General Help in Understanding Pressure. PLEASE HELP

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Discussion Overview

The discussion revolves around the concept of pressure in fluids, particularly in the context of fluid flow through a pipe. Participants explore the relationship between dynamic and static pressure, as well as the implications of pressure changes when fluid moves from wider to narrower sections of a pipe. The inquiry includes both theoretical understanding and practical applications, with a focus on clarifying misconceptions about static pressure.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about why static pressure decreases when dynamic pressure increases as fluid flows from a wider to a narrower section of a pipe.
  • The same participant questions the nature of static pressure, asking whether it acts equally in all directions or only horizontally and vertically, and how it interacts with the pipe and fluid.
  • Another participant introduces the concept of energy balance, comparing it to the potential and kinetic energy of a dropped ball or a pendulum, suggesting that static and dynamic pressures change to maintain energy balance.
  • There is a mention of how the impact of fast-moving water can cause injury due to the concentration of energy on a small area, contrasting it with slower-moving water that may exert force differently.
  • A question is raised about the behavior of a manometer and why the height changes if total pressure is constant throughout the flow.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views regarding the nature of static pressure and its behavior in fluid dynamics. The discussion remains unresolved with ongoing questions and clarifications needed.

Contextual Notes

Participants express uncertainty about the definitions and implications of static and dynamic pressure, as well as the conditions under which pressure measurements are taken. There are unresolved questions about the relationship between pressure changes and energy conservation in fluid flow.

Physicsman89
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I thought I understood pressure in fluids, but it seems like its starting to make less sense to me.

Let's consider a flow through a pipe where the rho g h term drops out. Therefore, we would only have the dynamic and static pressure. This is where I start to lose it. I have been told that when the flow of water goes from a wider section of a pipe to skinnier one. the dynamic pressure increases and static pressure decreases. The dynamic pressure going up makes sense, but I don't understand why the static pressure goes down. I think it may be that I don't understand exactly what static pressure is.

Does the static pressure act equally in all three dimensional directions or does it act only horizontally and vertically? Does it act on the pipes pressing outward or does it act horizontally pressing the fluid forward? I don't understand why the pressure wouldn't increase if the fluid is moving faster because i think getting hit by something fast would hurt more than getting hit by something slow.

Could someone please give a good basic description of what it is and how it changes because I can't find any simple answers. I would really appreciate it!
 
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To add to that, if you're using a manometer, why would the height change if the total pressure is constant throughout the entire flow?
 
Anyone?
 
It's energy balance.

You could compare it to dropping a ball from a height h.
At h the ball has a potential energy but no kinetic energy.
from the moment the ball is dropped, the potential energy decreases but since the ball is picking up speed, the kinetic energy increases.

Or a pendulum with rising and falling potential and kinetic energy as the bob sweeps along its arc.

The static and dynamic pressure work the same way to balance the energy equation.
As the diameter of the pipe cahnges from small to big or visa versa the dynamic and static pressures change also.

You get hurt from a fast moving stream of water because the water is being stopped by a particular small part of your body, and all that energy concentrated in one spot. A slow moving stream of water of the same amount of energy could possibly sweep you off your feet.
 

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