- #26

Ken G

Gold Member

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We do, that's what pressure is. Many people think pressure is a force per unit area, but not always-- it only becomes a force when you strip away half the stuff, leaving the force from the other way unbalanced. But pressure usually isn't like that, it's only like that at a boundary and only if you look at only have the story. So you can think of pressure two different ways-- either say pressure is a force per unit area, and only exists at a boundary, so when you talk about it in the interior, you imply you have stripped away one side and created a boundary. That's what we mean when we say "a piston exerts a pressure", so it's best used for external pressure. Or, we can say that pressure is a momentum flux per unit area, coming from both sides, so exists in the interior of the fluid or solid, and is best thought of as an interior pressure. Your confusion is between these two rather different, yet quantitatively equal, meanings of pressure.But since the fluid is static, the fluid layer at that depth must exert some sort of reaction force to the weight of the fluid. Why don't we consider this when we calculate pressure at a given point in a fluid.