A comet travels in a highly elliptical orbit around the Sun. Describe and explain how the orbital speed of the comet changes as it moves from its furthest point from the Sun to its closest point to the Sun.

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 comet’s speed increases as it moves from aphelion to perihelion. The orbital radius decreases, so by v=2πr/T a smaller r gives a larger v for the same period. Gravitational force is stronger at smaller r, pulling the comet faster. The loss of mass from ice sublimation near the Sun also reduces the comet’s inertia, allowing it to accelerate further.

Examiner tips

  • Mention the increase in speed and the decrease in radius first.
  • Use the equation v=2πr/T or explain the stronger gravitational pull.
  • Show the link between mass loss and increased speed if you include it.
  • Keep the answer concise and use correct UK spelling.

Common mistakes

  • Confusing aphelion with perihelion.
  • Using the wrong formula (e.g., v=√(GM/r) without explanation).
  • Omitting the mass loss effect or stating it incorrectly.

Mark scheme (4 marks)

  1. The orbital speed increases (as the comet moves closer to the Sun)
  2. The orbital radius decreases as the comet approaches the Sun
  3. From v = 2πr/T, a smaller orbital radius means a greater orbital speed (for a given period) / gravitational force is greater at smaller radius, increasing speed
  4. The comet's mass also decreases near the Sun as ice melts and vaporises, which further contributes to its speed being greater

Key terms in this question

orbital speed · highly elliptical · comet

Related

More Motion in the universe questions

▶ Try answering this question with AI marking (free) →