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I was reading this question: What is the counter-force to suction force?

OP says pressure ends up uniform everywhere after some time.

I don't see why unless there is a dissipation of energy. If there is sound, for example, in a closed reverberant room, the pressure waves will decay only through absorption of the walls, being imperfect reflectors.

But maybe I am taking air to be a perfect conductor of sound while it is not really, losing focus (high frequencies) over time.

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Sound waves as we know them are low-amplitude solutions to the equations of hydrodynamics. A small inhomogeneity can become a sound wave. A large inhomogeneity is non-linear (by definition, something is only linear when you can make linear approximations, which requires things to be small). Non-linear effects dissipate energy, because dissipation exists in the full hydrodynamic equations.

It should also be clear that free work is being wasted. Because a pressure difference (for example, vacuum on one side and pressure on the other) has lower entropy than homogenous pressure.

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If we have an open space and air at different pressures within it, then the pressure will equalize over time. This process does not take much energy, and it takes zero energy for an inviscid gas (used in fluid dynamics to analyze, assumes no viscosity - which is friction losses from particle collisions/interactions)

So it’s like a bunch of perfectly elastic balls bouncing around, they lose no energy and bounce off each other. They will hit each other until the average velocity is the same in any relatively large volume. And since molecules are tiny, even what we think of as a small volume would count as “relatively large”. This is the process of pressure equalization.. collisions. It is also the basis of pressure force.. collisions against a surface.

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