Newton's Third Law

    OCR
    GCSE

    Newton's Third Law postulates that forces always occur in interaction pairs; if object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A. Crucially, these forces must be of the same fundamental type and act on different bodies, meaning they never cancel each other out within the context of a single object's motion. This principle is the theoretical basis for the conservation of momentum and explains the mechanics of propulsion, recoil, and static interactions.

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    Objectives
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    Exam Tips
    3
    Pitfalls
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    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating forces act on two different objects
    • Award 1 mark for stating forces are equal in magnitude and opposite in direction
    • Credit responses that identify the forces are of the same type (e.g., both gravitational or both contact)
    • Award 1 mark for applying the syntax: 'Force of Object A on Object B is equal to Force of Object B on Object A'

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the forces are equal and opposite, but you must specify they act on *different* objects"
    • "Be careful—you described weight and normal force on the book. These act on the *same* object, so they are not a Third Law pair"
    • "Good use of the 'A on B' structure. Now explain how this leads to the recoil effect observed"
    • "Remember that action and reaction are simultaneous; avoid implying one causes the other to happen later"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating forces act on two different objects
    • Award 1 mark for stating forces are equal in magnitude and opposite in direction
    • Credit responses that identify the forces are of the same type (e.g., both gravitational or both contact)
    • Award 1 mark for applying the syntax: 'Force of Object A on Object B is equal to Force of Object B on Object A'

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Use the 'A on B, B on A' sentence structure to guarantee you identify different objects correctly
    • 💡Check the type of force; if one is gravitational (weight) and the other is contact (normal force), they are NOT a Third Law pair
    • 💡Do not use the terms 'action' and 'reaction' unless you explicitly clarify that they happen at the exact same time

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing Newton's Third Law pairs with balanced forces acting on a single object (e.g., weight and normal reaction on a stationary book)
    • Implying a time delay between the 'action' and 'reaction' force rather than stating they are simultaneous
    • Failing to specify that the interaction pair must consist of forces of the same fundamental type

    Key Terminology

    Essential terms to know

    Interaction pairs and simultaneity
    Vector nature of forces (magnitude and direction)
    Distinction between equilibrium and Third Law pairs
    Conservation of momentum

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Describe
    Identify
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG P4","title":"Investigation of Acceleration","relevance":"Understanding tension pairs in the string connecting trolley and mass"}

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