One of the most basic things students do in a physics lab is to collect data and use that to build a model. Most of these models come in the form of a mathematical function. But here is the problem.

When a relationship between two variables is defined by a curve it means that the gradient, or rate of change, is always varying. An average speed for a journey can be found from a distance-time graph ...

In a distance-time graph, the gradient of the line is equal to the speed of the object. The greater the gradient (and the steeper the line) the faster the object is moving.