Describe how the gravitational force acting on a planet determines its orbital motion around the Sun, and explain why planets closer to the Sun orbit faster than planets further away.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Gravity provides the centripetal force that keeps the planet in its orbit.
The gravitational force is directed towards the Sun, acting inward and causing the planet to travel in a curved path.
Planets closer to the Sun experience a stronger gravitational force.
A greater gravitational force gives a greater centripetal acceleration, so the planet must travel faster to maintain a stable orbit at that smaller radius.
The gravitational force is directed towards the Sun, acting inward and causing the planet to travel in a curved path.
Planets closer to the Sun experience a stronger gravitational force.
A greater gravitational force gives a greater centripetal acceleration, so the planet must travel faster to maintain a stable orbit at that smaller radius.
Examiner tips
- Use the phrase ‘centripetal force’ to link gravity to orbital motion.
- Show the direction of the force (towards the Sun).
- Explain the relationship between distance, force strength and orbital speed.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing gravitational force with the planet’s weight. Using ‘gravity pulls the planet down’ instead of ‘towards the Sun’. Failing to mention that the force is centripetal and causes a curved path.
Mark scheme (4 marks)
- Gravity provides the centripetal force that keeps the planet in its orbit
- The gravitational force is directed towards the Sun / acts inward, causing the planet to travel in a curved/circular path
- Planets closer to the Sun experience a stronger gravitational force
- A greater gravitational force means a greater centripetal acceleration / the planet must travel faster to maintain a stable orbit at that smaller radius
Key terms in this question
gravitational force · orbital motion
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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