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.

Looking at the above picture we see how the tangent is drawn for a distance-time graph which does not have a constant speed. After which you calculate the slope for the tangent and you can get the slope of the line.

For instructions on how to caculate slope, refer to distance-time graphs.

Continue to the next lesson: (Vectors) Position and Displacment