UPSC Prelims 2001 Q109: Science & Technology – Physics
The mass of a body on Earth is 100 kg (acceleration due to gravity, gₑ = 10 m/s²). If
acceleration due to gravity on the Moon = gₑ/6, then the mass of the body on the moon is
- (a) 100/6 kg
- (b) 60 kg
- (c) 100 kg
- (d) 600 kg
Answer: (c) 100 kg
Explanation
Mass of a 100 kg body on the Moon
| Quantity | On Earth → on the Moon |
|---|---|
| Mass | 100 kg → 100 kg — the amount of matter does not change |
| g | 10 m/s² → 10/6 ≈ 1.67 m/s² |
| Weight (W = mg) | 1,000 N → about 167 N |
| What changes | Only weight, not mass |
Given: a body of mass 100 kg on Earth, with g on the Moon one-sixth of g on Earth. Mass is the amount of matter in a body and stays the same everywhere, so it is still 100 kg. Only its weight falls to one-sixth.
Answer: (c) 100 kg.
Why each option is right or wrong
- (a) Incorrect. 100/6 would be the change in weight, not mass.
- (b) Incorrect. Mass does not change to 60 kg.
- (c) CORRECT. Mass stays 100 kg on the Moon.
- (d) Incorrect. Mass does not increase on the Moon.
Key point
Mass is measured with a beam balance (same everywhere); weight with a spring balance (changes with g).
Remember
Mass is constant; weight = mg changes.
Common mistakes
- Dividing mass by 6, mixing it with weight.
- Thinking mass depends on the local gravity.
- Multiplying the mass by 6 by mistake.
Exam tip
When a question says mass, ask: has any matter been added or removed?
Sources
- NCERT Class IX — Science, chapter 'Gravitation' (mass and weight)
- NCERT Class XI — Physics Part I, chapter 'Gravitation'