A student states: 'The gravitational field around the Earth is uniform everywhere.' Evaluate this statement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The gravitational field strength at the Earth's surface is approximately 9.8 N/kg. At the top of a mountain it is slightly less. In orbit, astronauts appear to float.
Model answer (5 marks)
The statement is incorrect – the field is not uniform everywhere.
1. Gravitational field strength decreases with increasing distance from the centre of the Earth.
2. Close to the Earth's surface the field can be approximated as uniform; field lines are approximately parallel and equally spaced.
3. Further from the Earth the field lines spread out, becoming radial.
4. A gravitational field exists around any object with mass, so the field exists in the space around the Earth and acts on any mass placed in it.
5. Therefore the claim of uniformity is only an approximation valid near the surface, not a general truth.
1. Gravitational field strength decreases with increasing distance from the centre of the Earth.
2. Close to the Earth's surface the field can be approximated as uniform; field lines are approximately parallel and equally spaced.
3. Further from the Earth the field lines spread out, becoming radial.
4. A gravitational field exists around any object with mass, so the field exists in the space around the Earth and acts on any mass placed in it.
5. Therefore the claim of uniformity is only an approximation valid near the surface, not a general truth.
Examiner tips
- Use the word ‘incorrect’ to show you understand the statement is wrong. Show the 4 key points from the mark scheme. Explain why the field is radial and why it decreases with distance. Mention the approximation near the surface to gain the full 5 marks.
Common mistakes
- Saying the field is uniform everywhere without qualification. Forgetting that the field decreases with distance. Not recognising the field is radial further from Earth. Omitting that the field exists around any mass.
Mark scheme (5 marks)
- The statement is incorrect / the field is not uniform everywhere
- Gravitational field strength decreases with increasing distance from the centre of the Earth
- Close to the Earth's surface the field can be approximated as uniform / field lines are approximately parallel and equally spaced near the surface
- Further from the Earth the field lines spread out / the field becomes radial
- A gravitational field exists around any object with mass / the field exists in the space around the Earth and acts on any mass placed in it
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
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