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S4 Physics: Pressure and Upthrust Paper 3 (Floating, Sinking and Density)
S4 Physics: Pressure and Upthrust Paper 3 (Floating, Sinking and Density)
This paper covers the density test for floating, sinking and suspension, plus iceberg freeboard, a boat entering sea water and a submarine diving.
Quick answer 1. floats, 20 N | 2. sinks, 10 N | 3. equal | 4. about 1,100 kg/m3 | 5. 12.2 percent | 6. 12,000 cm3 in, 8,000 cm3 out | 7. unchanged | 8. same upthrust, shallower draught | 9. same upthrust, greater weight | 10. 5 N
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Physics – Pressure and Upthrust |
| Level | Challenge |
02 Questions (10 items)
- FoundationA 0.004 m3 block of density 500 kg/m3 is put in water. Does it float or sink, and what is the upthrust?
- FoundationA 0.001 m3 aluminium block of density 2,700 kg/m3 is fully immersed. Does it sink, and what is the upthrust?
- FoundationAn iron block and a wooden block of the same volume are both fully immersed. Which feels the greater upthrust?
- FoundationAn object stays suspended at any depth in brine of density 1,100 kg/m3. What is its density?
- IntermediateAn iceberg of density 900 kg/m3 floats in sea water of density 1,025 kg/m3. What percentage of its volume shows above water?
- IntermediateA 0.02 m3 block of density 600 kg/m3 floats. What volumes are immersed and exposed, in cm3?
- IntermediateA floating ice cube melts in a glass. Does the water level rise, fall or stay the same?
- ChallengeA boat moves from river water (1,000 kg/m3) to sea water (1,025 kg/m3). How do the upthrust and draught change?
- ChallengeA submarine dives by taking in water. How do its upthrust and weight change?
- Error redoA 500 g block of density 500 kg/m3 floats. A student uses the full volume 0.001 m3 and gets 10 N. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 500 is less than 1,000, so it floats; upthrust equals weight = 500 x 0.004 x 10 = 20 N. | For a floating body upthrust equals its weight, not total volume times pg. |
| 2 | Weight = 2,700 x 0.001 x 10 = 27 N, upthrust = 1,000 x 0.001 x 10 = 10 N, so it sinks. | A density above water means it sinks; upthrust depends only on displaced volume. |
| 3 | They are equal, since the displaced volume and therefore pVg are the same. | When fully immersed, upthrust depends on volume only, not on the material. |
| 4 | Suspension needs object density equal to liquid density, so about 1,100 kg/m3. | Suspended, floating and sinking correspond to equal, less and greater density. |
| 5 | Immersed fraction = 900 / 1,025 = 0.8780, so exposed = 12.2 percent. | The immersed fraction is the density ratio; only a small part shows. |
| 6 | Immersed = 600 / 1,000 x 0.02 = 0.012 m3 = 12000 cm3; exposed = 8000 cm3. | Use the density ratio for the immersed volume, then subtract from the total. |
| 7 | It stays the same: the ice displaces water equal to its weight, and the melt water fills exactly that space. | A floating body displaces its own weight in liquid, which settles the question. |
| 8 | Upthrust is unchanged because it still equals the weight; denser water means less displaced volume, so the draught is shallower. | For a floating body the weight fixes the upthrust; density changes only the displaced volume. |
| 9 | Its volume is unchanged so the upthrust stays the same; taking in water raises its weight above the upthrust, so it sinks. | A submarine changes its weight, not its displaced volume, unlike a fish with a swim bladder. |
| 10 | Upthrust = weight = 0.5 x 10 = 5 N (half immersed); 10 N would need full immersion. | A floating body displaces only its own weight of water, so the full volume doubles the answer. |
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.
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