Orbital motion, natural and artificial satellites

    AQA
    GCSE

    Orbital motion is governed by the gravitational force acting as a centripetal force, pulling satellites towards the centre of the primary body. Candidates must understand that for circular orbits, while speed remains constant, velocity changes continuously due to the changing direction, implying acceleration. The topic necessitates an understanding of the relationship between orbital radius and orbital speed; specifically, that a decrease in orbital radius requires an increase in speed to maintain a stable orbit. Mastery involves applying these principles to both natural satellites (moons, planets) and artificial satellites (geostationary, polar), linking physical principles to functional applications.

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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 gravity provides the force that maintains the orbit
    • Award 1 mark for explaining that velocity changes because the direction of motion is constantly changing
    • Credit responses that state speed remains constant while velocity changes (acceleration)
    • Award 1 mark for linking a decrease in orbital radius to an increase in gravitational attraction (Higher Tier)
    • Award 1 mark for stating that orbital speed must increase to maintain a stable orbit if the radius decreases (Higher Tier)

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified that direction changes, but you must use the word 'velocity' to gain the mark for acceleration"
    • "Remember, for a stable orbit, if you move closer to the Earth, gravity is stronger, so you must move faster"
    • "Ensure you specify that gravity is the force acting towards the centre of the circle"
    • "Good use of the term 'acceleration' — now explain it in terms of vector quantities"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that gravity provides the force that maintains the orbit
    • Award 1 mark for explaining that velocity changes because the direction of motion is constantly changing
    • Credit responses that state speed remains constant while velocity changes (acceleration)
    • Award 1 mark for linking a decrease in orbital radius to an increase in gravitational attraction (Higher Tier)
    • Award 1 mark for stating that orbital speed must increase to maintain a stable orbit if the radius decreases (Higher Tier)

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked why a satellite accelerates at constant speed, explicitly mention that velocity is a vector quantity involving direction
    • 💡For Higher Tier questions on changing orbits, use the sequence: radius decreases → gravity increases → speed must increase to prevent spiraling in
    • 💡Do not confuse the force of gravity (weight) with mass; gravity acts as the centripetal force here

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that the 'speed' of a satellite changes in a circular orbit, rather than its 'velocity'
    • Failing to identify gravity as the specific force causing the acceleration, often using vague terms like 'centripetal force' without the source
    • Incorrectly suggesting that satellites in higher orbits (larger radius) travel faster; the opposite is true

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

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