Conceptual problems with pressure, me to clarify

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    Conceptual Pressure
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Discussion Overview

The discussion revolves around the concept of pressure in fluids, specifically addressing why air pressure is included in calculations of pressure at various depths in water. Participants explore the relationship between air pressure and water pressure, as well as gauge pressure.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why air pressure is factored into pressure-depth calculations, particularly noting that there is no air underwater.
  • One participant asserts that total pressure at any depth is the sum of air pressure and water pressure, using an analogy of weights on a scale to illustrate this point.
  • Another participant emphasizes that the water must support the weight of the air above it, suggesting that the analogy provided is a precise description rather than a mere analogy.
  • A follow-up question is raised about whether gauge pressure is simply the total pressure minus air pressure.
  • Some participants clarify that air pressure is not constant at different depths, referencing pilots who use barometers to measure altitude.
  • There is a discussion about the interpretation of the original question regarding the force of air at different depths, with some participants expressing confusion over the phrasing and intent of the question.
  • One participant introduces a hypothetical scenario involving a closed pipe submerged in water, suggesting that air pressure considerations still apply.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of including air pressure in depth calculations. While some agree on the importance of air pressure, others challenge the clarity of the original question and its assumptions. The discussion remains unresolved regarding the interpretation of pressure at various depths.

Contextual Notes

There are unresolved assumptions regarding the phrasing of the original question and the implications of air pressure at different depths. The discussion also reflects varying interpretations of gauge pressure and its relationship to total pressure.

jenzao
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Why is air pressure always factored in when calculating pressure at a given depth of water?
Why is the force from air pushing down on the surface of water the same at 1m as it is at 100m?
there is no air underwater, so why do we still need this value when doing pressure-depth calculations?
 
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Total pressure at any depth in the water is sum of air pressure and water pressure. The air pressure at the surface, independent of depth, has to be added into get the total.
For a simple analogy, put a ten pound weight on a scale - it will read ten pounds. Put another ten pound weight on top of the first and the scale will read twenty pounds.
 
The water must be able to sustain the weight of the air above it.

Thus, mathman's analogy is hardly just an analogy, rather, it is more of a precise description.

As I am sure he knows full well.
 
this was explained really well, thank you. just quick follow-up.. Is gauge pressure simply pressure minus the air pressure? (101325Pa)?
 
Yes.

Btw, air pressure is not the same at 1m as at 100m. Pilots use barometers (air pressure sensors) to measure their altitude.
 
russ_watters said:
Btw, air pressure is not the same at 1m as at 100m. Pilots use barometers (air pressure sensors) to measure their altitude.

I'm sure (!? uh -ohh better to stop typing now) that he meant 1 meter or 100 meters depth.
 
gmax137 said:
I'm sure (!? uh -ohh better to stop typing now) that he meant 1 meter or 100 meters depth.
I think he meant something like that, yes.

I quote:
Why is the force from air pushing down on the surface of water the same at 1m as it is at 100m?

This question makes no sense. There is no surface at 1m (depth?) or 100m. I'm guessing that you meant why you always have to add up the air pressure at the surface of the water whether we're talking about (total) pressures at 1m depth or 100m depth. I think the OP understands the answer already, but just to be sure. If you put three hamburgers on top of each other and you squash the top one, the bottom one will also get squashed, and not only the first (and second). :)

I'm hungry now.
 
gmax137 said:
I'm sure (!? uh -ohh better to stop typing now) that he meant 1 meter or 100 meters depth.
Perhaps, but does it really matter...?

If you drop a closed 100m pipe into the water, you can still use a barometric altimeter in it.
 

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