A very succinct title.
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’?
ETA: And, of course, I can’t edit the title typo.
Weight I’d say. Compression doesn’t really come into it…just think of the mass of the water as you descend.
-XT
What would compression have to do with it? A hundred feet of water is going to weigh a lot more than ten feet of water.
Water can be compressed. Specifically, it has a bulk modulus of 2.2 x 10[sup]9[/sup] Pa.
Of course, this has nothing (or very little) to do with why pressure increases with depth.
I don’t get it though. Water isn’t compressed, so what difference does the weight of additional water piled on top make? The water at 100’ is still the same density. What changed?
Concrete (effectively) is also incompressible. Would you rather have 2" or 30’ on top of you? Compressibility has to do with volume change, it doesn’t apply here.
The vertical component of pressure on a submerged body is equal to the weight of the column of water vertically above its projected area. Likewise, the horizontal component is the pressure force on a vertical projection of the area.
Put your hand on a table place one brick on your hand. Now add a second brick. then a third and fourth. Keep adding until you have 20 on your hand. The bricks do not compress but your hand will fell the weight.
YOu are confusing weight with compression.
And I know someone will point out if you stack 20 bricks on your hand they will fall over.
You’re correct, of course. Air pressure I can see changing since air is compressible, but water isn’t, and although weight and compression are indeed different, I have a hard time visualizing the concept.
Your concrete analogy hits the nail on the head, but my brain has to think hard about water. Don’t know why, and in all my life I never even considered this question until this morning.
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.
No, both a person at depth and the water at that depth with have the same pressure exerted on them. Equal and opposite and all that.
Water can be compressed, do you really think that water just remains water under extreme gravity? if poured a bunch onto a neutron star it would maintain the same density it has on earth?
water doesnt compress much under normal earth conditions but it is completely compressible. if it wasnt then sound waves either woulndt move through water at all or they would travel at light speed…no even faster than light yeah whatever…the point is whoever started this whole uncompressible nonsense is probably the same moron who started the whole drink 8, 8 oz glasses of water a day even if you weigh 100 pounds or 300 pounds you need exactly 64 oz of water per day.
You are mistaking density for pressure. The pressure in water is the force per unit area that the water exerts on things immersed in it. In an open pool of water (like the ocean) the force at a particular depth comes from the weight of the water above that depth. It isn’t necessary for the density of the water to change for the pressure to change.
Imagine yourself standing with a 100-pound weight on your shoulders. That weight would put a downward pressure on your shoulders. There would be an upward pressure from the floor onto the soles of your feet. These pressures would exist even though you are not compressible.
A fluid is a bit different from the example I just gave because the pressure is in all directions. For example, the pressure at a depth of 10 feet in fresh water would be about 19 pounds per square inch. This pressure would be the same regardless of direction - up, down or sideways. The same idea applies, though - the pressure is caused by something above pushing down, and is met by something below holding it up.
I think what you need is more of a tradesman’s perspective. One of the tasks I might be asked to do in a shift is provide a means to automatically detect the level of fluid in a tank. A bit of knowledge that greatly simplifies the task for me is that I know the hydrostatic pressure of the water is going to be roughly equal to .43 pounds per square inch per foot for water. For a different liquid we would need to know the specific gravity of the liquid. If I install a pressure gauge near the bottom of the tank I can do some math to determine the level of the liquid.
The important part here is that each foot of water in the column weighs .43 per square inch. So ten feet of water exert 4.3 psi on my gauge. The units are important here. The 4.3 is for each square inch. To find the weight pressing on a certain depth of the tank I need to know the area at that part of the tank as well as the pressure at that point. Force = pressure x area so if my 4.3 psi is pressing on 100 in^2 I get 430 pounds of water. If I raise the level of water in the tank the psi on the gage goes up and the force applied to the bottom of the tank goes up. The water may compress ever so slightly at the bottom but it doesn’t really factor in to the calculation. The weight of the column of water pressing on that area is what is really important to me.
I hope that helps a bit. It’s not the nitty-gritty science but it’s fairly intuitive way to look at it.
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.
The idea that water is incompressible comes from, I think, a comparison with air. In doing many calculations involving small changes in pressure, the compressibility of air (and of gasses in general) is an important factor in the calculations. For water (and liquids in general), it’s not so much–and it’s reasonable to assume that, for purposes of specific calculations, water is incompressible, as the eror intorduced by the asumption is negligible. Voila, a bit of misinformation is born.
“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.
But by an amount that, as zut notes, can be ignored in most cases - including consideration of why pressure increases with depth. (Googling suggests that the density increase is less than 1% per 2000m of depth.)
I completely disagree. Simplifications are critical in all applications. This happens to be an extremely useful approximation for many many applications. Might as well call Newton a retard for thinking his laws are true.
Leaffan, keep in mind that while the water may not be compressible, the person in it certainly is.