The idea of average speed
Teaching Guidance for 5-11 11-14
Wrong Track: If it takes Bill an hour to travel the 40 miles from Glasgow to Edinburgh he must be travelling at 40 mph.
Right Lines: For any journey, the total time and total distance allow us to calculate an average speed.
Describing real journeys
Thinking about the learning
The child's point of view sounds very sensible. However on a journey of 40 miles it is unlikely that Bill would be moving at the same speed throughout. He might need to take a bus to the train station. He'll probably spend some time waiting for the train. On the train he'll probably keep up a high speed but the train might make several stops during which time Bill's speed will reduce to zero and then pick up again. So in effect, Bill might never have maintained a speed of exactly 40 mph during his journey. The value 40 mph is his average speed.
Thinking about the teaching
The journey described above is a real life story of a real journey. It's a journey travelled daily by hundreds of commuters between two of Scotland's great cities.
It is helpful to try to place physics calculations in real life contexts such as this in order to illustrate the complexity of speed calculations. In a laboratory it might just be possible to consider an object moving at a constant speed but in most scenarios all that is possible is a calculation of an average speed.
We'd suggest that whenever you're after a calculation of an average speed you use data from a journey that has taken place (even if that journey is only in the imagination).