Units and notation

Thinking about units

Here are some careful thoughts on choosing particular ways of writing the units for speed.

It is common for people (adults and children) to think of speeds as being measured in miles per hour. Pupils may also meet speeds measured in kilometre per hour and metre per second.

It is possible to convert between different units. However, the conversion from millimetre / second to centimetre / second or metre / second should be avoided if it is likely to get in the way of pupils' understanding of the basic concept. There is limited value in expecting pupils at this stage to convert between different unit systems (for example between kilometre / hour and mph). If necessary, a conversion table can be provided.

At this level the most commonly used units for speed will be metres per second, often written as metre / second and sometimes metre second^-1. While these all have the same meaning, they will not be equally accessible to pupils.

The notation itself may cause a problem, and it is useful to liaise with the maths department to establish consistency on this issue. The notation metre / second is probably the most straightforward to use, since it is easy to see how the unit metres divided by seconds comes directly from the formula used to calculate speed and can act as a link to it. You might even choose to write metresecond.

Teacher Tip: Choose notation wisely, and liaise with maths colleagues.