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School-college Physics Notes: Forces & motion 5.1 Vehicle stopping distance

GCSE level Physics exam revision notes on Forces & Motion Part 5

5.1 Introduction to Road safety - defining & explaining the stopping distance & speed of a road vehicle - diagram and table of values

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics [forces-motion-5- updated Mar 26th 2026]

[KEY POINTS and learning objectives for this page, after initial notes]

INDEX of physics notes: reaction times, stopping distances of road vehicles, Newton's 2nd Law, KE calculations

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5.1 Introduction to stopping distances and the speed of road vehicles

What is the formula for stopping distance?

When driving a car you obviously need to be alert to any sudden change in your situation, particularly if you have to do an emergency brake to come to a halt.

In this situation you want to stop the car (or any other road vehicle) in the shortest time as possible to effect an appropriate emergency stop!

This means applying the maximum force on the brake pedal.

 

The longer it takes for you to react and longer it takes to come to a halt, the greater the risk of crashing into an object in your path. Everybody's 'thinking' reaction time to a situation requiring a quick physical response is different, though typically in the range 0.2 to 0.8 seconds. In biology you may have studied the nervous system including the reflex arc (GCSE level biology notes).

The distance it takes to stop a road vehicle in an emergency situation is given by the following formula:

STOPPING DISTANCE = THINKING DISTANCE + BRAKING DISTANCE

The thinking distance is how far you travel during your reaction time, which is the time interval from you perceiving a hazard and actually starting to take action e.g. apply the brakes.

The braking distance is the actual distance you travel from when you first apply the brakes until you come to a halt.

The stopping distance is the total time required from initial visual stimulus to actually stopping moving.

The chart above gives typical or average values for thinking distances, braking distances and stopping distance and are quoted from the UK Highway Code guidance booklet.

You can see the thinking distance is quite an appreciable portion of the total stopping distance, particularly at lower speeds, BUT, look how dramatically the total stopping distance increases with increase in speed.

These values should be doubled for wet roads and multiplied by 10 for ice coated roads. Snow would be somewhere in between, but where?, so just take great care in driving in any of these adverse driving conditions.

Later in this page I've used this data to do some graphs and calculations relating stopping distance to speed and the kinetic energy of a car.

INDEX of physics notes on reaction times, stopping distances of road vehicles, Newton's 2nd Law, braking friction force, KE calculations

Key points force and motion: Road safety - defining stopping distance

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A clear and exam-focused summary of stopping distance in the context of GCSE Physics road safety, tailored to major UK exam boards: WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

Stopping Distance – Definition

Stopping distance is the total distance a vehicle travels from the moment a driver spots a hazard to the moment the vehicle comes to a complete stop.

It is made up of two components:
Stopping Distance = Thinking Distance + Braking Distance

Components of stopping distance explained

Thinking Distance

  • Distance travelled while the driver reacts to a hazard.
  • Depends on:
    • Reaction time (typically 0.7–1.5 seconds)
    • Driver alertness (fatigue, distractions, alcohol/drugs)
    • Speed of the vehicle

Braking Distance

  • Distance travelled after brakes are applied until the vehicle stops.
  • Depends on:
    • Vehicle speed
    • Road conditions (wet, icy, gravel)
    • Tyre and brake condition
    • Vehicle mass

Typical Exam Board content about stopping distance

Key Focus Areas
Define and calculate stopping distance; explain factors affecting thinking and braking distance
Apply stopping distance to road safety scenarios; understand effects of speed and driver condition
Extended tier: analyse stopping distance graphs; apply to motion equations
Required practicals on reaction time; interpret graphs and evaluate safety implications
Use stopping distance in context of forces and energy; calculate braking force using F = ma
Gateway Science: explain stopping distance with examples; apply Newton’s laws and energy transfer

Graphical Interpretation of stopping distance

  • Thinking distance increases linearly with speed.
  • Braking distance increases non-linearly (quadratically) with speed.
  • Total stopping distance curve steepens with higher speeds.

Keywords, phrases and learning objectives for the physics of road vehicles - stopping distance

Know that stopping distance = thinking distance + braking distance and the factors that might affect them.

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INDEX of physics notes on reaction times, stopping distances of road vehicles, Newton's 2nd Law, braking friction force, KE calculations

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