Explain why planets closer to the Sun travel at a greater orbital speed than planets that are further away from the Sun.

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

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

The eight planets in our solar system all travel in slightly elliptical orbits around the Sun. Their orbital speeds differ depending on their distance from the Sun.

Model answer (4 marks)

Planets closer to the Sun have a smaller orbital radius, so their circumference is shorter.

Because the Sun’s gravity is stronger nearer to it, the orbital period of a planet is shorter.

Orbital speed is given by v = 2πr/T. A smaller r and a much smaller T mean that the ratio 2πr/T is larger.

Thus planets nearer the Sun travel at a greater orbital speed than those further away.

Examiner tips

  • Use the formula v = 2πr/T to link radius, period and speed. Show that r is smaller and T is even smaller, so v increases. Keep the answer concise and use the exact wording from the mark scheme.
  • common_mistakes
  • :
  • Confusing orbital speed with orbital period. Saying “planets closer to the Sun move faster because they are closer” without explaining the period effect.

Mark scheme (4 marks)

  1. Orbital speed is inversely proportional to the orbital period (or: orbital speed = 2πr/T)
  2. Planets closer to the Sun have a smaller orbital radius
  3. Planets closer to the Sun also have a shorter orbital period
  4. The decrease in orbital period is greater than the decrease in orbital radius, so the orbital speed is greater (or: the combined effect of smaller r and much smaller T results in a greater orbital speed)

Key terms in this question

orbital speed

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