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S3 Physics: Pressure and Buoyancy (Paper 2: intermediate)
S3 Physics: Pressure and Buoyancy (Paper 2: intermediate)
The usual slip here is swapping formulas and trying to use F / A inside a liquid. Internal liquid pressure depends only on depth and density, and buoyancy is always found from the displaced liquid.
Direct answers The word displaced is the key: only the submerged part displaces liquid, so the part above the surface does not count.
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 force of 400 N acts on 0.02 m2. What is the pressure?
- FoundationThe pressure is 5 000 Pa and the force is 250 N. What is the contact area?
- IntermediateOil has a density of 800 kg/m3 and g is 10 N/kg. What is the pressure at a depth of 3 m?
- FoundationIn the same liquid, what happens to the pressure if the depth doubles?
- ChallengeA metal block is fully immersed in water and displaces 0.002 m3. With g = 10 N/kg, what is the buoyant force?
- IntermediateWhat does Archimedes principle state?
- ChallengeAn object of weight 60 N floats on water. What mass of water does it displace?
- IntermediateA diver descends to 5 m in water. With g = 10 N/kg, what pressure does the water exert?
- ChallengeIce has a density of 900 kg/m3. What fraction of its volume is under water when it floats?
- Error redoWhat is wrong with saying that buoyancy keeps increasing with depth?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 20 000 Pa | p = 400 / 0.02 = 20 000 Pa; a small area raises pressure quickly. |
| 2 | 0.05 m2 | Rearrange: A = F / p = 250 / 5 000 = 0.05 m2. |
| 3 | 24 000 Pa | p = rho g h = 800 x 10 x 3 = 24 000 Pa; oil is less dense than water so the pressure is lower. |
| 4 | It doubles | Since p = rho g h is proportional to h, doubling depth doubles the pressure. |
| 5 | 20 N | F = rho g V = 1 000 x 10 x 0.002 = 20 N. |
| 6 | The buoyant force equals the weight of the liquid displaced. | So the volume and density of the displaced liquid give the buoyancy, whatever the object is made of. |
| 7 | 6 kg | The 60 N buoyancy equals the weight of displaced water, so its mass is 60 / 10 = 6 kg. |
| 8 | 50 000 Pa | p = 1 000 x 10 x 5 = 50 000 Pa, so every extra metre adds 10 000 Pa. |
| 9 | Nine tenths, or 0.9 | Buoyancy equals weight, so 900/1 000 = 0.9 of the volume is submerged. |
| 10 | Once fully submerged the displaced volume cannot grow, so the buoyant force stays constant. | Greater depth raises the pressure everywhere, but the difference between top and bottom depends on volume, so buoyancy stops changing. |
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.
Keep the two pressure formulas apart: p = F / A for solids, p = rho g h for liquids. For buoyancy always reason with the displaced liquid rather than guessing from how heavy an object feels.Tutor Registration →
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