A student places a wooden block and a lump of iron of the same mass into a tank of water. The wooden block floats on the surface, whilst the iron sinks to the bottom. Explain why the two objects behave differently, even though they have the same mass.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Wood has a density lower than that of water. Iron has a density much greater than that of water.
Model answer (4 marks)
1. Density is mass per unit volume. For the same mass, iron occupies a smaller volume than wood because its particles are packed more closely.
2. The iron therefore displaces less water than the wooden block.
3. The buoyant force equals the weight of the displaced water. For the iron this is less than its weight, so a net downward force remains and it sinks.
4. For the wooden block the displaced water weighs exactly the block’s weight, giving an equal upward buoyant force and the block floats.
2. The iron therefore displaces less water than the wooden block.
3. The buoyant force equals the weight of the displaced water. For the iron this is less than its weight, so a net downward force remains and it sinks.
4. For the wooden block the displaced water weighs exactly the block’s weight, giving an equal upward buoyant force and the block floats.
Examiner tips
- Use the definition of density to link mass, volume and packing of particles. Show the relationship between displaced water, buoyant force and weight. Explain the outcome for each object in one sentence each.
Common mistakes
- Confusing density with mass; stating both objects have the same density. Ignoring the role of displaced water in determining buoyancy. Failing to recognise that the wooden block displaces water equal to its own weight before fully submerging.
Mark scheme (4 marks)
- Density depends on how closely packed the particles are (spacing of particles / volume they occupy), so iron has a smaller volume than wood for the same mass
- The iron displaces less water than the wooden block because it has a smaller volume
- The weight of water displaced by the iron is less than the weight of the iron, so the upthrust is less than its weight and there is a resultant downward force, causing it to sink
- The wooden block displaces a weight of water equal to its own weight before it fully submerges, so the upthrust equals its weight and it floats
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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