Speed, Distance and Time

Distance against Time

\[ \mbox{(Average) Speed} = \frac{\mbox{Distance}}{\mbox{Time}} \]

So if you took 4 hours to travel 200 kilometers, you would have traveled at a speed of 50 km/h.

But it is important to realize that if you are given details of two separate legs of a journey, you cannot just average the given speeds to find the average speed. For example, if a family travel from Portsmouth to Brighton at a speed of 70 km/h and return from Brighton to Portsmouth at a speed of 40 km/h, the average speed is most certainly not

half of (70+40) = 55 km/h

Many problems at GCSE deal with idealized situations where people travel for long distances at constant speed.

If we were to describe a journey by plotting it on a distance-time graph