My speed is not your speed

A fly past

Wrong Track: Everyone agrees – it's on the aeroplane speedometer. We're doing 500 kilometres per hour.

Right Lines: Alice and Bob are sitting next to each other for the whole flight. Alice does not record any change of distance between her and Bob. But Charlie, in a fighter jet, records Alice and Bob's speed as 300 kilometre / hour as he overtakes, putting an extra 300 kilometre between himself and Alice for each hour of flying time.

Speed is always from someone's point of view

Thinking about the learning

Most pupils are comfortable with the concept of speed, though they may not have a neat definition for it. However, comparisons and calculations of speed may raise difficulties.

So can under-defined questions. Speed is always with respect to a point of view, and sometimes we need to carefully state the point of view to avoid ambiguity.

Thinking about the teaching

Take care to specify a point of view when you ask for a speed. The speed specifies the rate at which one object moves away from another.

So you need to locate yourself – perhaps you're moving along with one of the objects.

If you report something as at rest, all we can work out from that is that you and it are moving along together – the distance between you and the object is not changing. We don't know of absolute point of view against which we can all agree that something is stationary.