Mars orbits the Sun in a nearly circular orbit. Gravity is a non-contact force that acts between the Sun and Mars. Explain why Mars travels in a circular orbit rather than moving in a straight line, and state the direction in which the gravitational force on Mars acts.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mars orbits the Sun at an average distance of approximately 228 million kilometres, completing one orbit every 687 Earth days.
Model answer (4 marks)
Mars would move in a straight line if no force acted on it, because of Newton’s First Law.
The Sun’s gravity pulls Mars towards the Sun, providing a centripetal force.
This force is always directed inward, so it continually changes the direction of Mars’s velocity.
Thus Mars follows a nearly circular orbit rather than a straight line.
The Sun’s gravity pulls Mars towards the Sun, providing a centripetal force.
This force is always directed inward, so it continually changes the direction of Mars’s velocity.
Thus Mars follows a nearly circular orbit rather than a straight line.
Examiner tips
- Use Newton’s First Law to justify the straight‑line motion in the absence of force. State the force direction as ‘towards the Sun’. Show that the force is centripetal and continuously changes velocity direction. Mention that gravity is a non‑contact force acting at a distance.
Common mistakes
- Saying the force acts ‘away from the Sun’ instead of ‘towards’. Forgetting to mention that the force changes the direction of velocity. Using vague terms like ‘gravity pulls’ without specifying direction or centripetal nature.
Mark scheme (4 marks)
- Without a resultant force, Mars would travel in a straight line (Newton's First Law / constant velocity in a straight line).
- The gravitational force acts towards the Sun (from Mars towards the Sun / centripetally inward).
- The gravitational force continuously changes the direction of Mars's velocity.
- Gravity acts at a distance / with no physical contact between Mars and the Sun, acting through a gravitational field.
Key terms in this question
non-contact force · gravitational force · circular orbit
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
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