AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Forces and braking
Practise Factors affecting braking distance 2. 14 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, explain the dangers of large decelerations and (Higher) estimate the forces involved in braking.
Grade by grade
What you need to be able to do, from the first marks up to the top 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.
5
Calculate the work done by a braking forceW = F s, where s is the braking distance.
5
Link braking force to decelerationThe greater the braking force, the greater the deceleration (F = m a).
6
Link speed to the braking force neededA faster vehicle needs a greater braking force to stop in the same distance.
6
Explain the dangers of large decelerationsThe brakes may overheat, and the vehicle may skid so the driver loses control.
7
Calculate a braking force from kinetic energyBraking force × braking distance = ½ m v2.
8
Estimate braking forces for typical road vehiclesUse typical masses and speeds with F = m a or work done = kinetic energy.
Notes
Energy transfer when braking
When a force is applied to the brakes, work is done by the friction force between the brakes and the wheels.
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.
Estimating braking forces grade 8+
Use typical values: car mass ~1000 kg, speeds ~10 m/s to 30 m/s, braking distances of tens of metres.
Energy method: braking force = ½ m v2 ÷ braking distance.
Motion method: find the deceleration with v2 − u2 = 2 a s or a = Δv ÷ t, then use F = m a.
Example: a 1000 kg car stopping from 20 m/s in 40 m: F = ½ × 1000 × 202 ÷ 40 = 5000 N (~5 kN).
Cheatsheet
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
Typical car braking force: a few thousand newtons grade 8+
How to answer each type of question
Describe the energy transfer during braking
2 marks4
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 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).
Explain the dangers of large decelerations
3 marks6
Say that a large deceleration needs a large braking force.
Give the danger to the brakes: overheating.
Give the danger to control: skidding.
Example. Explain the dangers of a car braking with a very large deceleration.
Show the model answer
A very large braking force is needed, so the brakes may overheat (1) and become less effective (1). The tyres may skid on the road, so the driver may lose control of the car (1).
Calculate a braking force using kinetic energy
4 marks7
Calculate the kinetic energy with Ek = ½ m v2.
Set work done by the brakes = kinetic energy.
Use W = F s and rearrange for F.
Example. A van of mass 2000 kg is travelling at 12 m/s. It brakes and stops in a braking distance of 24 m. Calculate the average braking force.
Show the model answer
Ek = 0.5 × 2000 × 122 (1) Ek = 144 000 J (1) 144 000 = F × 24 (1) F = 6000 N (1)
Estimate a braking force
3 marks8
Use the values given, or state sensible estimates.
Find the deceleration (or the kinetic energy).
Use F = m a (or F = Ek ÷ distance) and round sensibly.
Example. A fully loaded bus has a mass of about 15 000 kg. Estimate the braking force needed to stop it from 10 m/s in a distance of 25 m.
Show the model answer
0 − 102 = 2 × a × 25, so the deceleration is 2 m/s2 (1) F = 15 000 × 2 (1) F ≈ 30 000 N (~30 kN) (1)
Shortcuts and 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.
Where marks are lost
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).
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.
In estimates, state the values you assume and give the answer to 1 or 2 significant figures.
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
Question 3Hard7 marks
A car of mass 1500 kg travelling at 25 m/s brakes to a stop. 80% of the car’s kinetic energy is transferred to the thermal energy stores of its four brake discs. Each disc has a mass of 6.0 kg. The specific heat capacity of the disc material is 460 J/kg °C.
(a) Calculate the kinetic energy of the car before braking.[2]
(b) Calculate the increase in temperature of the brake discs. Assume that the energy is shared equally between the four discs. Use the Physics Equations Sheet.[3]
(c) Explain why braking repeatedly while driving down a long, steep hill can be dangerous.[2]
Show the answer and mark scheme
(a)Answer: 468 750 J
Ek = 0.5 × 1500 × 252
468 750 (J)
(b)Answer: 34 °C
energy transferred to the discs = 0.80 × 468 750 = 375 000 (J)
375 000 = (4 × 6.0) × 460 × Δθ
Δθ = 34 (°C)
(c)Answer: The brakes may overheat and become less effective, so the car takes longer to stop.
the brakes can overheat / get very hot
so the brakes become less effective / the braking force decreases, so the braking distance increases