Name the forces involved.

thielen24

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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sophiecentaur

Gold Member
One of the Fluids (?) - air - is very compressible, surely.

thielen24

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.

sophiecentaur

Gold Member
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.

thielen24

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.

sophiecentaur

Gold Member
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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