Speed-time graphs show up almost every year in Combined Science Physics (5076/5078), usually in Paper 2 Section A as a structured question worth 6 to 9 marks. The good news: the marking is very mechanical. If you know exactly what the gradient means, what the area means, and how the examiner wants you to phrase your description, these are some of the most reliable marks on the whole paper.
Here is the method our tutors use, straight from the papers we sat ourselves.
First, check which graph you are looking at
Students lose marks before they even start because they confuse distance-time with speed-time graphs. Train yourself to read the y-axis label first, every single time.
| Graph | Gradient tells you | Area under graph tells you |
|---|---|---|
| Distance-time | Speed | Nothing useful |
| Speed-time | Acceleration | Distance travelled |
A horizontal line on a distance-time graph means the object is stationary. A horizontal line on a speed-time graph means constant speed, which is a completely different thing. If you write "the object is at rest" for a flat line at 12 m/s, that is an instant zero.
The three things examiners ask
1. Describe the motion
This is the part most students rush. The mark scheme wants you to go section by section, quoting the time interval and using precise vocabulary.
Good phrasing looks like this:
- "From 0 to 10 s, the object accelerates uniformly from rest to 20 m/s."
- "From 10 s to 25 s, the object travels at a constant speed of 20 m/s."
- "From 25 s to 30 s, the object decelerates uniformly and comes to rest."
Notice three habits worth copying:
- Always quote the time interval.
- Always use the word uniformly (or "at a constant rate") when the line is straight.
- Always quote a value with a unit where the graph gives you one.
If the line is curved and getting steeper, say "accelerating at an increasing rate". If it is curved and flattening off, say "accelerating at a decreasing rate". Do not say "decelerating" for a flattening curve if the speed is still rising, that is a very common trap.
2. Calculate the acceleration
Acceleration is the gradient. Show the substitution, not just the answer.
Acceleration = change in speed ÷ time taken = (20 − 0) ÷ 10 = 2 m/s²
Mark scheme habits:
- Read coordinates off gridlines, never off the middle of a square.
- Use a large triangle across the straight section so small reading errors matter less.
- A negative gradient gives a negative acceleration. Write "−4 m/s², i.e. a deceleration of 4 m/s²" to be safe.
- Units are m/s², and they carry a mark surprisingly often.
3. Find the distance travelled
Distance is the area under the graph. Split the shape into triangles and rectangles, work out each one, then add.
Using the example above:
- Triangle (0 to 10 s): ½ × 10 × 20 = 100 m
- Rectangle (10 to 25 s): 15 × 20 = 300 m
- Triangle (25 to 30 s): ½ × 5 × 20 = 50 m
- Total distance = 450 m
Write the shapes out as separate lines. If you make an arithmetic slip in one, you still keep the method marks.
If the question then asks for average speed, use total distance ÷ total time = 450 ÷ 30 = 15 m/s. Do not average the speeds shown on the graph, that is not the same thing and it is a classic wrong answer.
Curved graphs and estimating area
Combined Science does not ask you to integrate. If the graph is curved, the question will usually say "estimate" and expect you to count squares. Work out what one square is worth first, for example 1 s × 2 m/s = 2 m, then count whole squares and count part-squares over half as one. State your working, for example "about 47 squares × 2 m = 94 m".
Quick checklist before you move on
- Did I read the y-axis label?
- Did I quote time intervals in my description?
- Did I include units on every value?
- Did I use gradient for acceleration and area for distance, not the other way round?
- Does my answer look sensible for a car, a cyclist or a falling object?
Link it to the rest of the syllabus
Speed-time graph questions are often bolted onto forces. Once the graph is flat, the resultant force is zero, so for a falling object that means weight equals air resistance and the object has reached terminal velocity. Examiners love this follow-up, so practise saying "the resultant force is zero, so the object moves at constant speed" in one clean sentence.
Practise five past-paper graph questions in one sitting and time yourself. Ten minutes for a nine-mark question is generous, and the speed you gain here buys thinking time for the harder electricity questions later in the paper.
Want a tutor to mark your kinematics working line by line and show you the exact phrasing that scores? Message us on WhatsApp at https://wa.link/dsgbkf to book a free trial class with Dojo.