AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Forces and braking
Practise Factors affecting braking distance 1. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Braking distance is how far a vehicle travels after the brakes are applied. You need to explain how speed, road and weather conditions and the condition of the brakes and tyres affect it, discuss what this means for safety, and estimate how emergency stopping distances change over a range of typical speeds.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name factors that increase braking distanceHigher speed, wet or icy roads, worn brakes and worn tyres.
4
Explain how wet or icy roads affect brakingThere is less friction between the tyres and the road, so the braking distance is longer.
5
Explain the effects of worn tyres and brakesWorn tyres grip the road less; worn brakes produce a smaller friction force.
6
Explain the effect of speed on braking distanceA faster vehicle has more kinetic energy, so more work must be done to stop it.
6
Explain the safety implications of braking distancesLower speeds and bigger gaps are needed in bad conditions and near hazards.
7
Estimate stopping distances at typical speedsA car on a dry road needs roughly 25 m to stop from 13 m/s and roughly 100 m from 31 m/s.
8
Explain why braking distance depends on speed squaredBraking force × braking distance = ½ m v2, so for a constant force, distance ∝ v2.
Notes
What braking distance is
Braking distance is the distance a vehicle travels under the braking force, from when the brakes are applied until it stops.
Braking depends on friction: between the brake pads and the discs on the wheels, and between the tyres and the road.
Factors that increase braking distance
Greater speed: the vehicle has more kinetic energy, so the brakes must do more work to stop it.
Adverse road and weather conditions: wet or icy roads (and loose gravel or leaves) reduce the friction between the tyres and the road. The tyres may skid, so the braking force is smaller.
Poor condition of the tyres: worn tyres with too little tread grip the road less, especially in the wet, because they cannot clear the water away.
Poor condition of the brakes: worn brakes cannot produce as large a friction force, so the deceleration is smaller.
Greater mass, e.g. a fully loaded vehicle: more kinetic energy at the same speed, so a longer braking distance for the same braking force.
Safety implications
In wet or icy weather, drivers should slow down and leave a bigger gap to the vehicle in front.
Speed limits are lower where hazards are likely, such as near schools, because stopping distance increases with speed.
Regular checks of the brakes and the tyre tread keep braking distances as short as possible.
Rough, high-friction road surfaces are often laid on the approach to junctions and crossings.
Estimating emergency stopping distances grade 7+
Stopping distance increases steeply with speed. Rough values for a car on a dry road: about 25 m at 13 m/s (30 mph), about 50 m at 22 m/s (50 mph), about 100 m at 31 m/s (70 mph).
On a wet road the braking distance can be about twice as long, and on ice it can be many times longer.
Cheatsheet
Braking distance: the distance travelled under the braking force
Increased by: greater speed, wet or icy roads, worn tyres, worn brakes, greater mass
Wet or icy roads and worn tyres → less friction between the tyres and the road
Worn brakes → smaller braking force → smaller deceleration
Car on a dry road: stopping distance ~25 m at 13 m/s, ~100 m at 31 m/s
Same braking force: braking distance ∝ speed2grade 8+
How to answer each type of question
Name and explain factors affecting braking distance
2 to 4 marks4
Name a factor that affects the vehicle or the road, not the driver.
Explain its effect on friction (or on the kinetic energy to be removed).
Say that the braking distance increases.
Example. Give two factors, other than speed, that could increase the braking distance of a car. Explain why each factor increases the braking distance.
Show the model answer
An icy road (1): there is less friction between the tyres and the road, so the braking force is smaller (1). Worn brakes (1): the brakes produce a smaller friction force, so the deceleration is smaller (1).
Explain safety advice
3 marks6
Say how the condition affects friction.
Say how this affects braking distance.
Link the advice (slower speed, bigger gap) to stopping safely.
Example. Explain why drivers are advised to drive more slowly and leave bigger gaps in wet weather.
Show the model answer
Wet roads reduce the friction between the tyres and the road (1) so the braking force is smaller (the tyres may skid) and the braking distance is longer (1). Driving more slowly reduces the stopping distance, and a bigger gap gives room to stop in (1).
Estimate a stopping distance
3 marks7
Thinking distance = speed × reaction time.
Braking distance from v2 − u2 = 2 a s, with a negative acceleration.
Add the two and round sensibly.
Example. A car travels at 30 m/s on a dry road. Estimate its stopping distance. Use a reaction time of about 0.7 s and a deceleration of about 6 m/s2 when braking.
Show the model answer
thinking distance ≈ 30 × 0.7 = 21 m (1) braking distance: 0 − 302 = 2 × (−6) × s, so s = 75 m (1) stopping distance ≈ 21 + 75 ≈ 100 m (1)
Explain the effect of doubling the speed
3 marks8
Kinetic energy depends on speed squared.
The work done by the brakes must equal the kinetic energy.
Work done = force × distance, so with the same force the distance changes in the same way as the kinetic energy.
Example. The speed of a car doubles. The braking force stays the same. Explain why the braking distance becomes four times as long.
Show the model answer
Ek = ½ m v2, so doubling the speed makes the kinetic energy four times as big (1). The work done by the brakes must equal the kinetic energy (1). Work done = force × distance, so with the same force the braking distance must be four times as long (1).
Shortcuts and memory tricks
Braking distance factors are about the vehicle and the road: speed, mass, brakes, tyres, surface.
Every factor comes back to one of two ideas: more kinetic energy to remove, or less friction to remove it with.
Rough guide: wet roads can double the braking distance.
Where marks are lost
Listing tiredness, alcohol or phones as factors that affect braking distance. They affect the thinking distance.
Writing 'the road is slippery' without explaining that there is less friction between the tyres and the road.
Writing 'bad tyres' without saying what is wrong (worn, too little tread) and what it does (less grip, less friction).
Forgetting that a heavier, loaded vehicle has a longer braking distance at the same speed and braking force.
Exam technique
Structure your answer: factor → effect on friction or kinetic energy → effect on braking distance.
In estimate questions, state any values you assume and round your answer sensibly.
Say 'friction between the tyres and the road' rather than just 'grip'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Give one factor that affects the thinking distance but not the braking distance.
Tiredness (or alcohol, drugs, distractions).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Which two factors increase the braking distance of a car? Tick (✓) two boxes.[2]
An icy road
A tired driver
Worn tyres
A driver who has drunk alcohol
(b) Give one factor that affects the thinking distance but not the braking distance.[1]
(c) Explain why a wet road increases the braking distance of a car.[2]
(c)Answer: There is less friction between tyres and road, so the braking force and deceleration are smaller.
there is less friction (grip) between the tyres and the road
so the braking force is smaller / the deceleration is smaller
Question 2Medium3 marks
A car with a roof rack loaded with luggage has a much greater mass than the same car when empty. Both have identical brakes.
Explain why the loaded car needs a greater braking distance than the empty car, when braking from the same speed with the same braking force.[3]
Show the answer and mark scheme
Answer: More mass means more kinetic energy at the same speed; with the same braking force, more distance is needed to do the extra work removing it.
the loaded car has a greater mass, so (for the same speed) it has more kinetic energy
work done by the brakes = braking force × braking distance; the brakes must do more work to remove this greater kinetic energy
since the braking force is the same, a greater distance is needed to do this extra work, so the braking distance is greater
Question 3Hard9 marks
Use these typical values for a car on a dry road: reaction time of the driver ~0.7 s maximum braking deceleration ~7 m/s2
(a) Estimate the stopping distance of a car travelling at a typical town speed of 13 m/s. Use the Physics Equations Sheet.[3]
(b) Estimate the stopping distance of a car travelling at a typical motorway speed of 31 m/s.[2]
(c) The motorway speed is about 2.4 times the town speed, but the stopping distance is more than 4 times as great. Explain why.[2]
(d) On an icy road the maximum braking deceleration is about one quarter of the value on a dry road. Explain what this means for a driver travelling at 31 m/s on an icy road.[2]
Show the answer and mark scheme
(a)Answer: 21 m
thinking distance = 13 × 0.7 = 9.1 (m)
braking distance = 132 ÷ (2 × 7) = 12 (m)
stopping distance = 21 (m)
(b)Answer: 90 m
(31 × 0.7) + 312 ÷ (2 × 7) = 21.7 + 68.6
90 (m)
(c)Answer: Braking distance ∝ speed², so it increases by about 2.4² ≈ 5.7 times, while thinking distance increases 2.4 times.
thinking distance is proportional to speed, but braking distance is proportional to (speed)2
so the braking distance increases by a factor of about 2.42 ≈ 5.7
(d)Answer: The braking distance is about four times longer (≈ 275 m), so the driver must slow down and leave a much bigger gap.
the braking distance is about four times greater (about 275 m)
so the driver should travel much more slowly / leave a much bigger gap to the vehicle in front