Movement is a fundamental feature of the world we live in; it is also inextricably linked with time. The measurement of time relies on change - monitoring the swing of a pendulum, perhaps - but conversely, any discussion of the motion of the pendulum must involve the concept of time. Taken together, time and change lead to the idea of rate, the quantity which tells us how much change occurs in a given time. Thus, for example, for our pendulum we might describe rate in terms of the number of swings per minute. Or, to take a familiar example from everyday life, we refer to a rate of change in position as speed and measure it as the distance travelled in a given time