Water can't be compressesd, so why does pressure increase with depth?

[Xema]

@Tom_Tildrum

… keep in mind that while the water may not be compressible, the person in it certainly is.

Yet compressibility - whether of the water or the person - has essentially nothing to do with the increase of pressure with depth.

[Tyrrell_McAllister]

@DSYoungEsq

THIS is what is not intuitive to the poster of the OP. The stack of bricks example makes it clear that a taller column of bricks weighs more, but if I put a second column of bricks next to my hand, so that the side of my hand is touching the second column, the side of my hand doesn’t feel the “weight” of that second column pressing on it. This is because solids don’t flow, so the downward pressure on the top doesn’t force the sides of the bottom brick to press outward.

But with water, water flows, so that the pressure from above forces the water at the bottom to try and move sideways. But more water is there, so it can’t go anywhere. If I’m at the bottom of that water, I will feel that attempt to press sideways coming on me from all directions.

Maybe think of a column of stacked ice-cubes. When they are all frozen, you essentially have the stack-of-bricks case. Now suppose you zap the ice-cube on the bottom of the column with a laser with just enough energy to melt it (but not vaporize it). If your hand is next to the bottom of the column, what happens? When the bottom ice-cube liquefies, the ice-cubes above it press down, causing the water from the bottom ice-cube to squirt out in all directions. In particular, some will splatter onto your hand, applying some amount of pressure. That’s how one can see that when a liquid is pressed from above, it presses out laterally.

In a sense, this happens precisely because the liquid is incompressible. Imagine the opposite extreme, where the liquid is perfectly compressible. The melted ice-cube would be squashed flat by the cubes above it before it had a chance to splatter sideways, so there would be no pressure applied laterally.

[sailor]

@Diogenes_the_Cynic

The “pressure” being talked about at those depths is not the water pressing on itself, but pressing on you. You are what is being compressed, not the water.

Nonsense. I do not have to be there at all. Or is this the equivalent of a tree falling in the forest but nobody being there to hear it?

[sailor]

@Leaffan

I understand that the weight of the water comes into play. But since liquids can’t be compressed, how can the water pressure at 100’ be any different than at 10’?

Compressibility don’t enter into it.

It is a liquid and that’s all that matters. I think your reasoning is that if you are inside a mountain or a mine or even a building, you are not holding up all the weight above you. True, but something else is. But this is possible with rigid materials, not with liquids. For example, water is not a good building material for that very reason. It does not hold up well.

[Gbro]

@rbroome

“liquids can’t be compressed” ?
water, just like anything else, is compressible. Water at depth is certainly compressed and weighs more per unit volume than at the surface.

Its been a number of years since I have dove as my wet suit shrunk:o
110 feet is the max I ever dove and there was a sensation of sluggishness in dives deeper than 75 ft. from my recollection. I used to think it was more psychological than physiological. Regulator noise was much more pronounced but that could have been psychologically induced by the heightened level of awareness:dubious:
If I am way off base here, what else would one seance different from say 15 ft and 100 ft depth with no thermocline to deal with? ( I have dove Lake Superior’s north shore with an easterly wind and dove to 80 ft before reaching the thermocline).

[Turek]

@Gbro

Its been a number of years since I have dove as my wet suit shrunk:o
110 feet is the max I ever dove and there was a sensation of sluggishness in dives deeper than 75 ft. from my recollection. I used to think it was more psychological than physiological. Regulator noise was much more pronounced but that could have been psychologically induced by the heightened level of awareness:dubious:
If I am way off base here, what else would one seance different from say 15 ft and 100 ft depth with no thermocline to deal with? ( I have dove Lake Superior’s north shore with an easterly wind and dove to 80 ft before reaching the thermocline).

At that depth, there’s a pretty good chance you were suffering from some degree of nitrogen narcosis.