A solid block of steel dropped into a swimming pool sinks straight to the bottom, yet a massive steel cargo ship floats easily on the ocean. Steel is much denser than water in both cases. What actually allows the ship to float when a solid lump of the very same metal cannot?
- The ship's engines constantly push it upward through the water, cancelling out its weight
- Salt dissolved in the ocean makes seawater dense enough on its own to hold up any object placed in it
- The paint and coatings on the hull trap a thin layer of air bubbles that make the surface float
- The ship's hollow hull encloses a large volume of air, lowering its overall average density below that of water, so it displaces enough water to generate an upthrust equal to its weight
Why D? And why not the others?
Correct answer: D. The ship's hollow hull encloses a large volume of air, lowering its overall average density below that of water, so it displaces enough water to generate an upthrust equal to its weight
By Archimedes' principle, an object floats when it displaces a weight of water equal to its own weight, generating an upthrust that balances gravity; a ship's hull is mostly hollow, enclosing a huge volume of air, so the average density of the whole vessel - steel plus all that enclosed air - drops well below the density of water, letting it displace enough water to float, whereas a solid lump of the same steel has no enclosed air and stays denser than water throughout, so it sinks. The option about engines is wrong because ships float perfectly well with their engines switched off; propulsion moves a ship forward, it doesn't hold it up. The option about salty water alone is wrong because while seawater's slightly higher density than fresh water does add a little extra upthrust, it is far too small an effect to keep a solid block of steel afloat, and ships also float in fresh water. The option about trapped air bubbles in the paint is wrong because the floating effect comes from the vast air-filled volume inside the entire hull structure, not from a microscopically thin coating on its outer surface.
Source: BBC Bitesize GCSE Physics: Forces -- density, upthrust and Archimedes' principle