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What affects the stopping distance of a vehicle, and how do safety features reduce injury?

Thinking distance, braking distance and stopping distance, the factors that affect them, and how vehicle safety features work.

A focused answer to WJEC GCSE Physics on stopping distances, covering thinking distance, braking distance, total stopping distance, the factors that affect each, the energy transfer in braking, and how crumple zones, airbags and seatbelts reduce injury.

Generated by Claude Opus 4.89 min answer

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

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  1. What this topic is asking
  2. Thinking, braking and stopping distance
  3. Factors that affect stopping distance
  4. Energy and forces in braking
  5. Try this

What this topic is asking

WJEC wants you to define thinking, braking and stopping distances, list the factors that affect them, and explain how vehicle safety features reduce injury. This sits in topic 2.1 (with links to 2.3 and 2.4) of Unit 2 of WJEC GCSE Physics (3420).

Thinking, braking and stopping distance

The thinking distance is simply speed multiplied by reaction time, so it increases in proportion to the speed. The braking distance depends on how much kinetic energy must be removed, which depends on the speed squared, so it grows much faster than the thinking distance as speed rises. This is why the published stopping distances in the Highway Code rise so steeply: doubling the speed roughly doubles the thinking distance but roughly quadruples the braking distance, so the total more than doubles.

During braking, the work done by the braking force must equal the kinetic energy of the car, so a larger kinetic energy needs the brakes to act over a longer distance for the same force. This connects stopping distances directly to topic 2.3 work and energy: the braking force does work, W=FdW = Fd, to transfer away the kinetic energy Ek=12mv2E_k = \tfrac{1}{2}mv^2.

Factors that affect stopping distance

Energy and forces in braking

Try this

Q1. A car travels at 15 m/s15\,\text{m/s} and the driver's reaction time is 0.7 s0.7\,\text{s}. Calculate the thinking distance. [2 marks]

  • Cue. Thinking distance =speed×time=15×0.7=10.5 m= \text{speed} \times \text{time} = 15 \times 0.7 = 10.5\,\text{m}.

Q2. State one factor that increases the braking distance of a car. [1 mark]

  • Cue. Higher speed, wet or icy roads, worn tyres or worn brakes (any one).

Exam-style practice questions

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

WJEC 20194 marksState the two parts of the total stopping distance of a car and give one factor that affects each.
Show worked answer →

A topic 2.1 State question. The total stopping distance is the thinking distance plus the braking distance (1 mark each for naming the two parts). The thinking distance is affected by the driver's reaction time (increased by tiredness, alcohol, drugs or distraction) (1 mark); the braking distance is affected by speed, road conditions or the state of the brakes and tyres (1 mark). Markers reward the two distances and a valid factor for each. A common error is to give a single distance with no breakdown.

WJEC 20213 marksExplain why a faster car has a much longer braking distance.
Show worked answer →

A topic 2.1 Explain question. A faster car has more kinetic energy (Ek=12mv2E_k = \tfrac{1}{2}mv^2) (1 mark). Because kinetic energy depends on the speed squared, doubling the speed gives four times the kinetic energy (1 mark), and the brakes must do more work to transfer it all away, so the braking distance is much longer (1 mark). Markers reward the kinetic energy, the speed-squared dependence and the longer distance. A common error is to say braking distance simply doubles when speed doubles.

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