Explain why a planet in a circular orbit around a star continues to travel in that circular path rather than moving off in a straight line.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Planets in our Solar System travel in nearly circular orbits around the Sun. Without a force acting on them, an object in space would travel in a straight line at constant speed.
Model answer (5 marks)
A planet in a circular orbit is attracted to the star by gravity, which provides a force directed towards the star.
This inward force is the centripetal force that continually changes the direction of the planet’s velocity.
Because the speed of the planet remains constant, the only effect of the force is to bend its path into a circle.
If the gravitational force were absent, the planet would move in a straight line at constant speed, as required by Newton’s first law.
Thus the continuous inward gravitational pull keeps the planet on a circular orbit rather than letting it fly off in a straight line.
This inward force is the centripetal force that continually changes the direction of the planet’s velocity.
Because the speed of the planet remains constant, the only effect of the force is to bend its path into a circle.
If the gravitational force were absent, the planet would move in a straight line at constant speed, as required by Newton’s first law.
Thus the continuous inward gravitational pull keeps the planet on a circular orbit rather than letting it fly off in a straight line.
Examiner tips
- Use the term ‘centripetal force’ and ‘gravity’ early. Show the link between force direction and change in velocity. Mention constant speed to justify circular path. Remember Newton’s first law for the ‘no force’ case.
Common mistakes
- Confusing centripetal force with centrifugal force. Forgetting to state that speed stays constant. Saying the planet moves in a straight line without mentioning inertia.
Mark scheme (5 marks)
- Gravity / gravitational force acts between the planet and the star
- The gravitational force acts towards the star / is directed inward (centripetal direction)
- This force continuously changes the direction of the planet's velocity / motion
- Without this force the planet would travel in a straight line (Newton's first law / inertia)
- The speed / magnitude of velocity stays constant (so the planet remains in orbit at the same distance)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
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- Decode the mark scheme abbreviations →
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