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S3 Physics: Pressure and Buoyancy (Paper 3: challenge)
S3 Physics: Pressure and Buoyancy (Paper 3: challenge)
In challenge questions first decide whether the pressure is in a solid or in a liquid, then decide which volume the buoyancy needs. Keep N, m and kg/m3 throughout; mixing centimetres and grams shifts answers by ten thousand.
Direct answers Write the two steps separately: find the pressure or buoyancy first, then compare to decide floating or sinking.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S3 Physics: Pressure and Buoyancy |
| Level | S3 Physics |
02 Questions (10 items)
- FoundationA cube of side 0.2 m weighs 200 N and rests flat on a table. What pressure does it exert?
- IntermediateThe same cube is placed in water (1 000 kg/m3, g = 10 N/kg). Does it float or sink, and what is the buoyant force?
- IntermediateA hydraulic press has a small piston of area 0.01 m2 under a 50 N force and a large piston of area 0.5 m2. What force can the large piston lift?
- ChallengeA submarine is 30 m under water with a window of area 0.02 m2. With g = 10 N/kg, what force does the water exert on the window?
- ChallengeA wooden block of density 600 kg/m3 and volume 0.005 m3 floats at rest. With g = 10 N/kg, what is the buoyancy and the submerged fraction?
- FoundationWith g = 9.8 N/kg, what is the pressure at 3 m depth in water?
- FoundationA boat rises so that it displaces 0.5 m3 less water. With g = 10 N/kg, how heavy is the cargo removed?
- ChallengeAn object of density 850 kg/m3 is put in oil of density 800 kg/m3 and in water of 1 000 kg/m3. What happens in each?
- IntermediateWhy does a bubble grow as it rises to the surface?
- Error redoHow should the claim that anything denser than water must sink be corrected?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 5 000 Pa | The contact area is 0.2 x 0.2 = 0.04 m2, so p = 200 / 0.04 = 5 000 Pa. |
| 2 | It sinks, with a buoyant force of 80 N. | Its volume is 0.008 m3, so the buoyancy when fully immersed is 80 N, less than its 200 N weight. |
| 3 | 2 500 N | The pressure 50 / 0.01 = 5 000 Pa is the same throughout, so the large piston feels 5 000 x 0.5 = 2 500 N. |
| 4 | 6 000 N | First find the pressure, 1 000 x 10 x 30 = 300 000 Pa, then multiply by 0.02 m2 to get 6 000 N. |
| 5 | 30 N and 60 per cent submerged | Buoyancy equals its weight, 600 x 10 x 0.005 = 30 N, and the submerged fraction is 600 / 1 000 = 0.6. |
| 6 | 29 400 Pa, which is 600 Pa less than the 30 000 Pa obtained with g = 10. | p = 1 000 x 9.8 x 3 = 29 400 Pa; always use the g value the question gives. |
| 7 | 5 000 N | Displacing 0.5 m3 less means 1 000 x 10 x 0.5 = 5 000 N less support, which is the weight removed. |
| 8 | It sinks in the oil and floats in the water. | Its density is above the oil but below the water, so it sinks in oil and floats in water, which is how density separation works. |
| 9 | The pressure falls as depth decreases, so the trapped gas expands. | With the same amount of gas and a lower external pressure, the volume increases, a day to day example of Boyle law. |
| 10 | A solid object denser than water sinks, but a hollow shape can displace enough water to float. | Floating depends on average density; a steel ship or a submarine floats by controlling the displaced volume. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Challenge questions usually take two steps: find a force from pressure, or find buoyancy from a volume, then compare to decide floating or sinking. Convert every unit to N, m and kg/m3 first.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.