AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy stores and systems: a vehicle slowing down
When a vehicle slows down, its store of kinetic energy decreases because its speed falls.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a vehicle slows down, its store of kinetic energy decreases because its speed falls. The energy is not lost: it is transferred by mechanical work done against resistive forces, mainly friction from the brakes and air resistance. Brake friction transfers energy to the thermal energy store of the brake discs and pads, while air resistance transfers energy to the thermal energy store of the surrounding air. The vehicle's gravitational potential energy store may also change if it moves up or down a slope.
What earns the marks
- The kinetic energy store of the vehicle decreases because the vehicle's speed decreases.
- Energy is transferred from the kinetic energy store of the vehicle to the thermal energy store of the brakes and surroundings.
- Mechanical work is done by the brakes against friction, and by air resistance against the motion of the vehicle.
- The total energy is conserved; the decrease in kinetic energy equals the increase in thermal energy stores, assuming no change in gravitational potential energy.
- If the vehicle moves along a slope, gravitational potential energy store changes must also be considered.
Common mistakes and how to put them right
Saying energy is 'used up' or 'lost' when a vehicle slows down.
Put it right: Energy is conserved and transferred to thermal energy stores of the brakes and surroundings.
Ignoring air resistance and attributing all energy transfer to the brakes.
Put it right: Both brake friction and air resistance transfer energy to thermal stores.
Forgetting that gravitational potential energy may also change if the vehicle is on a slope.
Put it right: State whether the vehicle is on a level road or a slope and include any change in height.
Exam tips
- Name the store that decreases and the store that increases, and state the transfer pathway, for example 'by mechanical work done by friction'.
- Use the conservation of energy to justify why the total energy remains constant even though the vehicle slows down.
- If a diagram or data is given, refer to it explicitly, such as brake temperature rising or stopping distance increasing.
Try the lesson
The start of the Energy stores and systems: a vehicle slowing down lesson, made from checked teaching notes for this specification point.
Have a go first
A car is moving along a road and then slows down as the driver applies the brakes. Which energy store of the car decreases as the car slows down?
Listen
When a vehicle slows down, its store of kinetic energy decreases because its speed falls. The energy is not lost: it is transferred by mechanical work done against resistive forces, mainly friction from the brakes and air resistance. Brake friction transfers energy to the thermal energy store of the brake discs and pads, while air resistance transfers energy to the thermal energy store of the surrounding air. The vehicle's gravitational potential energy store may also change if it moves up or down a slope. A strong answer earns marks for these points. The kinetic energy store of the vehicle decreases because the vehicle's speed decreases. Energy is transferred from the kinetic energy store of the vehicle to the thermal energy store of the brakes and surroundings. Mechanical work is done by the brakes against friction, and by air resistance against the motion of the vehicle. The total energy is conserved; the decrease in kinetic energy equals the increase in thermal energy stores, assuming no change in gravitational potential energy. A common slip: Saying energy is 'used up' or 'lost' when a vehicle slows down. Instead, energy is conserved and transferred to thermal energy stores of the brakes and surroundings. Exam tip: Name the store that decreases and the store that increases, and state the transfer pathway, for example 'by mechanical work done by friction'.
Key words
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Conservation of energy
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Mechanical work and friction
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Thermal energy and dissipation
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Energy stores and transfers
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See every definition
- Conservation of energy
- The principle that the total energy is conserved, so the decrease in kinetic energy equals the increase in thermal energy stores, assuming no change in gravitational potential energy.
- Mechanical work and friction
- Energy is transferred by mechanical work done against resistive forces, mainly friction from the brakes and air resistance.
- Thermal energy and dissipation
- Brake friction transfers energy to the thermal energy store of the brake discs and pads, while air resistance transfers energy to the thermal energy store of the surrounding air.
- Energy stores and transfers
- The kinetic energy store of the vehicle decreases because its speed falls, and the energy is transferred to the thermal energy store of the brakes and surroundings.
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