# 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.

> OCR GCSE Physics A: Gateway Science (J249) — P1.1 The particle model · Explain · 4 marks

> Wood has a density lower than that of water. Iron has a density much greater than that of water.

## Mark scheme (4 marks)

1. 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
2. The iron displaces less water than the wooden block because it has a smaller volume
3. 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
4. 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

- [Revision notes for OCR GCSE Physics A: Gateway Science (J249)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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