What Is a Speed Diagram?

Speed diagrams, sometimes called velocity diagrams, are used to describe the correlation between distance, speed and time. In the diagram, we have the time along the first axis ($x$-axis) and the distance along the second axis ($y$-axis). It’s possible to calculate the average speed of different periods of time on the graph. For example, the graph being a horizontal line means there is no movement, because time passes during this period while the distance traveled stays the same. This is how you have to think when you interpret a speed diagram.

We will do some calculations based on this diagram below. Before that, let’s have a quick look at the diagram together. The car is not moving from $0.75$ to $1.2$ hours, as marked by the horizontal line. You can also see that the graph is steeper before the break than after the break, which means that the car was driving faster before the break. Due to the break, the average speed for the entire trip is lower than the speed in both the intervals where the car is actually moving.

When a problem talks about a trip that goes somewhere and back again, the graph will still continue to rise. This is because the second axis shows how far we have traveled, not how far we are from our starting point.