Explain why planets closer to the Sun orbit at greater orbital speeds than planets further away.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The planets in our Solar System all orbit the Sun in stable, roughly circular orbits. However, Mercury completes an orbit in about 88 days whilst Neptune takes approximately 165 years.
Model answer (4 marks)
The Sun’s gravity provides the centripetal force that keeps a planet in orbit.
1. The gravitational force is F=GMm/r², so it is larger when the distance r is smaller.
2. For a circular orbit the required centripetal force is mv²/r.
3. A larger gravitational force therefore demands a larger orbital speed v.
4. Planets close to the Sun also travel a shorter circumference, so the higher speed and shorter path give a much shorter orbital period.
1. The gravitational force is F=GMm/r², so it is larger when the distance r is smaller.
2. For a circular orbit the required centripetal force is mv²/r.
3. A larger gravitational force therefore demands a larger orbital speed v.
4. Planets close to the Sun also travel a shorter circumference, so the higher speed and shorter path give a much shorter orbital period.
Examiner tips
- Show the two equations (F=GMm/r² and mv²/r) and link the larger force to a larger v.
- Mention that the shorter path also contributes to the shorter period.
Common mistakes
- Confusing gravitational force with centripetal force; they are the same in this context.
- Forgetting to note that the orbital path is shorter for inner planets.
Mark scheme (4 marks)
- Gravitational force / gravitational attraction between the Sun and a planet acts as the centripetal force
- Gravitational force is greater for planets closer to the Sun (because the distance is smaller)
- A greater centripetal force requires a greater orbital speed (for a stable circular orbit)
- Closer planets also travel a shorter orbital path / circumference, so the combination of higher speed and shorter path results in a much shorter orbital period
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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