Momentum

    OCR
    GCSE

    Momentum is a vector quantity defined as the product of mass and velocity ($p=mv$), necessitating precise directional analysis during calculation. The Principle of Conservation of Momentum asserts that in a closed system, the total momentum before an event equals the total momentum after, a fundamental law governing collisions and explosions. Advanced application links Newton's Second Law to the rate of change of momentum ($F = \Delta p / \Delta t$), providing the theoretical basis for impact safety features such as crumple zones and airbags by analyzing the relationship between force, time, and impulse.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for correct substitution into p = m × v with mass in kg and velocity in m/s
    • Credit responses that explicitly state 'total momentum before = total momentum after' in conservation questions
    • For Higher Tier collision problems, award marks for correctly assigning a negative sign to velocities moving in the opposite direction
    • In safety explanations, candidates must link increased contact time to a reduced rate of change of momentum to score full marks
    • Award 1 mark for the final answer with correct significant figures, usually matching the data provided

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You calculated the momentum correctly, but check your units—mass must be in kg."
    • "Remember that momentum is a vector; if the object rebounds, the velocity changes sign."
    • "Your explanation of the airbag is vague. Use the phrase 'rate of change of momentum' to secure the marks."
    • "Good use of the conservation principle. Ensure you show the step where you equate initial and final momentum explicitly."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correct substitution into p = m × v with mass in kg and velocity in m/s
    • Credit responses that explicitly state 'total momentum before = total momentum after' in conservation questions
    • For Higher Tier collision problems, award marks for correctly assigning a negative sign to velocities moving in the opposite direction
    • In safety explanations, candidates must link increased contact time to a reduced rate of change of momentum to score full marks
    • Award 1 mark for the final answer with correct significant figures, usually matching the data provided

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 'Show that' questions, you must display the unrounded value from your calculation before giving the final answer to prove you didn't work backwards
    • 💡Always draw a simple diagram with arrows indicating direction before starting a conservation of momentum calculation to ensure signs are correct
    • 💡For 6-mark questions on safety features, structure your answer: Identify the feature → State it increases collision time → State this reduces rate of change of momentum → Conclude this reduces force on the passenger

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Failing to convert mass from grams to kilograms before calculating momentum
    • Ignoring the vector nature of momentum by treating velocities as scalar quantities during head-on collisions
    • Confusing 'reduced momentum' with 'reduced rate of change of momentum' when explaining how airbags or crumple zones work
    • Rearranging the equation F = (mv - mu)/t incorrectly, often swapping the initial and final momentum

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Vector nature of momentum and calculation ($p=mv$)
    Conservation of momentum in closed systems (collisions and explosions)
    Relationship between force and rate of change of momentum (Newton's Second Law)
    Impulse and impact forces in safety contexts

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Show
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG P3","title":"Investigation of Force and Acceleration","relevance":"Provides experimental context for Newton's Second Law, linking F=ma to rate of change of momentum"}

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