A spacecraft travels from Earth to Mars. During the journey, the spacecraft moves away from Earth and closer to Mars. Explain how the weight of the spacecraft changes during this journey, and why its mass remains the same throughout.

OCR A-Level Physics A (H556) — 2.1 Physical quantities and units · Explain · 5 marks · View as Markdown

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

Earth has a much stronger gravitational field than Mars. In deep space, far from any planet, the gravitational field strength is approximately zero.

Model answer (5 marks)

Weight depends on the gravitational field strength, not on mass alone.
1. As the spacecraft moves away from Earth, the field strength of Earth decreases, so its weight decreases.
2. In deep space, far from any planet, the field strength is approximately zero, so the weight is approximately zero.
3. As the spacecraft approaches Mars, the field strength of Mars increases, so its weight increases again.
4. The mass of the spacecraft remains unchanged because mass is the amount of matter it contains and does not depend on the gravitational field strength or its location.

Examiner tips

  • Use the term ‘gravitational field strength’ and ‘weight’ clearly; explain the change in each region.
  • Show the sequence: Earth → deep space → Mars, linking field strength to weight.
  • Remember mass is constant – state this explicitly.
  • Use the word ‘approximately’ when referring to zero field strength in deep space.

Common mistakes

  • Confusing weight with mass; writing that mass changes.
  • Using ‘gravity’ instead of ‘gravitational field strength’ and not linking it to weight.
  • Claiming the spacecraft’s weight is zero everywhere in space without noting the small residual field of the Sun.

Mark scheme (5 marks)

  1. Weight depends on gravitational field strength (not mass alone)
  2. As the spacecraft moves away from Earth, the gravitational field strength decreases, so the spacecraft's weight decreases
  3. In deep space (far from any planet) the gravitational field strength is approximately zero, so the weight of the spacecraft is approximately zero
  4. As the spacecraft approaches Mars, gravitational field strength increases, so its weight increases again
  5. Mass remains the same because mass is the amount of matter in the spacecraft and does not depend on gravitational field strength / location

Key terms in this question

weight · mass

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