The Earth orbits the Sun in a nearly circular orbit. Gravity is described as a non-contact force. Explain how gravity keeps the Earth in its circular orbit around the Sun, and why the Earth is said to be continuously accelerating even though its orbital speed remains constant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Earth takes approximately 365 days to complete one orbit of the Sun at an average distance of approximately 150 million kilometres.
Model answer (4 marks)
Gravity acts on the Earth through a field, pulling it towards the Sun without contact. This gravitational force is the resultant force that continually changes the direction of the Earth’s velocity, keeping it in a circular path. Acceleration is the rate of change of velocity; velocity is a vector with magnitude and direction. Because the direction of the Earth’s velocity is constantly changing, its velocity is continuously changing, so the Earth is continuously accelerating even though its speed remains constant.
Examiner tips
- Use the term ‘gravitational field’ to show understanding of non‑contact force.
- Explain that acceleration refers to change in velocity vector, not just speed.
- Mention that the force is the resultant force keeping the orbit circular.
- Keep the answer concise and directly linked to the mark scheme points.
Common mistakes
- Confusing speed with velocity, or saying the Earth is not accelerating because speed is constant.
- Failing to mention that gravity acts through a field or that the force is the resultant force.
- Using vague language such as ‘gravity pulls the Earth’ without explaining the direction change of velocity.
Mark scheme (4 marks)
- Gravity acts on the Earth at a distance / without physical contact (through a field) pulling it towards the Sun
- This gravitational force acts as the resultant force that continuously changes the direction of the Earth's velocity
- Acceleration is the rate of change of velocity, and velocity is a vector quantity with both magnitude and direction
- Because the direction of the Earth's velocity is continuously changing, the velocity is continuously changing, so the Earth is continuously accelerating
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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