A moon orbits a planet in a slightly elliptical orbit at a near constant orbital speed. State what provides the force that keeps the moon in its orbit. Describe how the orbital speed of the moon would change if it moved to a new stable orbit with a larger orbital radius but the same orbital period, and explain what this tells you about the relationship between orbital speed and orbital radius.

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)

Gravity (gravitational force) provides the force that keeps the moon in its orbit.
The orbital speed would increase at the larger orbital radius.
Because orbital speed = 2π × orbital radius ÷ time period, and the orbital period stays the same, a larger orbital radius gives a greater orbital speed.
Orbital speed is directly proportional to orbital radius (when the orbital period is constant).

Examiner tips

  • Use the exact phrase ‘gravity (gravitational force)’. State the speed change clearly: ‘would increase’. Show the formula v = 2πr/T to justify the relationship. Mention the proportionality explicitly.”]
  • common_mistakes
  • :
  • Saying the speed would decrease or staying the same. Using the wrong formula (e.g. v = √(GM,/r)).\nOmitting the word ‘directly’ in the proportionality statement.”}

Mark scheme (4 marks)

  1. Gravity (gravitational force) provides the force that keeps the moon in its orbit.
  2. The orbital speed would increase (be greater) at the larger orbital radius.
  3. Because orbital speed = 2π × orbital radius ÷ time period, and the orbital period stays the same, a larger orbital radius gives a greater orbital speed.
  4. Orbital speed is directly proportional to orbital radius (when the orbital period is constant).

Key terms in this question

orbital speed · orbital radius · orbital period

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