Here is my understanding of pressure, stemming from this physics.stackexchange answer and the idea that pressure in water at a certain depth is completely based on the gravitational acceleration the body experiences and the mass of the column impressing a force on some cross-section of area. I realize this generally extends to liquids of constant densities, but discussing water is probably the only case necessary for the conceptual ideas.

So given a cube of water (or any liquid of constant density), if a molecule enters this cube in some manner, its pressure suddenly increases and another water molecule may escape in any direction. This is why pressure acts in all directions.

The reason why pressure increases as depth increases, as I understand it, is that the amount water that is attempting to enter some defined cube of water at a certain depth increases with depth. The water closer to the surface that is trying to enter lower depths is trying to do so because it is experiencing the force of gravity. If this is true, then we can say that water not experiencing a gravitational field does not experience a preferred direction to enter, and so this concept of pressure breaks down.

I asked my high school physics teacher the following: If we consider a blob of water not experiencing any force field, then does our current concept of pressure throughout a body of water break down?

He said that it actually increases towards the center of mass of this blob of water, so that 'depth' still matters. I'm unsure about this. Could someone explain whether this is the case, and if so, what is physically happening to make that the case?

  • $\begingroup$ You probably need to ask your physics teacher to explain his statement more fully. I also teach physics, and any answer that I would give you would be "out of context" relative to the discussion that you and your teacher already had. $\endgroup$ – David White Nov 30 '16 at 3:47

"...it actually increases towards the center of mass of this blob of water..." This actually happens in a star due to self-gravitation. In our ordinary blob of water this effect is completely negligible. In the absence of gravity (due to some large mass, such as earth) pressure inside the blob would assume a uniform positive value. Recall that pressure variation with depth is written $p_0+\rho g h$. $p_0$ is exactly that pressure that would be obtained in the absence of gravity.


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