Explain why the gravitational field strength decreases as a spacecraft travels away from the Earth's surface, and describe what happens to the weight of an astronaut inside the spacecraft during this journey.

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).

A spacecraft launches from Earth and travels in a straight line away from the planet into deep space.

Model answer (5 marks)

1. The gravitational field strength (g) produced by the Earth is inversely proportional to the square of the distance from the Earth's centre (g∝1/r²). 2. As the spacecraft moves away from the Earth, r increases, so g decreases. 3. The gravitational force on the spacecraft and the astronaut is F=mg, where m is mass and g is the local field strength. 4. Because g falls, the force of gravity on the astronaut – his weight – falls. 5. The astronaut’s mass does not change, so the reduction in weight is solely due to the weaker field.

Examiner tips

  • Use the 1/r² relationship to justify the decrease in g.
  • Show the weight formula W=mg and explain the effect of a lower g.
  • Mention that mass stays constant to show the change is only in force.

Common mistakes

  • Confusing mass with weight or saying the astronaut’s mass decreases.
  • Forgetting to state that g decreases with distance.
  • Using the wrong formula for weight (e.g., W=mg²).

Mark scheme (5 marks)

  1. Gravitational field strength decreases as distance from the centre of the Earth increases
  2. Because the gravitational force exerted by the Earth on the spacecraft/astronaut decreases with distance
  3. Weight is the force due to gravity acting on the astronaut (W = mg)
  4. As gravitational field strength decreases, the astronaut's weight decreases
  5. The astronaut's mass remains constant throughout the journey

Key terms in this question

gravitational field strength · weight

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