# Explain why, as a spacecraft travels from the Earth to the Moon along the line joining their centres, the magnitude of the net gravitational field strength first decreases, reaches zero at one point, and then increases again.

> IB DP Physics Higher Level (2023 syllabus) — D.1 Gravitational fields · Explain · 4 marks

> A spacecraft travels in a straight line from the surface of the Earth to the surface of the Moon. Both bodies exert gravitational forces on the spacecraft throughout its journey.

## Mark scheme (4 marks)

1. The gravitational field strengths due to Earth and Moon act in opposite directions (towards each respective body), so the net field is the vector difference of the two contributions.
2. Close to Earth the Earth's contribution dominates, so the net field points towards Earth and is large; as the spacecraft moves away from Earth its contribution decreases (field strength ∝ 1/r²).
3. At one specific point, the field strengths due to the Earth and the Moon are equal in magnitude but opposite in direction, so the net gravitational field strength is zero.
4. Beyond this neutral point, as the spacecraft approaches the Moon, the Moon's field dominates and the net field increases in magnitude, now directed towards the Moon.

## Key terms

- [gravitational field strength](https://www.gradenine.co.uk/glossary/gravitational-field-strength)
- [net gravitational field](https://www.gradenine.co.uk/glossary/net-gravitational-field)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-as-a-spacecraft-travels-7b686d01) · Published by Druglandscape Ltd.