Momentum (Physics only)

    AQA
    GCSE

    Momentum is defined as the product of an object's mass and velocity, representing a vector quantity that is strictly conserved within closed systems. The principle of conservation of momentum dictates that the total momentum before an interaction—such as a collision or explosion—must equal the total momentum after, provided no external forces act. This topic extends Newton's Second Law by defining force as the rate of change of momentum, a concept critical for analyzing impact forces and the mechanics of safety features like crumple zones and airbags.

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    Objectives
    3
    Exam Tips
    3
    Pitfalls
    5
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for explicitly stating 'total momentum before = total momentum after' in conservation calculations.
    • Award 1 mark for correct substitution into p = mv, ensuring mass is in kg and velocity in m/s.
    • Credit responses that correctly assign a negative sign to velocity vectors moving in the opposite direction to the defined positive.
    • For Higher Tier safety explanations, award marks for linking increased collision time to a reduced rate of change of momentum and consequently reduced impact force.

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for explicitly stating 'total momentum before = total momentum after' in conservation calculations.
    • Award 1 mark for correct substitution into p = mv, ensuring mass is in kg and velocity in m/s.
    • Credit responses that correctly assign a negative sign to velocity vectors moving in the opposite direction to the defined positive.
    • For Higher Tier safety explanations, award marks for linking increased collision time to a reduced rate of change of momentum and consequently reduced impact force.

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always define a positive direction (e.g., 'to the right') at the start of a collision problem to avoid sign errors.
    • 💡When explaining seatbelts or airbags, explicitly state F = (change in momentum) / time; marks are often awarded for the equation even in written explanations.
    • 💡Check units carefully; masses in grams must be converted to kilograms before calculating momentum.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Neglecting the vector nature of momentum by failing to assign a negative sign to velocities in the opposite direction.
    • Confusing momentum (kg m/s) with moment of a force (Nm) or kinetic energy (J).
    • In safety questions, stating that safety features 'absorb impact' or 'reduce energy' without referencing the extension of collision time or reduction in rate of change of momentum.

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    State
    Determine
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    Practical Links

    Related required practicals

    • •{"code":"Technique","title":"Investigation of collisions","relevance":"Using air tracks and light gates to verify conservation of momentum"}

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