Factors affecting braking distance 1 — AQA GCSE Physics
Test yourself on Factors affecting braking distance 1 with AQA GCSE practice questions.
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Factors affecting braking distance 1 explained
The braking distance depends on the friction force available at the tyres and at the brakes.
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On a wet or icy road, a film of water or ice between tyre and surface lowers the friction, so the maximum braking force is smaller. Worn tyres with shallow tread cannot clear water, and worn brake pads or discs grip the wheel less firmly. In each case the same kinetic energy, ½ m v², has to be removed by a smaller force. Since work done = braking force × braking distance, a smaller force means the distance must be larger. Greater mass works the same way, because more kinetic energy has to be removed in the first place. Worn pads also overheat and fade sooner, so the braking force can fall further part way through a long stop.
Adverse road conditions include wet or icy conditions. Poor condition of the vehicle is limited to the vehicle's brakes or tyres.
Adverse road conditions include wet or icy conditions, which reduce friction between the tyres and the road. For the purpose of this specification, poor vehicle condition is strictly limited to the vehicle's brakes or tyres. This distinction matters in exam questions: driving on a wet road and a tired driver both lengthen stopping distance, but for different reasons. Worn tyres cannot channel water away, so the tyre rides on a film of water and friction drops; worn pads cannot press on the disc as firmly, so braking force falls. Both leave the driver's reaction time untouched. Questions often give one factor of each kind and ask which distance it changes. Fitting new brakes or tyres shortens the braking distance.
Students should be able to: explain the factors which affect the distance required for road transport vehicles to come to rest in emergencies, and the implications for safety estimate how the distance required for road vehicles to stop in an emergency varies over a range of typical speeds.
An emergency stop question usually wants factors, mechanisms and a safety conclusion. Deal with each factor by saying which distance it changes and why: a wet road or worn tyres reduce friction and lengthen the braking distance; tiredness, alcohol or a phone lengthen the reaction time and so the thinking distance. Then estimate, using thinking distance = speed × reaction time and scaling the braking distance with the square of the speed. The safety implication follows from the numbers rather than from advice: because stopping from 30 m/s takes roughly six times the distance needed from 10 m/s, speed limits and larger gaps in poor conditions matter far more than drivers expect.
Your focus
- Explain why a wet road increases the braking distance, starting from the friction at the tyres.
- Link a reduced braking force to a longer braking distance using work done = force × distance.
- Give one road condition and one vehicle condition that lengthen the braking distance, and say why each does.
Show all 9 objectives
- Select, from a list of options, the two changes that would increase a car's stopping distance.
- Explain why worn tyres lengthen the braking distance on a wet road but not on a dry one.
- State which of the specified factors act on thinking distance and which act on braking distance.
- Explain, for two named factors, how each changes the distance a vehicle needs to stop in an emergency.
- Estimate stopping distances at 10 m/s, 20 m/s and 30 m/s and use them to justify a lower speed limit near a school.
- Justify leaving a larger gap in wet conditions using the effect of reduced friction on braking distance.
Factors affecting braking distance 1 exam tips
Marking Points
- one mark for naming wet or icy road conditions as an adverse condition
- one mark for naming worn brakes or worn tyres as poor vehicle condition
- one mark for stating that the friction force between the tyres and the road is reduced
- one mark for linking the smaller braking force to the longer distance needed to remove the kinetic energy
- one mark for selecting wet or icy conditions as an adverse road factor
- one mark for selecting worn brakes or worn tyres as a vehicle factor
- one mark for rejecting new brakes or new tyres, which shorten the braking distance
- one mark for stating that tiredness changes the thinking distance while a wet road changes the braking distance
- one mark for naming a factor and stating whether it changes the thinking or the braking distance
- one mark for the mechanism, reduced friction at the tyres or a longer reaction time
- one mark for an estimate using thinking distance = speed × reaction time
- one mark for scaling the braking distance with the square of the speed
- one mark for a safety implication that follows from the figures, such as leaving a longer gap in the wet
Examiner Tips
- 💡Build the chain: condition, then friction, then braking force, then distance.
- 💡Use the words 'friction between the tyres and the road' rather than just 'grip'.
- 💡If the question offers marks for an explanation, bring in work done = force × distance.
- 💡In tick-two questions, eliminate the options that would shorten the distance before choosing.
- 💡Remember that for this specification, poor vehicle condition is limited to brakes and tyres.
- 💡If asked to explain how worn tyres affect stopping distance, specify that they increase braking distance but do not affect thinking distance.
- 💡Structure a long answer as factor, mechanism, distance affected; repeat for each factor.
- 💡Quote at least one number in a safety discussion; comparisons of distances earn credit.
- 💡Say explicitly which distance you are changing each time you name a factor.
Common Mistakes
- answering 'bad weather' alone, which the mark scheme ignores
- saying ice makes the car heavier or the brakes colder
- explaining a braking factor through the driver's thinking distance
- claiming the brakes stop working altogether on an icy road
- ticking 'fitting new tyres' as something that increases the stopping distance
- assuming poor vehicle condition includes factors other than brakes and tyres, when the specification limits it to these two
- listing fog as a factor affecting braking distance, when it affects what the driver can see (thinking distance) rather than the grip of the tyres
- listing factors with no mechanism attached to any of them
- applying the reaction time argument to a wet road
- offering advice such as 'drive carefully' in place of a physical reason
- estimating with a reaction time or a vehicle mass far outside typical values