What forces are involved in a container trapping air in water?

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Discussion Overview

The discussion revolves around the forces involved when a container traps air in water, particularly focusing on the dynamics of compressible and incompressible fluids and the resultant forces acting on the container. The scope includes theoretical modeling and mechanical implications.

Discussion Character

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

Main Points Raised

  • One participant questions whether there are substantial upward forces besides buoyancy when a bowl-shaped container traps air in water.
  • Another participant notes that air is very compressible, suggesting that compressibility may play a role in the forces involved.
  • A participant clarifies that both air and water are somewhat compressible and questions if their interaction could create a normal force countering the external force applied to the container.
  • One participant argues that the compressibility of water can be ignored due to its much higher modulus compared to air, suggesting that viscosity effects are secondary in a wide bowl scenario.
  • Another participant expresses that the problem could have mechanical engineering implications, although they believe it may not be a novel consideration.
  • A later reply proposes modeling the situation as a massive piston falling into a gas volume, emphasizing the need for appropriate equations to describe the forces and pressures involved.

Areas of Agreement / Disagreement

Participants express differing views on the significance of compressibility and viscosity in the scenario, indicating that multiple competing perspectives remain without a clear consensus on the primary forces at play.

Contextual Notes

The discussion highlights the complexity of modeling the scenario, including factors such as fluid viscosity, changing volume and pressure of the air, and the surface area in contact with the air, which remain unresolved.

Who May Find This Useful

Readers interested in fluid dynamics, mechanical engineering, or theoretical modeling of forces in fluid systems may find this discussion relevant.

thielen24
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Imagine you apply an external force to an upside-down bowl shaped container, moving it into a flat surface of water and trapping air in its underside. Are there any substantial upward forces besides the buoyant force? If the impulse is high enough, is there a considerable force produced by the incompressibility of the either of the liquids?
 
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One of the Fluids (?) - air - is very compressible, surely.
 
What I meant by "one of the fluids" was either the air or the liquid below it. Surely they are both somewhat compressible, but does the interaction between two compressible fluids cause a normal force to counter the external downward force? To me this seems like a question that would need to be modeled using many parameters (such as the viscosity of the fluids, the changing volume and thus pressure of the air, the changing surface area in contact to the air) to visualize and solve, but I may be over complicating a simple situation.
 
The modulus of the water is so much higher than that of air that I reckon you can ignore any compressibility of water.
In a wide bowl, even the effect of viscosity would be secondary.
I don't actually fancy doing the sums myself but I guess I could make a stab at it if I really had to, making the model as simple as possible to start with. My first approach would be to consider a cylindrical bowl (parallel sides) and see what the gas laws give you when the mass of water flows up into the space. The water will flow at a speed determined by density and pressure difference.
 
It's not really necessary... I was just thinking about this last night trying to go to bed. The question could have some nice mechanical engineering implications, though probably nothing that hasn't already been considered.
 
I think the problem boils down to a massive piston (possibly varying mass) falling into a volume of gas. Get the right equation to describe forces / pressures and masses and then get a smart brain to solve it.
 

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