AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Forces › Forces and motion › Forces and braking
Practise Factors affecting braking distance 2. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
When a vehicle brakes, work done by friction between the brakes and the wheels reduces its kinetic energy and heats the brakes. You need to explain this energy transfer, link braking force to speed and deceleration, and explain the dangers of large decelerations.
Key facts
Braking: work done by friction reduces kinetic energy, and the brakes get hotter
Work done by brakes = braking force × braking distance = kinetic energy lost
Ek = ½ m v2
Greater speed → greater braking force needed to stop in the same distance
Greater braking force → greater deceleration (F = m a)
Large decelerations: brakes overheat, skidding, loss of control
Notes
Energy transfer when braking
diagram
When a force is applied to the brakes, work is done by the friction force between the brakes and the wheels.
Braking: friction between the pads and the disc does work, transferring kinetic energy to thermal energy.
This work done reduces the kinetic energy of the vehicle.
The energy is transferred to the thermal energy store of the brakes, so the temperature of the brakes increases.
Work done by the brakes = braking force × braking distance = kinetic energy lost.
Braking force, speed and deceleration
The greater the speed of a vehicle, the greater the braking force needed to stop it in a certain distance, because it has more kinetic energy.
The greater the braking force, the greater the deceleration of the vehicle (F = m a).
For the same braking force, a vehicle with a greater mass has a smaller deceleration.
Dangers of large decelerations
Large decelerations need large braking forces, which may make the brakes overheat and work less well.
The wheels may lock and the tyres skid, so the driver may lose control of the vehicle.
Large decelerations also mean large forces on the people inside, which can cause injuries.
How to answer each type of question
Describe the energy transfer during braking
2 marksGrade 4
Say that work is done by friction between the brakes and the wheels.
Name the stores: from kinetic to thermal (the brakes get hotter).
Example. Describe the energy transfer when a moving car brakes to a stop.
Show the model answerHide the model answer
Work is done by the friction force between the brakes and the wheels (1). Energy is transferred from the kinetic energy store of the car to the thermal energy store of the brakes, so the brakes get hotter (1).
Don’t lose marks
Saying the kinetic energy is 'lost' or 'used up'. It is transferred to the thermal store of the brakes (and the surroundings).
Forgetting to square the speed in Ek = ½ m v2.
Just writing 'it is dangerous' for large decelerations. Name the dangers: brakes overheating, skidding, loss of control.
Mixing up the braking distance (m) with the braking force (N).
More tips
Memory tricks
Braking chain: friction → work done → kinetic energy decreases → brakes heat up.
Braking force × braking distance = ½ m v2 links force, distance, mass and speed in one line.
Double the speed means four times the kinetic energy, so four times the braking force to stop in the same distance.
Exam technique
Use the terms 'work done', 'kinetic energy store' and 'thermal energy store'.
Ek = ½ m v2 and W = F s must both be recalled; they are often combined in 4-mark questions.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Describe the energy transfer when a vehicle brakesEnergy is transferred from the kinetic store of the vehicle to the thermal store of the brakes.
Grade 5
Calculate the work done by a braking forceW = F s, where s is the braking distance.
Grade 5
Link braking force to decelerationThe greater the braking force, the greater the deceleration (F = m a).
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What happens to the temperature of the brakes?
It increases.
A car’s brakes exert a force of 4500 N over a distance of 15 m. Calculate the work done by the brakes. Use the equation: work done = force × distance
67 500 J
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) When the brakes of a moving car are applied, work is done by the friction force between the brakes and the wheels. What happens to the kinetic energy of the car? Tick (✓) one box.[1]
It decreases
It increases
It stays the same
(b) What happens to the temperature of the brakes?[1]
(c) The brakes of a car exert a braking force of 6000 N while the car travels 20 m. Calculate the work done by the brakes. Use the equation: work done = force × distance[2]
Show the answer and mark scheme
(a)Answer: It decreases
(b)Answer: It increases.
it increases
(c)Answer: 120 000 J
W = 6000 × 20
120 000 (J)
Question 2Medium7 marks
A car of mass 1200 kg is travelling at 15 m/s. The driver brakes and the car stops in a braking distance of 18 m.
(a) Calculate the kinetic energy of the car before braking.[2]
(b) Calculate the average braking force.[3]
(c) Describe the energy transfer that takes place as the car brakes.[2]
Show the answer and mark scheme
(a)Answer: 135 000 J
Ek = 0.5 × 1200 × 152
135 000 (J)
(b)Answer: 7500 N
work done by the brakes = kinetic energy lost = 135 000 J
135 000 = F × 18
F = 7500 (N)
(c)Answer: Work done by friction in the brakes transfers energy from the car’s kinetic store to the thermal store of the brakes.
work done by the friction force (between the brakes and the wheels) reduces the kinetic energy of the car
energy is transferred to the thermal energy store of the brakes / the temperature of the brakes increases