(Physics only) Students will be required to interpret graphs relating…

The idea

Graphs of stopping distance against speed are curved because the braking part grows with the square of the speed while the thinking part grows in proportion to it. Reading one is a matter of care: check the axis quantity and unit, read up from the speed to the curve and across, and compare different vehicles only at the same speed. A loaded lorry sits above a car on such a graph because its greater mass means more kinetic energy, ½ m v², at the same speed, so a given braking force needs a longer distance to remove it. A line for a wet road sits above the dry line for the same vehicle because the friction available at the tyres is smaller. Where two curves are drawn, describe the gap between them as well as each curve, because the difference usually widens as the speed rises.

What earns the marks

• Reading a value correctly from the curve, with the unit from the axis.
• Comparing two vehicles at the same speed rather than at different speeds.
• Describing the shape, that stopping distance rises more steeply as speed increases.
• Linking the greater mass of a lorry to greater kinetic energy and so a longer braking distance.

Common mistakes and how to put them right

• Describing a curved stopping distance graph as directly proportional.

• Comparing a car at one speed with a lorry at a different speed.

• Misreading the axis scale when the gridlines are worth two or five units each.

• Saying the lorry has worse brakes rather than explaining with mass and kinetic energy.

Exam tips

• Draw the construction lines on the graph in pencil; examiners award reading marks for a visible method.
• Quote both coordinates when you describe a point, for example '72 m at 30 m/s'.
• When comparing vehicles, write one sentence for the similarity and one for the difference.