AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Appreciation of momentum conservation issues in the context of ethical…

    Full specification wording

    Momentum: Appreciation of momentum conservation issues in the context of ethical transport design.

    Momentum conservation shapes transport safety.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 6 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Momentum conservation shapes transport safety. In a crash, total momentum is conserved, but the force on occupants depends on how quickly momentum changes, since force is the rate of change of momentum. Crumple zones, seat belts and airbags increase the time taken for the vehicle and occupants to change momentum, reducing the average force for the same momentum change. Ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact.

    For example: A car of mass 1200 kg travelling at 15 m s⁻¹ has momentum 1.8 × 10⁴ kg m s⁻¹; if it stops in 0.30 s the average force is about 6.0 × 10⁴ N, but extending the stopping time to 1.5 s reduces the average force to about 1.2 × 10⁴ N.

    What earns the marks

    • Link conservation of momentum to the total momentum of a vehicle and its occupants before and after a crash.
    • Explain that force equals the rate of change of momentum, so a longer collision time reduces the average force.
    • Describe how crumple zones, seat belts and airbags increase the time over which momentum changes.
    • Discuss ethical trade-offs between occupant safety, vehicle mass, cost, fuel use and environmental impact.
    • Use data or calculations to compare average forces for different collision times at the same momentum change.

    Common mistakes and how to put them right

    • Claiming that safety features reduce the momentum change.

      Put it right by explaining that they increase the time taken, reducing the force for the same momentum change.

    • Confusing conservation of momentum with conservation of kinetic energy in a crash.

      Put it right by stating that kinetic energy is dissipated in deformation and heating.

    • Treating ethical design as only a technical issue.

      Put it right by recognising that it also involves balancing safety, cost, mass and environmental consequences.

    Exam tips

    • Use the relationship between force, momentum change and time to justify each safety feature you describe.
    • Support ethical discussion with specific consequences, such as increased mass raising fuel consumption.
    • Keep momentum calculations separate from ethical evaluation so each part of the answer is clear.

    Try the lesson

    The start of the Momentum: Appreciation of momentum conservation issues in the context of ethical transport design. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In a crash test, a car is designed to crumple on impact. Why does crumpling reduce the force on the passengers?

    Listen

    Momentum conservation shapes transport safety. In a crash, total momentum is conserved, but the force on occupants depends on how quickly momentum changes, since force is the rate of change of momentum. Crumple zones, seat belts and airbags increase the time taken for the vehicle and occupants to change momentum, reducing the average force for the same momentum change. Ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact. For example, a car of mass 1200 kg travelling at 15 m s⁻¹ has momentum 1.8 × 10⁴ kg m s⁻¹; if it stops in 0.30 s the average force is about 6.0 × 10⁴ N, but extending the stopping time to 1.5 s reduces the average force to about 1.2 × 10⁴ N. A strong answer earns marks for these points. Link conservation of momentum to the total momentum of a vehicle and its occupants before and after a crash. Explain that force equals the rate of change of momentum, so a longer collision time reduces the average force. Describe how crumple zones, seat belts and airbags increase the time over which momentum changes. Discuss ethical trade-offs between occupant safety, vehicle mass, cost, fuel use and environmental impact. A common slip: Claiming that safety features reduce the momentum change. Instead, explaining that they increase the time taken, reducing the force for the same momentum change. Exam tip: Use the relationship between force, momentum change and time to justify each safety feature you describe.

    See it

    A top-down flowchart with six boxes. The first box says the vehicle and occupants are moving with total momentum. An arrow leads to a second box saying that in a crash total momentum is conserved. An arrow leads to a third box saying a momentum change over a short time gives a large average force. An arrow leads to a fourth box saying crumple zones, seat belts and airbags increase the time taken to change momentum. An arrow leads to a fifth box saying a longer time gives a smaller average force for the same momentum change. A final arrow leads to a sixth box saying ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact.
    It shows how conserving momentum in a crash, extending the collision time with safety features and weighing ethical trade-offs link together.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Momentum change over a short time gives a large average force
    2. 2. Crash: total momentum conserved
    3. 3. Vehicle and occupants moving: total momentum
    4. 4. Ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact
    5. 5. Longer time gives a smaller average force for the same momentum change
    6. 6. Crumple zones, seat belts and airbags increase the time taken to change momentum

    Key words

    Say what each word means before you turn the card.

    Momentum conservation in road collisions

    Try answering aloud or in your head before turning the card.

    Force as rate of change of momentum

    Try answering aloud or in your head before turning the card.

    Vehicle safety engineering

    Try answering aloud or in your head before turning the card.

    Ethical and environmental trade-offs in transport design

    Try answering aloud or in your head before turning the card.

    See every definition
    Momentum conservation in road collisions
    In a crash, total momentum is conserved, though the force on occupants depends on how quickly momentum changes.
    Force as rate of change of momentum
    The force on occupants depends on how quickly momentum changes, since force is the rate of change of momentum.
    Vehicle safety engineering
    Crumple zones, seat belts and airbags increase the time taken for the vehicle and occupants to change momentum, reducing the average force for the same momentum change.
    Ethical and environmental trade-offs in transport design
    Ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart with six boxes. The first box says the vehicle and occupants are moving with total momentum. An arrow leads to a second box saying that in a crash total momentum is conserved. An arrow leads to a third box saying a momentum change over a short time gives a large average force. An arrow leads to a fourth box saying crumple zones, seat belts and airbags increase the time taken to change momentum. An arrow leads to a fifth box saying a longer time gives a smaller average force for the same momentum change. A final arrow leads to a sixth box saying ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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