Plotting velocity against time
Velocity–time graphs are used to show how velocity varies with time.
The two axes
Here velocity goes on the y axis and time on the x axis.
Only the y axis has changed — displacement has become velocity. But that changes everything the graph can tell you.
The data
| Time t (s) | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Velocity v (m s−1) | 0 | 3 | 6 | 9 | 12 | 15 | 18 |
The graph
| t (s) | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| v (m s⁻¹) | 0 | 3 | 6 | 9 | 12 | 15 | 18 |
Why a straight line?
Because in every second the velocity changes by the same amount — 3 m s−1. So this is motion with a uniform (constant) acceleration.
Do not confuse the two graphs
A straight line on a displacement–time graph means uniform velocity. A straight line on a velocity–time graph means uniform acceleration.
So always check the label on the y axis before reading a graph.