For the gradient of a straight line in SEC G3 Physics K323 practical work, choose two well-separated points on the line you drew. Use their coordinates even if they are not individual experimental readings. The published 2027 syllabus guidance calls for a gradient triangle whose hypotenuse spans at least half the length of your line.
Read the horizontal and vertical scales before calculating. Then divide the change in the vertical quantity by the change in the horizontal quantity. The units follow the same order.
- Follow the question’s instructions for drawing the line.
- Choose two readable points far apart on that line.
- Subtract coordinates in the same order for both axes.
- Give the gradient with units and the requested precision.
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Separate the measurements from the fitted line
The plotted dots show the individual readings. A straight best-fit line represents the trend you drew through the data. A measurement above or below that line has a different vertical coordinate from the line at the same horizontal position. If you choose it for your triangle, your calculation describes a different slope.
Choose coordinates you can read accurately from the grid. A wide triangle reduces the relative effect of a small coordinate-reading error. You can choose convenient points that fall on your line even when you did not record measurements at those positions.
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Worked example | A displacement–time gradient
Suppose two readable points on your fitted line are 1.0 s with 0.40 m and 5.0 s with 2.00 m. Assume the triangle spans enough of the fitted line. The vertical change is 1.60 m and the horizontal change is 4.0 s. The gradient is 0.40 m/s. Since this is displacement against time, it represents velocity.
Now suppose the actual measurement at 5.0 s was 2.08 m, above the line. Use 2.00 m if it is the line’s value at the chosen coordinate. The 2.08 m reading remains part of the original data table.
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CHAPTER 3 OF 4 · Back to contents
Check your result
Draw your triangle clearly and mark the coordinates you used. Check that the sign agrees with the direction of the line as you read from left to right. If the question asks you to use the gradient to find another quantity, write the equation connecting them before substituting.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Can I use a plotted point?
You can use it if it lies on your fitted line and your two points give a sufficiently large triangle.
What if the graph is curved?
Use the method requested. If you need a gradient at one point on a curve, the calculation may require a tangent there.