# A space probe is launched from Earth and travels to Mars. A scientist states that the probe has the same mass on Mars as it does on Earth, but a different weight. Explain why the scientist is correct, and describe what happens to the probe's motion when it is in free-fall above the surface of Mars compared to free-fall above the surface of Earth.

> OCR A-Level Physics B: Advancing Physics (H557) — 2.1 Physical quantities and units · Explain · 5 marks

> Mars has a weaker gravitational field strength than Earth. The gravitational field strength on Earth is 10 N/kg, whereas on Mars it is approximately 4 N/kg.

## Mark scheme (5 marks)

1. Mass is a measure of the amount of matter in an object and does not depend on the gravitational field / mass stays the same on Mars and Earth
2. Weight depends on / is calculated using gravitational field strength (weight = mass × gravitational field strength)
3. Mars has a weaker / smaller gravitational field strength than Earth, so the probe weighs less on Mars
4. In free-fall, the probe accelerates due to gravity / the acceleration equals the gravitational field strength
5. The probe accelerates more slowly / has a smaller acceleration in free-fall above Mars than above Earth because the gravitational field strength is weaker on Mars

## Key terms

- [mass](https://www.gradenine.co.uk/glossary/mass)
- [weight](https://www.gradenine.co.uk/glossary/weight)
- [free-fall](https://www.gradenine.co.uk/glossary/free-fall)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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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