Stopping distances: thinking distance and braking distance, the factors that affect each, and the link between braking, work done and road safety.

A focused answer to AQA GCSE Physics 4.5.6, covering stopping distance as the sum of thinking and braking distance, the factors that affect each part, and how braking transfers kinetic energy to heat through work done by the friction force.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Stopping distance
Thinking distance
Braking distance
Braking, work and energy
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What this dot point is asking

AQA wants you to define stopping distance as the sum of thinking distance and braking distance, list the factors affecting each, and explain how braking transfers the car's kinetic energy through work done by friction.

Stopping distance

Thinking distance

A typical reaction time is a few tenths of a second, but the distance covered in that time can be large at speed because the car keeps moving at its full speed until the brakes are applied. The thinking distance is directly proportional to speed (from $s = vt$ with a fixed reaction time), so doubling the speed doubles the thinking distance. The factors that lengthen the reaction time, such as tiredness, alcohol, drugs and distraction, all affect the driver rather than the car, so they increase only the thinking distance and leave the braking distance unchanged. This is a distinction AQA tests directly, so always check whether a given factor affects the driver (thinking) or the car and road (braking).

Braking distance

It is worth noting the difference in how the two distances grow with speed. Thinking distance is found from $s = vt$ , where $t$ is the reaction time, so it is directly proportional to speed: double the speed and the thinking distance doubles. Braking distance depends on the kinetic energy, $E_k = \frac{1}{2}mv^2$ , which is proportional to the square of the speed, so doubling the speed roughly quadruples the braking distance. This is why speed limits matter so much and why the gap between a safe and an unsafe following distance widens rapidly on faster roads.

Braking, work and energy

A large braking force is needed to stop a fast car in a short distance, but there is a limit: too large a braking force can cause the tyres to skid (lock up), which actually lengthens the stopping distance and means the driver loses steering control. This is why modern cars use anti-lock braking systems. The work done by the braking force equals the force multiplied by the braking distance, and this equals the kinetic energy removed, which gives a neat link between this dot point and the work-energy and kinetic-energy equations.

Try this

Q1. State the two parts that make up the stopping distance. [2 marks]

Cue. Thinking distance and braking distance.

Q2. Give one factor that increases the thinking distance and one that increases the braking distance. [2 marks]

Cue. Thinking distance: tiredness, alcohol, drugs or distraction. Braking distance: wet or icy roads, worn tyres or worn brakes.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksA car is travelling at

20\,\text{m/s}

when the driver sees a hazard. The driver's reaction time is

0.70\,\text{s}

. Calculate the thinking distance, and explain why the braking distance would more than double if the car had instead been travelling at

40\,\text{m/s}

Show worked answer →

The thinking distance is the distance travelled during the reaction time at the original speed, found using $s = vt = 20 \times 0.70 = 14\,\text{m}$ (2 marks). The braking distance more than doubles when the speed doubles because the kinetic energy of the car is given by $\frac{1}{2}mv^2$ , which depends on the square of the speed (1 mark). Doubling the speed gives four times the kinetic energy, and the braking force must do four times as much work to stop the car, so for a roughly constant braking force the braking distance becomes about four times as large, not twice (1 mark). Markers reward the thinking distance calculation and the link from the $v^2$ dependence of kinetic energy to the braking distance.

AQA 20214 marksDescribe the energy transfer that takes place when a car brakes, and explain why heavy braking from a high speed can cause the brakes to overheat.

Show worked answer →

When the brakes are applied, a friction force acts between the brake pads and the discs (or wheels) (1 mark). This friction force does work, transferring energy from the car's kinetic store to the thermal store of the brakes, so the brakes get hot (1 mark). Braking from a high speed means the car has a large kinetic energy (because $E_k = \frac{1}{2}mv^2$ ), so a large amount of energy must be transferred to the thermal store in a short time (1 mark). If this happens too quickly the brakes cannot lose heat to the surroundings fast enough, so their temperature rises sharply and they can overheat (1 mark). Markers reward identifying friction doing work, the kinetic-to-thermal transfer, and the link to a large kinetic energy at high speed.

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Sources & how we know this

AQA GCSE Physics (8463) specification — AQA (2016)