Calculating the Instantaneous speed When we are calculating speed from the graph for a uniform motion the speed which get as an answer is actually the average speed and since in uniform motion the average speed is same as that of instantaneous speed so its correct,right? Now if this is right if we calculate the variable speed with the help of a tangent isnt that also average speed?
 A: Uniform motion is a special case. The instantaneous speed is the same at all times. If you draw a graph, it is a figure where the slope is the same at all points. That is a line. 
For a line, the average slope between two points is the same as slope of the tangent at any point. So yes, the average speed and the instantaneous speed at each point are the same for uniform motion. 
But that only works for uniform motion.
A: Average velocity between two points in time is given by the slope of the secant line between the two corresponding points on the position vs. time graph.
Instantaneous velocity is found at a point in time. It is given by the slope of the tangent line at that point on the position vs. time graph.
In the case on constant velocity, the slope of the secant line between any two points and the slope of the tangent line at any point are always the same. If the velocity is not constant then it is not generally true that the slope of the secant line between any two points will be equal to the slope of the tangent line at some other point.
Something to keep in mind, in saying average velocity you have to specify between which time points you are referring to. For instantaneous velocity you have to specify at what point in time. There is not a single average or instantaneous velocity.
