Overview
What you’ll learn
- Split any stopping scenario into thinking + braking distance.
- List the factors for each part and say which the driver controls.
- Explain braking as a kinetic-to-thermal energy transfer.
- Show why braking distance scales with speed squared.
- Estimate decelerations and forces for typical UK speeds and masses.
Specification checklist (AQA 8463 · Edexcel 1PH0 · OCR Gateway J249)
Statements are paraphrased from the shared DfE subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only, and statements badged Physics only are examined in separate (triple) Physics, not Combined Science. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 9.1State that the stopping distance of a vehicle is the sum of the thinking distance (distance travelled during the driver’s reaction time) and the braking distance (distance travelled under the braking force).
- 9.2Describe typical values: a reaction time of around 0.2–0.9 s, and typical stopping distances at UK road speeds.
- 9.3Explain the factors that increase thinking distance: greater speed and any factor that increases reaction time (tiredness, alcohol, drugs, distractions such as mobile phones).
- 9.4Explain the factors that increase braking distance: greater speed, poor road conditions (wet or icy), poor tyre or brake condition, and greater vehicle mass.
- 9.5Explain that braking transfers energy from the kinetic store of the vehicle to the thermal store of the brakes, and that a larger braking force means a larger deceleration.
- 9.6Explain the dangers of large decelerations (brakes overheating, loss of control) and why braking distance grows with the square of the speed.
- 9.7Estimate the forces and decelerations involved in road vehicles using F = m a and the equations of motion for typical speeds and masses.
Key points
- 1Stopping distance = thinking distance + braking distance — reaction phase, then braking phase.
- 2Thinking distance = speed × reaction time (~0.2–0.9 s): rises with speed, tiredness, alcohol, drugs, distractions.
- 3Braking distance rises with speed, poor weather (wet/icy), worn tyres/brakes, greater mass.
- 4Braking transfers energy from the kinetic store of the car to the thermal store of the brakes.
- 5Doubling the speed doubles the thinking distance but quadruples the braking distance.
- 6Very hard braking risks overheated brakes and loss of control — large decelerations are dangerous.
- 7Estimate with a = Δv ÷ t, F = m a and braking distance = v² ÷ (2 a).
Notes · 1 of 3
The two phases of stopping
First you travel while deciding; then the brakes do their work.
From seeing a hazard to standing still, a car travels stopping distance = thinking distance + braking distance. The thinking distance is covered at unchanged speed during the driver’s reaction time (typically 0.2–0.9 s). The braking distance is what the brakes then need to kill the remaining speed. At 30 m/s (about 67 mph) with a 0.7 s reaction time, thinking distance alone is 30 × 0.7 = 21 m — longer than many learners expect.
Notes · 2 of 3
What stretches each distance
Some factors the driver chooses; some the road imposes.
| Increases thinking distance | Increases braking distance |
|---|---|
| Higher speed | Higher speed (squared!) |
| Tiredness | Wet or icy roads (less grip) |
| Alcohol and drugs | Worn tyres or brakes |
| Distractions (mobile phones) | Greater vehicle mass (fully loaded) |
The driver controls the speed and their state; the road controls grip. Advertisers’ stopping figures assume alert drivers on dry tarmac — the exam expects you to say what changes when those assumptions fail.
Notes · 3 of 3
Braking as an energy transfer
The brakes are a kinetic-to-thermal converter — and they can only take so much.
Braking applies a large friction force; the wheel (or disc) slows and the kinetic store of the car drains into the thermal store of the brakes. The temperature rise is why hard braking from high speed can make brakes glow — and a very large deceleration risks loss of control and faded, overheated brakes. Because kinetic energy is Eₖ = ½ m v², the energy the brakes must absorb grows with the square of speed: doubling speed multiplies the braking energy (and so, roughly, the braking distance) by four.
Revise
Key terms
The words examiners expect you to use precisely.
- Stopping distance
- Thinking distance + braking distance — the total travelled from hazard to halt.
- Thinking distance
- Distance covered during the driver’s reaction time, at unchanged speed.
- Braking distance
- Distance covered while the brakes bring the vehicle to rest.
- Reaction time
- The delay between seeing a hazard and starting to respond — typically 0.2–0.9 s.
- Deceleration
- Negative acceleration — velocity decreasing with time, in m/s².
- Braking force
- The friction force the brakes apply to slow the wheels, in newtons.
Show 2 more key termsShow fewer key terms
- Kinetic store
- The energy store of a moving object, emptied into the brakes when stopping.
- Brake fade
- Loss of braking effectiveness when the brakes overheat after repeated hard stops.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying reaction time is part of the braking distance.
The two phases are separate — braking has not started during reaction time.
Instead: Thinking distance is travelled at full speed; braking distance starts when the brakes bite.
Mistake: Saying braking distance doubles when speed doubles.
Braking distance scales with v².
Instead: Double speed → four times the braking distance (for the same deceleration).
Show 8 more mistakesShow fewer mistakes
Mistake: Listing “higher speed” under thinking-distance factors only.
Speed feeds both distances.
Instead: Speed raises thinking distance (linearly) and braking distance (squared).
Mistake: Saying wet roads make drivers react faster.
Grip affects braking, not the driver’s nerves.
Instead: Wet/icy roads reduce friction, stretching the braking distance.
Mistake: Saying a heavier car needs more thinking distance.
Mass does not touch reaction time.
Instead: Greater mass stretches the braking distance — more kinetic energy for the brakes to shed.
Mistake: Forgetting units when multiplying speed by reaction time.
Metres per second times seconds gives metres.
Instead: Keep speed in m/s and time in s so the distance comes out in m.
Mistake: Saying brakes convert kinetic energy into fuel.
It goes to heat.
Instead: Braking transfers kinetic energy to the thermal store of the brakes (and surroundings).
Mistake: Claiming large decelerations are always safer.
Beyond grip and comfort, hard braking risks skidding, overheating and loss of control.
Instead: There is a maximum safe deceleration set by grip, tyres and conditions.
Mistake: Using mph speeds in metre calculations.
Unit chaos.
Instead: Convert mph to m/s first (30 mph ≈ 13 m/s; 60 mph ≈ 27 m/s) — or the question will supply m/s.
Mistake: Assuming tiredness only matters at night.
Reaction time stretches whenever the driver is impaired.
Instead: Tiredness, alcohol, drugs and distractions all lengthen reaction time at any hour.
Check yourself
Quick quiz
24 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Define stopping distance
Stopping distance equals
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier and Physics-only questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a GCSE paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 9 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 9: thinking and braking distance, reaction-time factors, road and vehicle factors, braking energy transfer, speed-squared scaling, and force estimates in emergency stops.
Mark scheme
PROChapter 9 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How is this chapter examined?
Paper 1
1 h 45 · 100 marks
Chapters 1–7: energy, electricity, particle model, atomic structure
Paper 2
1 h 45 · 100 marks
Chapters 8–18: forces, waves, magnetism and electromagnetism, space physics
Tiers
Foundation (grades 1–5) · Higher (grades 4–9)
Higher assumes all Foundation content; the equation sheet is printed
Want a full paper? Try the Paper 1-style (Foundation), Paper 1-style (Higher), Paper 2-style (Foundation), Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
How do I remember which factors go where?
Ask “does it change the driver or the road?” Driver factors (tiredness, alcohol, phones) stretch thinking distance. Road and vehicle factors (ice, tyres, mass) stretch braking distance. Speed stretches both.
Why does braking distance scale with speed squared?
Two ways to see it: the brakes must remove Eₖ = ½mv², which quadruples when speed doubles; or the deceleration distance s = v² ÷ (2a), where the squared speed sits on top.
Are published stopping distances exact?
No — they assume an alert driver on a dry road with good brakes and tyres. Real conditions stretch them; that comparison is a favourite exam question.
Is this chapter examined on both papers?
Stopping distances sit in the Forces topic (AQA Paper 2 and equivalents). It appears on Foundation and Higher papers of all three boards.
Does QuizQuip have any connection to the exam boards?
No. AQA, Pearson Edexcel and OCR are named only to identify the specifications this content supports. QuizQuip is not affiliated with, endorsed by, or connected to AQA, Pearson Edexcel or OCR, and no past-paper or specimen material is reproduced.
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Chapter 10: Momentum and impacts (Higher)