Explain why an astronaut's weight changes when they travel from the surface of the Earth to the surface of the Moon, but their mass does not change.

AQA A-Level Physics (7408) — 3.7.2 Gravitational Fields · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The Moon has a smaller gravitational field strength than the Earth. The gravitational field strength on Earth is approximately 10 N/kg and on the Moon it is approximately 1.6 N/kg.

Model answer (5 marks)

Mass is the amount of matter in an object and does not change with location.

Weight is the force of gravity acting on an object.

Weight = mg, where m is mass and g is the gravitational field strength.

On the Moon g ≈ 1.6 N kg⁻¹, which is much smaller than the Earth's g ≈ 10 N kg⁻¹.

Therefore the astronaut’s weight is less on the Moon, but his mass remains unchanged.

Examiner tips

  • Use the definition of mass and weight; show the formula W=mg; compare g values; keep answer concise and to the point.
  • Remember to state that mass is invariant and weight depends on g; this structure matches the mark scheme.

Common mistakes

  • Confusing mass with weight; writing that mass changes on the Moon.
  • Using the wrong units for gravitational field strength or forgetting to include the formula W=mg.

Mark scheme (5 marks)

  1. Mass is the amount of matter in an object
  2. Mass does not depend on location / mass is the same everywhere / mass is not affected by gravitational field strength
  3. Weight is the force of gravity acting on an object
  4. Weight depends on gravitational field strength (W = mg)
  5. The gravitational field strength on the Moon is less than on Earth, so the astronaut's weight is less on the Moon

Key terms in this question

mass · weight

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