A comet travels in a highly elliptical orbit around the Sun. As the comet moves closer to the Sun, its speed increases. Explain why the comet speeds up as it approaches the Sun and why it slows down again as it moves away from the Sun.

Eduqas GCSE Physics — 10.1 Solar system; stability of orbital motions; satellites · 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 Sun’s gravity pulls the comet toward it. When the comet is near the Sun the gravitational force has a component in the direction of its motion, so it does positive work on the comet and its kinetic energy increases – the comet speeds up. As the comet moves away, the same gravitational force now acts opposite to its motion, doing negative work and reducing its kinetic energy – the comet slows down.

Examiner tips

  • Use the word ‘gravity’ and ‘work’ to link force and speed changes.
  • Show the direction of the force relative to the motion for both inbound and outbound parts.

Common mistakes

  • Confusing the Sun’s gravity with a ‘force of attraction’ that always speeds up the comet.
  • Forgetting to mention that the force acts opposite to the motion when the comet is receding.

Mark scheme (4 marks)

  1. The Sun's gravity/gravitational force acts on the comet
  2. As the comet moves closer to the Sun, the gravitational force does work on the comet / the gravitational force has a component in the direction of motion, causing it to speed up
  3. As the comet moves away from the Sun, the gravitational force acts in the opposite direction to its motion
  4. This opposing gravitational force decelerates the comet / causes it to slow down

Key terms in this question

elliptical orbit

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