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I was doing this experiment: i take a plastic cup, put in a container of water, it floats – then when I put a coin in the water, it sinks and rests at the bottom.

So to make the cup sink, i fill it with water – and the cup sinks, but it sinks until it's totally submerged with water and stops ( doesn't sink to the bottom )

Why did that happen? Why didn't it sink like the coin?

PS: I am aware of the presence of this thread, and it doesn't answer my question , so please do not mark duplicate.

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  • $\begingroup$ If you fill the cup with water and push it (just the cup - no coin) to the bottom, does it stay at the bottom or does it float up to the surface again? $\endgroup$
    – John Rennie
    Commented Sep 17, 2013 at 17:04
  • $\begingroup$ PS if the cup floats up from the bottom then its density is slightly less than water as gregsan suggests. If the cup stays on the bottom then you're seeing the cup pinned to the surface by surface tension. $\endgroup$
    – John Rennie
    Commented Sep 17, 2013 at 17:14
  • $\begingroup$ @johnRennie when i push the cup it floats again $\endgroup$
    – user28324
    Commented Sep 17, 2013 at 18:59
  • $\begingroup$ If you put the coin in the cup, does it sink? How big does the cup need to be before it will not sink when it has a coin in it? $\endgroup$
    – New Alexandria
    Commented Sep 18, 2013 at 12:30

1 Answer 1

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this will happen if the cup material's density is very close to that of water itself. completely submerged, the buoyant force on the cup is very similar to its own weight, so it appears to neither sink nor float underwater.

Polystyrene cups are about 1g/cm³. these will do. heavier duty plastic cups will definitely sink faster, although not as fast as a coin.

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  • $\begingroup$ So if i used something lighter , with a density of 0.4 g/cm3 for example, and same volume as the cup and weight of the plastic cup, will it sink when filled with water? $\endgroup$
    – user28324
    Commented Sep 17, 2013 at 19:05
  • $\begingroup$ a cup of less dense material cannot both be the same volume and weight of a cup made of denser material. a 0.4 g/cm3 cup of the same volume will be lighter, and will very clearly float after being totally submerged. $\endgroup$
    – gregsan
    Commented Sep 17, 2013 at 19:47

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