Describing Motion

To calculate the average speed of an object, you need to know the distance that the object travelled and the time it took for that distance to be travelled.

  • S = Speed in metres per second (m/s)
  • D = Distance in metres (m)
  • T = Time in seconds (s)

These triangles above are used to help us remember the formula. Sometimes we may be asked to work out the distance or time, not only the speed. So, we need to know how to rearrange the formula to work out the various subjects.


Physical QuantitySymbolUnitUnit Symbol
SpeedSMetres per secondm/s


Work out the speed of a jogger who ran 60 m in 20 s

60m ÷ 20 s = 3 m/s

The speed = 3 m/s

The unit for speed is metres per second (m/s). This concept can be displayed in a graph: distance-time graphs

Distance Time Graph

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As you can see by the graph, the time (s) is always on the x axis and the distance (m) is always on the y axis. The direction and slope of the lines tell us information about the speed of the object, as shown in the above graph.

Remember: the speed of the object = gradient of the line.

Relative motion

The relative motion takes into account direction and speed. For example, sometimes when you are in a car that is driving at a fast speed and you look out of the window, the cars outside seem to be driving at a slow pace. Even though, all the cars are actually driving at a fast speed.

We need to know how to work out relative speed.

The table below tells you how to do this:

SituationFormula for relative speed
Objects moving in an opposite direction towards or away from each otherAdd the two speed values together
Objects moving in the same direction towards or away from each otherFastest speed – slowest speed