Edexcel IGCSE Physics · Spec 1.2
Speed & Distance-Time Graphs
Speed, and how to read and interpret distance-time graphs.
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Speed & Distance-Time Graphs
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Explained
Speed, and what a distance time graph shows
Speed is distance travelled per unit time. Average speed is the total distance divided by the total time, and the word average matters, because an object rarely travels at one steady speed for a whole journey.
Reading the graph
On a distance time graph, distance is on the vertical axis and time on the horizontal. The gradient of the line gives the speed: a steeper line means a greater speed.
- A horizontal line means the distance is not changing, so the object is stationary.
- A straight sloping line means constant speed.
- A curve means the speed is changing.
A horizontal line is the one worth pausing on. It means stopped, not moving steadily. On a velocity time graph a horizontal line means something completely different, which is where most confusion in this topic comes from.
The two graphs are not interchangeable
On a distance time graph the gradient gives speed and the area under the line means nothing at all. On a velocity time graph the gradient gives acceleration and the area gives distance.
Before you read anything off a graph, check the vertical axis label. Everything you do next depends on it. The velocity time graph page covers the other case in detail.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Physics record that confusion between velocity time and distance time graphs was evident in responses, and that many candidates stated that acceleration changed without describing how or why.
Two habits follow from that. Name which graph you are looking at before interpreting it, and when describing motion, say what is happening and then say what the shape of the line tells you.
Finding speed from a gradient
Choose two points that are far apart on the straight section, read off the change in distance and the change in time, and divide the first by the second.
Using widely spaced points reduces the effect of any small error in reading the graph. Using the origin as one of your points is fine only if the line actually passes through it.
Describing a journey
A question showing several sections usually wants each described in turn: travelled at constant speed for the first part, then stopped for a period, then travelled back towards the start.
A line sloping downwards means the distance from the starting point is decreasing, so the object is returning. Say that, rather than saying it is going backwards, which describes direction rather than what the graph shows.
Spec 1.2
What you need to know
- Describe what speed means
- Use average speed equals distance divided by time
- Interpret distance-time graphs
Active recall
Quick check
Answer each question before opening the answer.
What does the gradient of a distance–time graph tell you?
The speed of the object.
What does a horizontal (flat) line on a distance–time graph mean?
The object is stationary (not moving).
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