AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
When a force is applied to the brakes of a vehicle, work done by the…
Full specification wording
Factors affecting braking distance 2: When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
Braking transfers energy from the vehicle's kinetic energy store to the thermal energy store of the brakes.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Braking transfers energy from the vehicle's kinetic energy store to the thermal energy store of the brakes. The brakes apply a friction force to the wheel, and the vehicle travels a braking distance while that force acts. Work done is force × distance, so the friction force does work against the vehicle's motion. This work reduces the kinetic energy store of the vehicle, slowing it down. Because energy is conserved, the energy removed does not disappear: it is transferred to the brakes and surroundings, so the brakes become hotter. A larger braking force does more work over the same distance, removing kinetic energy faster and increasing brake temperature more. This is why brakes can overheat and fade during repeated heavy braking.
What earns the marks
- Identify the friction force between the brakes and the wheel as the force that acts to slow the vehicle.
- State that work done = force × distance moved in the direction of the force, and apply it to the braking distance.
- Explain that the work done by friction reduces the kinetic energy store of the vehicle.
- Explain that the energy transferred is not lost but heats the brakes, increasing their temperature.
- Link greater braking force or greater speed to more work done and a larger temperature rise in the brakes.
Common mistakes and how to put them right
Saying energy is used up or destroyed.
Put it right: Energy is conserved and is transferred to the thermal energy store of the brakes and surroundings.
Confusing force with energy.
Put it right: The friction force does work, and work done is measured in joules, while force is measured in newtons.
Ignoring the braking distance when calculating work done.
Put it right: Work done by the friction force depends on both the force and the distance travelled while braking.
Exam tips
- Use the equation work done = force × distance and state the distance as the braking distance.
- Describe the energy transfer chain: kinetic energy store of vehicle → thermal energy store of brakes and surroundings.
- Use the phrase temperature of the brakes increases rather than saying heat is created.
Try the lesson
The start of the Factors affecting braking distance 2: When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases. lesson, made from checked teaching notes for this specification point.
Have a go first
A car is moving along a road. The driver presses the brake pedal, so a friction force acts between the brakes and the wheel. What happens to the kinetic energy of the car and to the temperature of the brakes?
Listen
Braking transfers energy from the vehicle's kinetic energy store to the thermal energy store of the brakes. The brakes apply a friction force to the wheel, and the vehicle travels a braking distance while that force acts. Work done is force × distance, so the friction force does work against the vehicle's motion. This work reduces the kinetic energy store of the vehicle, slowing it down. Because energy is conserved, the energy removed does not disappear: it is transferred to the brakes and surroundings, so the brakes become hotter. A larger braking force does more work over the same distance, removing kinetic energy faster and increasing brake temperature more. This is why brakes can overheat and fade during repeated heavy braking. A strong answer earns marks for these points. Identify the friction force between the brakes and the wheel as the force that acts to slow the vehicle. State that work done = force × distance moved in the direction of the force, and apply it to the braking distance. Explain that the work done by friction reduces the kinetic energy store of the vehicle. Explain that the energy transferred is not lost but heats the brakes, increasing their temperature. A common slip: Saying energy is used up or destroyed. Instead, energy is conserved and is transferred to the thermal energy store of the brakes and surroundings. Exam tip: Use the equation work done = force × distance and state the distance as the braking distance.
See it
Key words
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Work done by a friction force
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Conservation of energy during braking
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Transfer from kinetic energy store to thermal energy store
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Braking force, braking distance and temperature rise
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See every definition
- Work done by a friction force
- The friction force between the brakes and the wheel does work against the vehicle's motion, and work done is force × distance.
- Conservation of energy during braking
- Energy is conserved, so the energy removed from the vehicle does not disappear but is transferred to the brakes and surroundings.
- Transfer from kinetic energy store to thermal energy store
- Braking transfers energy from the vehicle's kinetic energy store to the thermal energy store of the brakes, so the brakes become hotter.
- Braking force, braking distance and temperature rise
- A larger braking force does more work over the same distance, removing kinetic energy faster and increasing brake temperature more.
Label the diagram
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1Box 1
2Box 2
3Box 3
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