Stopping Distance, Thinking Distance and Braking Distance — AQA GCSE Physics
The total stopping distance of a vehicle is made up of the thinking distance and the braking distance.
Stopping distance
$$\text{stopping distance} = \text{thinking distance} + \text{braking distance}$$
Thinking distance
The thinking distance is the distance travelled during the driver's reaction time (from seeing a hazard to applying the brakes). It is increased by:
- tiredness,
- alcohol and drugs, and
- distractions (e.g. using a phone).
A faster speed also increases the thinking distance (more distance covered in the reaction time).
Braking distance
The braking distance is the distance travelled while the brakes are applied (the vehicle decelerating). It is increased by:
- higher speed,
- poor road conditions (wet, icy),
- poor vehicle condition (worn brakes or tyres), and
- a greater mass.
Braking distance increases more sharply with speed — roughly with the square of the speed.
Energy and braking
When a vehicle brakes, the kinetic energy is transferred to thermal energy in the brakes (which can get very hot). Larger decelerations (sudden braking) require larger forces and risk skidding or brakes overheating.
Exam tips
- Stopping distance = thinking + braking distance.
- Thinking distance ↑ with reaction time (tiredness, alcohol, distraction).
- Braking distance ↑ with speed, poor conditions, worn brakes/tyres.
- Braking transfers kinetic energy → thermal energy in the brakes.