of a space is equal to a length times a length, so the basic unit of area is the area of a square that has side-lengths of 1 metre. The unit is \(\text{metres}\times \text{metres}\), written as \(\text{m}^2\), and said as "square metres".

We can also build volume out of length. The volume of a space is equal to a length times a length times a length, so the basic unit of volume is the area of a cube that has side-lengths of 1 metre. The unit is \(\text{metres}\times \text{metres} \times \text{metres}\), written as \(\text{m}^3\), and said as "cubic metres".

We tend to use the litre as the unit for measuring liquids (although they sell small vegetables by the litre at markets in Europe). The conversion is

The milk carton on the left contains 1 Litre of milk.

It gets really cumbersome measuring times in seconds, so we have invented many larger units such as the minute (60 seconds), the hour (60 minutes), the day (24 hours), the week (7 days), and so on. If you tried to say that something was happening in 14 days using seconds, you'd have to say it would happen in 1,209,600 seconds, which wouldn't mean very much to anyone.

It's more convenient to measure the speed of faster objects like this racing car, or an aeroplane in kilometres per hour (abbreviated km/h). We can convert kilometres per hour to metres per second as follows:

1. There are 1,000 metres in a kilometre. So one kilometre per hour is 1,000 metres per hour.
2. There are 3,600 seconds in one hour. So one kilometre per hour becomes \(\dfrac{1,000}{3,600} \approx 0.278\) metres per second.

A force is a push or a pull. The metric unit of force is the Newton (abbreviated N). A Newton is the amount of force required to make a 1 kg object accelerate at a rate of 1 \(\text{m/s}^2\). So, one Newton is equal to 1 kg \(\text{m/s}^2\). A Newton is derived from all three of the units: metre, kilogram and second.