To calculate the velocity of an object, the of the position vs time graph should be calculated.

By definition we have to:

Where,

dr / dt: rate of change of the position of an object with respect to time

Therefore, we know that in a position graph with respect to time, both variables are proportional.

The proportionality constant is the speed of the object.

Mathematically, this constant represents the slope of the line.

To calculate the velocity of an object, the slope of the position vs. time graph should be calculated.