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So I was having a doubt of why metal block sinks in water. I was wondering why water molecules that have the capacity to exert pressures supporting the water columns at depth of say Mariana Trench cannot lift a block of iron.

With a bit of help, I came up with this explanation:
Demonstration of the fact

  1. The block is placed in water$\implies$ Pressure on the surface molecules increases.
  2. Lateral pressure is equal to upward pressure. (As aso explained in this question and this question)
  3. Moecules experiencing the extra pressure shift in other directions to regions of lower pressure(1st layer of molecules is shown by darker blue line) and the block sinks a little.
  4. The shifted water, that above the second layer(& above the block) and the weight of the block increase pressure on $2^{nd}$ layer of molecules and the process(sinking) continues.
  5. The reverse happens for floating.

These questions (1 and 2) seem to agree that pressure difference is the cause of upthrust.

  • Is my explanation correct?

I am hoping it is correct, which brings us to the real question.
The same block is put in the centre of a U-tube (some kind of teleportation or maybe it is suspended by string that later dissolves) in this way
The arrangement

  • Given sufficient height of arms of the U-tube(columns), will even metal float?(irrespective of density differences?)
Aurelius
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1 Answers1

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This doesn't have to do with pressure, it has to do with density. Ocean water varies in density from about 1.03 g/cc at the surface to 1.08 g/cc at the ocean floor. Iron, meanwhile, is 7.9 g/cc. Hence it will never float on water.

However, it might interest you to know that iron will float on mercury, which has a density of about 13 g/cc. You can see images and videos of this.

RC_23
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  • So a jet of water cannot lift iron ? – Aurelius Sep 17 '23 at 18:08
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    A jet of water operates on an entirely different principle than buoyancy. In a jet, it is the momentum of the water being transferred to the object (e.g. a piece of iron) which supports the object's weight. You could even do this with a jet of air if powerful enough. – RC_23 Sep 17 '23 at 18:10
  • Sorry my bad, but if link has a metal block placed on a hole can't pressure lift it? – Aurelius Sep 17 '23 at 18:21
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    I'm not sure what that picture is, but for example the reason pressurized oil can move a piston is it is only pressurized on one side, not the other. For a submerged item, the pressure is very nearly the same on all sides. There is a slight pressure difference between top and bottom that causes buoyancy, but this will never be enough to lift a solid item unless the density of the solid is lower than that of the surrounding fluid – RC_23 Sep 17 '23 at 18:33
  • so is my explanation of upthrust wrong? – Aurelius Sep 17 '23 at 18:35
  • Surface tension will allow an iron needle to float on water. – MaxW Sep 17 '23 at 18:41
  • Good point______ – RC_23 Sep 17 '23 at 19:04