# Plastic bottle floating and sinking

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.

• 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? Commented Sep 17, 2013 at 17:04
• 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. Commented Sep 17, 2013 at 17:14
• @johnRennie when i push the cup it floats again Commented Sep 17, 2013 at 18:59
• 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? Commented Sep 18, 2013 at 12:30