Is Water Pressure Equal in an Upside Down Submerged Cup?

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Discussion Overview

The discussion revolves around the concept of water pressure in an upside-down submerged cup within a larger container of water. Participants explore the principles of hydrostatics, pressure differences, and the effects of air pressure on the water in the cup.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the water in the upside-down cup experiences the same pressure as the surrounding water, and if there are differences in pressure between the top and bottom of the cup.
  • Another participant argues that if pressures were equal, there would be no net pressure force acting on the water in the cup.
  • It is noted that water pressure increases with depth, suggesting that pressure may not be uniform in this scenario.
  • A participant mentions that the water remains in the cup due to the vacuum created when the cup is inverted, as long as the integrity of the cup is maintained.
  • Some participants discuss the role of air pressure inside the cup, stating that it exerts a downward force on the water, while external air pressure exerts an upward force, contributing to the net pressure force.
  • There is a suggestion that if the cup is small enough, the pressure difference may be negligible.
  • One participant emphasizes that the issue may primarily involve air pressure rather than water pressure, as long as air cannot enter the cup.
  • Another participant clarifies that pressure at higher elevations is generally less than at lower elevations, unless influenced by other factors.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between air pressure and water pressure in the context of the inverted cup. There is no consensus on whether the pressures are equal or how they interact.

Contextual Notes

Some assumptions about the system, such as the size of the cup and the conditions under which air enters the cup, remain unresolved. The discussion also highlights the complexity of pressure dynamics in this scenario.

LocktnLoaded
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In grade school I remember the physics experiment of the cup of water turned upside down on a card, and kept the water in the cup, my question is based on the same principle except that the cup is turned upside down in a bigger container of water, my question is , is the water in the cup at the same pressure as in the bigger container of water, or is it slightly different?
If it is slightly different , is the pressure in the cup any different from the top of the cup compared to the bottom of the cup?
Also if anyone knows a good link on this , I'd appreciate a heads up, Thanks
 
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If the pressures were the same then there would be no net pressure force on the water in the glass and the force of gravity would be "unchallenged."
 
Water pressure grows from the surface to the bottom.
 
LocktnLoaded said:
In grade school I remember the physics experiment of the cup of water turned upside down on a card, and kept the water in the cup, my question is based on the same principle except that the cup is turned upside down in a bigger container of water, my question is , is the water in the cup at the same pressure as in the bigger container of water, or is it slightly different?
If it is slightly different , is the pressure in the cup any different from the top of the cup compared to the bottom of the cup?
Also if anyone knows a good link on this , I'd appreciate a heads up, Thanks

Well, if you turn upside down a cup of water on the top of a big container of water, then the water escapes out the cup and it becomes empty. Am I loosing something?.
 
When the cup has water in it and is lifted upside down in say a pan of water, as long as the cup doesn't break the field of the pan water and let in air, it keeps the water at the highest level of the cup because of the vacuum, that's what was meant.
 
LocktnLoaded said:
When the cup has water in it and is lifted upside down in say a pan of water, as long as the cup doesn't break the field of the pan water and let in air, it keeps the water at the highest level of the cup because of the vacuum, that's what was meant.

I see. Employ Hidrostatics. If the cup length is small enough, there won't be approximately any pressure difference.
 
Tide says, If the pressures were the same then there would be no net pressure force on the water in the glass and the force of gravity would be "unchallenged."
But wouldn't it be held in the glass by vacuum and not pressure, thus elimiting the build up of pressure?
 
Lock,

The air pressure inside the glass is not zero so it has to exert a downward force on the water. Likewise, the air below exerts an upward force on the water. The net pressure force is the sum of those forces (one negative and the other positive) and must be nonzero in order to sustain the weight of the water.
 
Tide said:
Lock,

The air pressure inside the glass is not zero so it has to exert a downward force on the water. Likewise, the air below exerts an upward force on the water. The net pressure force is the sum of those forces (one negative and the other positive) and must be nonzero in order to sustain the weight of the water.

the problem is air pressure not water pressure. as long as air cannot get in, the pressure inside will not change.
 
  • #10
Tide is referring to the pressure at the water surface
just under the glass "bottom" (which is now on top) as "air pressure".
You "always" get a small amount of air inside the glass.

The pressure at higher elevations is always less than at lower,
unless there are drastic changes in the speed (here, vtop=vbottom=0)
 

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