Understanding Pressure Imbalance in a U-Tube: Exploring Fluid Mechanics

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Discussion Overview

The discussion revolves around the mechanics of pressure in a U-tube with differing diameters on each side, focusing on the forces acting on the liquid within the tube. Participants explore concepts related to fluid mechanics, pressure balance, and the implications of tube geometry on liquid behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the liquid is not pushed to the left side despite greater atmospheric pressure on the right side, suggesting a misunderstanding of the forces involved.
  • Another participant clarifies that the upward force from the sides of the tube also contributes to the net force on the liquid, indicating that the net force is zero.
  • A request for a free body diagram is made to better understand the force balance in the U-tube scenario.
  • One participant expresses gratitude for a helpful external resource that clarifies the reaction forces at play when the tube narrows.
  • There is a discussion about the relationship between force and mass, with a participant suggesting that the analogy used may not directly apply to the U-tube situation.
  • Another participant emphasizes that the net force on the fluid is crucial for understanding its motion, challenging the notion that atmospheric pressure is the only force acting on the fluid.

Areas of Agreement / Disagreement

Participants exhibit a mix of understanding and confusion regarding the forces acting on the liquid in the U-tube. While some points of clarification are made, there remains no consensus on the initial question posed about the pressure imbalance and its implications.

Contextual Notes

Some participants express uncertainty about the balance of forces and the need for visual aids, indicating that assumptions about the system's behavior may not be fully articulated or understood.

Who May Find This Useful

This discussion may be of interest to individuals studying fluid mechanics, particularly those exploring concepts of pressure, force balance, and the effects of geometry on fluid behavior.

tsochiu
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There is a u-tube which its diameter on the left hand side is smaller than the right hand side.
It is filled with liquid.
As the atmosphere pressure is equal in the two side, and force = pressure*area.
I want to ask, why the liquid in the u-tube is not pushed to the left side as the force from atmosphere on the right side is larger than left? Or what is wrong in my concept?
 
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While it's true that the force of the atmosphere is greater on the right side surface, that's not the only force acting on the liquid: The sides of the tube also exert an upward force on the right hand side. (The details depend on the exact shape of the tube.) The net force on the liquid will be zero.
 
Sorry, I cannot understand it clearly, could you give me an example on how the force is balanced? With a free body diagram will be better. Thank you very much
 
Why don't you provide a diagram of the U-tube in question.

You might find this discussion helpful: http://scubageek.com/articles/wwwparad.html"
 
Last edited by a moderator:
Your web page is really helpful. I can understand what is happening in the u-tube now.
That is when the u-tube start to narrow,the wall of the u-tube will give a reaction force toward the liquid which will cancel out the extra force due to the wider mouth.
Thanks for your help, it solve my great problem with pressure.
 
If you push on an object with twice the force with which you push on an object of half the mass, which will accelerate faster?
 
tsochiu said:
That is when the u-tube start to narrow,the wall of the u-tube will give a reaction force toward the liquid which will cancel out the extra force due to the wider mouth.
Exactly.

russ_watters said:
If you push on an object with twice the force with which you push on an object of half the mass, which will accelerate faster?
Regardless of the mass of the fluid, if there's a net force on it, it will accelerate. The problem here is in thinking that the only force on the fluid is that due to atmospheric pressure.
 
It was an analagous situation, not an analysis of this situation.
 
I guess I didn't see the analogy.
 

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