A natural satellite orbits a planet in a slightly elliptical orbit. Describe how the orbital speed and orbital period of the natural satellite would each change if the satellite moved into a new stable orbit with a greater orbital radius, and explain why a greater orbital radius requires a greater orbital period.

Pearson Edexcel International GCSE Physics (4PH1) — 8.1 Motion in the universe · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The orbital speed decreases when the orbital radius increases.
The orbital period increases when the orbital radius increases.
A greater orbital radius means a longer path / greater circumference for one complete orbit.
The satellite also travels more slowly at the greater orbital radius, so the combination of a longer path and lower speed means the orbital period is greater.

Examiner tips

  • Use the exact terms "orbital speed" and "orbital period". Show the relationship: speed ∝ 1/√r and period ∝ √r. Explain both the longer path and the slower speed. Keep the answer concise to fit the 4 marks.
  • common_mistakes
  • :
  • Confusing speed with velocity or not mentioning the slower speed. Failing to state that the path length increases. Using vague wording like "the orbit gets bigger" instead of "greater orbital radius".

Mark scheme (4 marks)

  1. The orbital speed decreases (when the orbital radius increases)
  2. The orbital period increases (when the orbital radius increases)
  3. A greater orbital radius means a longer path / greater circumference for one complete orbit
  4. The satellite also travels more slowly at the greater orbital radius, so the combination of a longer path and lower speed means the orbital period is greater

Key terms in this question

orbital speed · orbital period · orbital radius · natural satellite

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