We know that pressure in fluids increases with depth but why does it happen, if pressure is caused by the collision of particles?
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2 Answers
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Pressure also has to do with the amount of fluid which is pushing onto the surface you are talking about. In a container, for example, a surface which is at a greater depth has a larger column of fluid above it (i.e. a greater weight pushing down on it.).
The pressure on a surface at some depth $h$ is calculated as: $$P=P_o +\rho gh$$ (where $g$ is the gravitational field/acceleration in that region and $P_o$ is the atmospheric pressure.)
If you mess around with this formula a bit you will realize that this quantifies pressure in terms of the weight of the fluid column above the surface you choose.
answered Oct 5, 2015 at 16:38
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$\begingroup$ You mean to say that if a body was immersed into a fluid the fluid column above the body acts perpendicularly downwards on the body. But when we study buoyant force we are said that the buoyant force is caused by the pressure differences at the top of the body and the lower part of the body. You mean to say that the body immersed experiences different magnitudes of pressure at the top and the bottom you mean to say that there is more pressure at the top than at the bottom, then the body should sink because there is actually more force trying to push it down. $\endgroup$ Commented Oct 5, 2015 at 16:46
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1$\begingroup$ You misunderstood me, there is more pressure at a lower part of the body. (It's counter-intuitive, but this doesn't necessarily have to act perpendicularly downwards on the body.) $\endgroup$ Commented Oct 5, 2015 at 16:48
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2$\begingroup$ @Faizan: Open a tube of toothpaste. Press on the side. Toothpaste comes out of the opening. That is perpendicular to the direction of your pressure. That's what happens in fluids - the pressure goes quite easily "around corners". $\endgroup$ Commented Oct 5, 2015 at 18:23
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$\begingroup$ That is an excellent example, @MikeDunlavey $\endgroup$ Commented Oct 6, 2015 at 12:09
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$\begingroup$ Well, what would happen if you would have a bucket of water in space with negligible force of gravity acting upon it, would it still exert the same pressure. @Mike_Dunlavey $\endgroup$ Commented Oct 7, 2015 at 11:15
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"Collision of particles" is just one way that particles can repel each other. They can just be near each other and not want to be so near. That's what happens in fluids -- particles get pushed closer, and they repel more, which is what pressure is.
