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Getting started: variables and graphs in physics
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Physics questions often give you a graph to plot, read or explain. These skills start at KS3 and earn marks in every GCSE paper.
Variables
Independent: the variable you change. Dependent: the one you measure. Control: the ones you keep the same.
Example: stretching a spring. Independent: force (weight added). Dependent: extension. Control: the same spring.
If the independent variable is categoric (for example type of material), draw a bar chart. If it is continuous (a number), draw a line graph.
Example: stretching a spring. Independent: force (weight added). Dependent: extension. Control: the same spring.
If the independent variable is categoric (for example type of material), draw a bar chart. If it is continuous (a number), draw a line graph.
Plotting a graph
Put the independent variable on the x-axis (across) and the dependent variable on the y-axis (up).
Label both axes with the quantity and unit. Choose a scale that uses more than half the grid.
Plot points with small crosses, then draw a line of best fit: a straight line or smooth curve passing as close as possible to the points, ignoring anomalies. Do not join the dots.
Label both axes with the quantity and unit. Choose a scale that uses more than half the grid.
Plot points with small crosses, then draw a line of best fit: a straight line or smooth curve passing as close as possible to the points, ignoring anomalies. Do not join the dots.
Reading a graph
A straight line through the origin shows the two variables are directly proportional: double one and the other doubles.
gradient = change in y ÷ change in x
On a distance-time graph the gradient is the speed.
gradient = change in y ÷ change in x
On a distance-time graph the gradient is the speed.
A distance-time graph is a straight line from (0 s, 0 m) to (10 s, 50 m). Find the speed.
- gradient = change in distance ÷ change in time
- = 50 ÷ 10
Answer: 5 m/s
Independent on x, dependent on y, both labelled with units. Draw a line of best fit. A straight line through the origin means directly proportional. Gradient = change in y ÷ change in x.
Check you have got it
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