Graph questions appear regularly in IGCSE Physics papers. They test whether you can present data visually, extract information from a graph and link graph features to physics concepts. Here is what you need to be able to do.
Plotting graphs
When given data to plot:
- Choose scales that use at least half the grid in each direction. Do not start both axes at zero unless the data requires it.
- Label axes with the quantity and the unit (e.g., “Time / s” on the x-axis).
- Plot points accurately. Use a sharp pencil. Each point should be within half a small square of its correct position.
- Draw a line of best fit. If the relationship is linear, use a ruler. If it is a curve, draw a smooth curve that follows the trend. Do not connect dots.
Reading values from graphs
When asked to “read off” or “determine from the graph”:
- Draw a line from the given value on one axis to the graph line, then across to the other axis.
- Show these construction lines clearly on the graph.
- Read the value to the precision of the scale.
Calculating gradient
Gradient = change in y / change in x.
- Choose two points on the line of best fit (not data points) that are far apart.
- Draw a large right-angled triangle between them.
- Read the coordinates of both points.
- Calculate: gradient = (y2 - y1) / (x2 - x1).
- Include the unit of the gradient (e.g., m/s if y is distance in m and x is time in s).
Calculating area under a graph
Area under a graph gives a physical quantity when the graph represents a rate.
- Speed-time graph: area = distance travelled.
- Force-time graph: area = impulse (change in momentum).
For straight-line sections, use the trapezium or triangle formula. For curved sections, count squares.
Common graph types
| Graph | x-axis | y-axis | Gradient means | Area means |
|---|---|---|---|---|
| Distance-time | Time | Distance | Speed | N/A |
| Speed-time | Time | Speed | Acceleration | Distance |
| Force-extension | Extension | Force | Spring constant | Energy stored |
| V-I characteristic | Current | Voltage | Resistance | N/A |
| Cooling curve | Time | Temperature | Rate of cooling | N/A |
Practice strategy
Find 5 past paper questions that require graph plotting and 5 that require graph interpretation. Work through each one and check against the mark scheme. Pay attention to the specific wording the mark scheme uses for gradient and area calculations.
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