We frequently hear about accidents caused by vehicles moving at high speeds. Due to this reason, there are different regulatory speed limits assigned to different roadways. We should obey these speed limits in order to prevent accidents, particularly in highways with high speed limits. What we mean by speed is the rate at which a given distance is traversed.

In other words, speed is the distance traversed in a unit time.

Vehicles moving in roadways cannot maintain the same speed throughout the travel time. The speedometer of a motor vehicle usually indicates only the speed of the vehicle at a particular instant. When there is heavy traffic, vehicles have to slow down, and when there are pedestrians crossing the road, the vehicles even have to stop. On the other hand, if there are only a few other vehicles in the road, the same speed can be maintained for a fairly long distance. Let us consider two such instances, one where the same speed is maintained, and the other where the speed changes with time, through the following examples. The distance traversed by a certain object at different instances in time is shown in the table below.

According to these data,

Distance traversed by the object during the first second = (3 – 0) = 3 m

Distance traversed during the next second = (6 – 3) = 3 m

Similarly, the distances traversed during each of the third, fourth, fifth and sixth seconds is also 3 m.

That is, the object has traversed 3 m during each 1 s time interval. Therefore, in this case we can say that the object has moved at a uniform or constant speed.

Now let us consider the motion of another object described by the following data table

The distance traversed by this object is 3 m in first second, 2 m in second second and 4 m in third second etc.

Therefore the distance traversed by this object during each interval of one second is not the same. That is, this object has not travelled with a uniform speed. In such instances, where an object does not travel with a uniform speed, it is useful to calculate the mean speed of the object during a given time interval. The mean speed of an object can be calculated by dividing the total distance travelled, by the corresponding time interval. Mean speed is also known as the average speed.

As another example, let us consider a vehicle that travelled a distance of 100 km from a location near Colombo to Peradeniya in 2 hours. In such a journey, a vehicle cannot maintain the same speed throughout the whole journey. However we could divide the total distance of 100 km by the total time duration of 2 hours as done above to calculate the average speed, which is 50 kilometers per hour.