Explain why the gravitational field strength at the surface of the Moon is less than the gravitational field strength at the surface of the Earth, and explain what effect this difference has on the mass and weight of a rock brought from the Moon to the Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Moon has a much smaller radius and a much smaller mass than the Earth.
Model answer (5 marks)
The gravitational field strength g at a planet’s surface is g=GM/r². The Moon’s mass M is much smaller than Earth’s, and its radius r is also smaller, but the reduction in mass dominates, giving a smaller g on the Moon.
When a rock is taken from the Moon to Earth its mass m stays the same, because mass is an intrinsic property.
Weight is W=mg. Since g is larger on Earth, the rock’s weight increases when it is on Earth.
Thus the rock is lighter on the Moon and heavier on Earth because the Moon’s weaker gravitational field gives a smaller weight for the same mass.
When a rock is taken from the Moon to Earth its mass m stays the same, because mass is an intrinsic property.
Weight is W=mg. Since g is larger on Earth, the rock’s weight increases when it is on Earth.
Thus the rock is lighter on the Moon and heavier on Earth because the Moon’s weaker gravitational field gives a smaller weight for the same mass.
Examiner tips
- Show the formula g=GM/r² and compare M and r for Earth and Moon; link to weight via W=mg; remember mass is unchanged; keep answer concise.
- Use the exact terms ‘gravitational field strength’, ‘mass’, ‘weight’, ‘g’, and ‘W=mg’.
- Structure answer: 1) explain g difference, 2) state mass unchanged, 3) explain weight change.
Common mistakes
- Confusing mass with weight; writing that the rock’s mass changes on Earth.
- Using the wrong formula for weight (e.g., W=mg²).
- Omitting the link between g and weight or not stating that mass is unchanged.
Mark scheme (5 marks)
- Gravitational field strength depends on the mass of the object creating the field
- The Earth has a greater mass than the Moon, so its gravitational field strength is greater
- The mass of the rock does not change when moved from the Moon to the Earth
- The weight of the rock increases when moved to the Earth
- Because weight = mass × gravitational field strength, and gravitational field strength is greater on Earth
Key terms in this question
gravitational field strength · mass · weight
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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