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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The probe’s mass is a property of the amount of matter it contains and is independent of the gravitational field; therefore its mass is the same on Earth and on Mars.
Weight is the force of gravity on an object and is calculated as weight = mass × gravitational field strength. Because the gravitational field strength on Mars (≈4 N kg⁻¹) is weaker than on Earth (10 N kg⁻¹), the probe’s weight on Mars is less.
In free‑fall the probe accelerates under gravity alone. The acceleration equals the local gravitational field strength. Thus the probe accelerates more slowly in free‑fall above Mars than above Earth because the field strength is smaller on Mars.
Weight is the force of gravity on an object and is calculated as weight = mass × gravitational field strength. Because the gravitational field strength on Mars (≈4 N kg⁻¹) is weaker than on Earth (10 N kg⁻¹), the probe’s weight on Mars is less.
In free‑fall the probe accelerates under gravity alone. The acceleration equals the local gravitational field strength. Thus the probe accelerates more slowly in free‑fall above Mars than above Earth because the field strength is smaller on Mars.
Examiner tips
- Use the exact formula weight = mass × g; state g values for Earth and Mars.
- Explain that mass is invariant, weight depends on g.
- Mention that acceleration in free‑fall equals g.
- Show the comparison of 4 N kg⁻¹ vs 10 N kg⁻¹.
Mark scheme (5 marks)
- 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
- Weight depends on / is calculated using gravitational field strength (weight = mass × gravitational field strength)
- Mars has a weaker / smaller gravitational field strength than Earth, so the probe weighs less on Mars
- In free-fall, the probe accelerates due to gravity / the acceleration equals the gravitational field strength
- 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 in this question
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- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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