Speed is defined as a measure of the distance an object travels in a given length of time.

The speed of an object can be determined from the equation below:


Speed = Distance travelled (meters) Time taken (seconds)


v = dt

Note: the symbol v can be used to denote speed or velocity, here it is used for speed.

Typically the unit for speed is given as metres per second (m/s). However, other units such as kilometres per hour for large distances and centimetres per second (cm/s) for small distances can be used.



Rearranging the equation

The equation for speed can be rearranged to give two other useful equations:


Distance travelled = Speed x Time taken


Time taken = Distance travelled Speed


Using the equation in the distance travelled (d) = Speed (v) x Time taken (t), we can put it in an equation triangle.


Average Speed

The equation for average speed is the same as that given for speed above:


Average speed = Distance travelled (meters) Time taken (seconds)


Thus, if a car travels 100 meters in 5 seconds, its average speed can be calculated as:

Average speed = 1005 = 20 meters per second

Average speed is used to give the speed of an object over a given interval of time; if however the speed of an object is required for a particular moment then the instantaneous speed is used.

Instantaneous speed is the speed of an object at a given moment. In this case the equation is similar to that of average speed but the time taken is a much smaller interval. A speedometer in the car gives an instantaneous speed as it gives the speed of the moving car at that specific time, whereas the average speed would be used as a measure of speed for the whole journey.