INSTANTANEOUS SPEED

Instantaneous speed is the speed at a particular moment in time. On a Speed-Time graph the instantaneous speed can be taken from any point which lies on the line. In contrast, instantaneous speed has to be measured in a different way on a distance-time graph.

On a Distance-time graph, if the speed is constant than the instantaneous speed is the same at 
any time, and is equal to the speed and at that time.
What if the speed is not constant? What do we do then?

The first step is to draw a tangent across the point in time that you wish to find the speed for. 

Then you calculate the slope for the new tangent you have drawn. . and viola!, you have the 
Instantaneous speed.