Explain why a planet in a circular orbit around the Sun remains at a constant distance from the Sun, even though a gravitational force acts on it.

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)

Gravity acts as a centripetal force on the planet
The gravitational force changes the direction of the planet’s velocity, acting at right angles to the velocity
The planet therefore moves in a curved, circular path around the Sun
The distance from the Sun stays constant because the planet’s speed (the magnitude of its velocity) remains unchanged

Examiner tips

Use the word ‘centripetal’ to link gravity and circular motion
Show that the force is perpendicular to velocity to explain constant speed
Mention that a constant speed in a circle gives a constant radius
Keep the answer concise – one sentence per point

Common mistakes

Confusing the force with the velocity (e.g. saying the force changes the speed rather than the direction)
Failing to state that the force is perpendicular to the velocity
Using vague terms like ‘keeps it moving’ instead of ‘keeps the distance constant’

Mark scheme (4 marks)

Gravity acts as a centripetal force on the planet
The gravitational force changes the direction of the planet's velocity / acts at right angles to the velocity
The planet moves in a curved / circular path around the Sun
The distance from the Sun stays constant because the planet's speed (magnitude of velocity) remains unchanged

Key terms in this question

gravitational force · circular orbit

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