Explain why the gravitational field strength at the surface of the Moon is weaker than the gravitational field strength at the surface of the Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Moon has a smaller mass and smaller radius than the Earth. Both objects produce gravitational fields.
Model answer (5 marks)
A gravitational field is a region where a mass experiences a force due to gravity.
The field strength (g) produced by a spherical body is given by g=GM/r², where M is the mass of the body and r is the distance from its centre.
The Moon has a smaller mass than the Earth, so the numerator GM is smaller.
The Moon’s radius is also smaller, so the distance r in the denominator is smaller, but the reduction in M dominates.
Consequently, the value of g at the Moon’s surface is smaller than that at the Earth’s surface, so the gravitational field strength on the Moon is weaker.
The field strength (g) produced by a spherical body is given by g=GM/r², where M is the mass of the body and r is the distance from its centre.
The Moon has a smaller mass than the Earth, so the numerator GM is smaller.
The Moon’s radius is also smaller, so the distance r in the denominator is smaller, but the reduction in M dominates.
Consequently, the value of g at the Moon’s surface is smaller than that at the Earth’s surface, so the gravitational field strength on the Moon is weaker.
Examiner tips
- Use the formula g=GM/r² to show dependence on mass and radius.
- Explain that the Moon’s smaller mass outweighs the effect of its smaller radius.
- Mention that the field strength is the force per unit mass.
- Keep the answer concise and use UK spelling.
Common mistakes
- Confusing the Moon’s smaller radius with a stronger field; the mass effect is more important.
- Using the wrong formula (e.g., g=GM/r instead of r²).
- Failing to mention that g is force per unit mass.
Mark scheme (5 marks)
- A gravitational field is a region where a mass experiences a force (due to gravity)
- Gravitational field strength depends on the mass of the object producing the field
- The Moon has a smaller mass than the Earth
- Gravitational field strength also depends on the distance from the centre of the body / the radius of the body
- Because the Moon has a smaller mass (and its surface is closer to its centre), the gravitational field strength at its surface is weaker / the force on a mass at the Moon's surface is smaller
Key terms in this question
gravitational field · gravitational field strength
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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