Speed and average speed

Speed describes how much distance an object travels per unit of time. A speed of $$5\ \text{m/s}$$ means that the object travels 5 metres each second.

For motion at constant speed, the speed $$v$$, distance travelled $$s$$ and time taken $$t$$ are related by:

If distance is measured in metres and time in seconds, speed is measured in metres per second, written $$\text{m/s}$$.

Average speed when speed varies

An object may travel at different speeds during a journey or stop for part of it. Its average speed describes the total distance travelled per unit of time over the whole journey:

The total time includes any time spent at rest. Average speed does not mean that the object travelled at that speed throughout the journey.

For example, suppose a person travels 120 metres in 20 seconds, rests for 10 seconds, then travels another 80 metres in 20 seconds. The total distance is 200 metres and the total time is 50 seconds, including the rest. The average speed is therefore $$200 \div 50 = 4\ \text{m/s}$$.