Moons

    OCR
    GCSE

    Moons are defined as natural satellites orbiting planets, distinct from artificial satellites, held in stable circular orbits by gravitational attraction. This topic requires analysis of circular motion mechanics, specifically identifying gravity as the centripetal force that causes a change in velocity (direction) while speed remains constant. Candidates must evaluate the inverse relationship between orbital radius and orbital speed required to maintain a stable orbit within a gravitational field.

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    Objectives
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    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining a moon as a natural satellite that orbits a planet
    • Award 1 mark for stating that gravitational force (or weight) provides the centripetal force acting towards the centre of the orbit
    • Credit responses that explain acceleration occurs because velocity changes due to changing direction, even if speed is constant
    • Award 1 mark for linking smaller orbital radii with higher orbital speeds required for stable orbits

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified gravity as the force, but you must specify it acts as a 'centripetal force' towards the centre"
    • "Be careful with the difference between speed and velocity — remember velocity includes direction"
    • "Good calculation of orbital speed, but check your units — did you convert the time period to seconds?"
    • "You've described the orbit well; to improve, explain what would happen to the orbital speed if the moon moved closer to the planet"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a moon as a natural satellite that orbits a planet
    • Award 1 mark for stating that gravitational force (or weight) provides the centripetal force acting towards the centre of the orbit
    • Credit responses that explain acceleration occurs because velocity changes due to changing direction, even if speed is constant
    • Award 1 mark for linking smaller orbital radii with higher orbital speeds required for stable orbits

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining orbital motion, explicitly state that 'velocity is a vector' to justify why a change in direction constitutes acceleration
    • 💡Memorise that for a stable orbit, if the radius decreases, the speed must increase; use this relationship for qualitative prediction questions
    • 💡Ensure you can calculate orbital speed using v = 2πr / T, paying close attention to converting time units into seconds

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that velocity is constant in a circular orbit, failing to recognise velocity is a vector quantity dependent on direction
    • Believing that gravity does not act on the Moon because it does not fall into the Earth (ignoring that it is in freefall)
    • Confusing the period of orbit with the period of rotation (spin) on its axis
    • Incorrectly suggesting that the centripetal force is a separate physical force rather than the resultant force provided by gravity

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Natural vs. Artificial Satellites
    Gravitational Centripetal Force
    Circular Orbital Motion Mechanics
    Relationship between Orbital Radius and Speed

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Calculate
    Suggest

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