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This question already has an answer here:

In Bernoulli's equation the pressure plus kinetic and potential energy is constant. But what is this pressure term, physically? Is it the pressure of the fluid on the walls of the container or is it the pressure exerted on the fluid by the surroundings (air/liquid as air is also fluid). Because these two types of pressure are entirely different. In torricelli's law it is the pressure exerted by the atmosphere on the fluid and in venturimeter it is the pressure that the fluid under consideration exerts on the tube. I want some logical answers so that I could easily see what this pressure is physically - the formulas I know.

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marked as duplicate by stafusa, Jon Custer, John Rennie, Kyle Kanos, Qmechanic Nov 26 '17 at 19:10

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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You don't need to have a solid surface or a free surface in order for fluid pressure to exist. More generally, each parcel of fluid within the bulk of the fluid exerts a pressure on the surrounding parcels of fluid, and the surrounding fluid exerts equal but opposite pressure on the fluid within the parcel. At a surface separating two immiscible fluids (like water and air), the pressures of the two media match one another at the boundary. At a solid wall, the pressure that the fluid exerts on the wall is equal to the pressure that the wall exerts on the fluid. So there is really only one kind of pressure.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Nov 25 '17 at 19:46
  • $\begingroup$ That would, of course, slow down the hole velocity. Are we getting close to the end of your questions? I feel that you should have been able to answer this question on your own. $\endgroup$ – Chet Miller Nov 25 '17 at 19:49
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If you see the derivation of the Bernoulli's equation, it is nothing but one of the application of Work-Energy theorem. The pressure at any point of fluid here is the force per unit area experienced at that point by the rest of the fluid.

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