Newtons Third Law

    AQA
    GCSE

    Newton's Third Law defines the nature of force interaction pairs, stating that forces always occur in pairs of the same type, equal in magnitude and opposite in direction, acting simultaneously on two different bodies. Candidates must rigorously distinguish these interaction pairs from balanced forces acting on a single body (Newton's First Law) to avoid the common error of assuming they cancel out. Mastery of this concept is the theoretical foundation for deriving the conservation of momentum and analyzing propulsion in systems ranging from rocketry to vehicular collisions.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that forces are equal in magnitude and opposite in direction
    • Credit responses that explicitly identify the two different bodies involved (e.g., 'Force of A on B and Force of B on A')
    • Award 1 mark for identifying that the interaction pair consists of forces of the same type (e.g., both gravitational)
    • In collision contexts, award marks for stating the force exerted by the smaller object is exactly equal to the force exerted by the larger object
    • Credit the correct drawing of vector arrows starting from the respective objects and pointing in opposite directions

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly identified the forces are equal, but you must specify they act on *different* objects to gain the mark"
    • "Be careful: Weight and Reaction force act on the same object, so they are not a Third Law pair. Look for the Earth pulling the object and the object pulling the Earth"
    • "Excellent identification of the pair. Now explain why the lighter object accelerates away faster despite the force being the same, referencing mass"
    • "Use the phrase 'Force of [Object A] on [Object B]' to ensure precision in your answer"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that forces are equal in magnitude and opposite in direction
    • Credit responses that explicitly identify the two different bodies involved (e.g., 'Force of A on B and Force of B on A')
    • Award 1 mark for identifying that the interaction pair consists of forces of the same type (e.g., both gravitational)
    • In collision contexts, award marks for stating the force exerted by the smaller object is exactly equal to the force exerted by the larger object
    • Credit the correct drawing of vector arrows starting from the respective objects and pointing in opposite directions

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Adopt the sentence structure 'The force of [Object A] on [Object B] is equal and opposite to the force of [Object B] on [Object A]' to ensure you describe different bodies
    • 💡When asked why a lighter object moves faster after a collision despite equal forces, explicitly link Newton's Third Law ($F_1 = -F_2$) to Newton's Second Law ($a = F/m$)
    • 💡Check the number of objects: if the forces act on the *same* object, it is an equilibrium problem, not a Third Law pair

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Conflating Newton's Third Law pairs with balanced forces (equilibrium) acting on a single object, such as Weight and Normal Contact Force on a stationary book
    • Assuming that a larger or faster object exerts a greater force during a collision than a smaller or slower object
    • Describing the forces vaguely as 'action and reaction' without specifying the objects involved
    • Failing to recognize that interaction pairs must be the same type of force (e.g., pairing a gravitational force with a contact force)

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

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    Practical Links

    Related required practicals

    • •{"code":"Required Practical 7","title":"Investigation of Force and Acceleration","relevance":"Demonstrates interaction forces and inertia, supporting theoretical understanding of N3L"}

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