GCSE Physics · Edexcel 1PH0 · Topic 2D
Stopping distances
Key points
- Stopping distance = thinking distance + braking distance.
- Thinking distance is how far the vehicle travels during the driver's reaction time. It increases with speed, tiredness, alcohol, drugs and distractions such as phones.
- Braking distance increases with speed, vehicle mass, wet or icy roads and worn tyres or brakes.
- Doubling the speed doubles the thinking distance but roughly quadruples the braking distance, because kinetic energy depends on .
- Brakes do work against the vehicle's kinetic energy: . Large decelerations mean large forces, which can injure passengers or overheat brakes.
- Typical reaction times are 0.2–0.9 s; they can be measured with a ruler-drop test.
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How far a car travels before it stops depends on the driver as well as the vehicle and road. This sub-topic applies forces, reaction time and energy to one of the most important real-life uses of physics.
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