UPSC Prelims 2004 Q128: Science & Technology – Physics
A weightless rubber balloon is filled with 200 cc of water. Its weight in water is equal to
- (a) 9.85 N
- (b) 9.8/10 N
- (c) 9.8/2 N
- (d) zero
Answer: (d) zero
Explanation
Weight of a water-filled balloon in water
| Item | Reasoning |
|---|---|
| Balloon | Weightless rubber skin, so only the water inside matters |
| Water inside | 200 cc of water has the same density as the surrounding water |
| Buoyant force | Equals the weight of the displaced water, so it cancels the weight of the water inside |
| Apparent weight | Zero |
A weightless balloon filled with 200 cc of water displaces 200 cc of water; the upthrust equals its weight, so its weight in water is zero.
Answer: (d) zero.
Why each option is right or wrong
- (a) Incorrect. 9.85 N would be a much larger weight.
- (b) Incorrect. 9.8/10 N is the weight in air, not in water.
- (c) Incorrect. 9.8/2 N is not the apparent weight in water.
- (d) CORRECT. The buoyant force balances the weight, so it is zero.
Key point
Archimedes: apparent weight = true weight - buoyant force; same density gives zero.
Remember
A water-filled balloon has zero weight in water.
Common mistakes
- Quoting the weight in air (about 2 N).
- Forgetting the upward buoyant force of the water.
- Confusing the mass with the weight of the water.
Exam tip
An object of the same density as the fluid floats and has zero apparent weight.
Sources
- NCERT Class IX Science — Gravitation
- NCERT Class XI Physics — Mechanical Properties of Fluids (Archimedes' principle)