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## Main Question or Discussion Point

I understand the meaning of pressure on a surface to be force acting per unit area.

But when it comes to understanding pressure in the context of fluids in motion or at rest I think I am having some trouble grasping the concept.

When we say the pressure of a gas in a container (which may be of any arbitrary blobby shape) is say 2 Pascals, what does it really mean?

1) Does it mean the pressure of the gas on ANY small area considered on the surface of the container == 2 pascals?

2) Or Does it mean the pressure on a thin small wafer (possible imaginary?) placed anywhere inside the gas == 2 pascals?

I am really confused!

To add to my problem of understand pressure, is a sentence from Chorin and Marsden's a Mathemtical introduction to fluid dynamics when they define an ideal fluid.

*"Lets us define an ideal fluid with the following property: For any motion of the fluid there is a function p(x,t) called the pressure such that if S is a surface in the fluid with a chosen unit normal n, the force of stress exerted across the surface S per unit area at x in S at time t is p(x,t) n i.e.

force across S per unit area=p(x,T)n"*

Here is my problem: Suppose we have an ideal fluid and a time and position varying pressure function as above. For a thin small wafer placed at x at time t, there are two equal and oppsite forces acting across its surface i.e. p(x,t)n and -p(x,t)n since the surface has 2 forces in the direction of the two normals to it.

This reasoning means the force on the thin small wafer is zero. Which means that the force at any point the the fluid at any time is zero. Which is totally counterintuitive.

Further the pressure function (as defined above) does not depend on the orientation of the element. i.e. the normal n

I would expect the force magnitude to change if we change the orientation of the elment centered x

Why would they define an ideal fluid like that?

But when it comes to understanding pressure in the context of fluids in motion or at rest I think I am having some trouble grasping the concept.

When we say the pressure of a gas in a container (which may be of any arbitrary blobby shape) is say 2 Pascals, what does it really mean?

1) Does it mean the pressure of the gas on ANY small area considered on the surface of the container == 2 pascals?

2) Or Does it mean the pressure on a thin small wafer (possible imaginary?) placed anywhere inside the gas == 2 pascals?

I am really confused!

To add to my problem of understand pressure, is a sentence from Chorin and Marsden's a Mathemtical introduction to fluid dynamics when they define an ideal fluid.

*"Lets us define an ideal fluid with the following property: For any motion of the fluid there is a function p(x,t) called the pressure such that if S is a surface in the fluid with a chosen unit normal n, the force of stress exerted across the surface S per unit area at x in S at time t is p(x,t) n i.e.

force across S per unit area=p(x,T)n"*

Here is my problem: Suppose we have an ideal fluid and a time and position varying pressure function as above. For a thin small wafer placed at x at time t, there are two equal and oppsite forces acting across its surface i.e. p(x,t)n and -p(x,t)n since the surface has 2 forces in the direction of the two normals to it.

This reasoning means the force on the thin small wafer is zero. Which means that the force at any point the the fluid at any time is zero. Which is totally counterintuitive.

Further the pressure function (as defined above) does not depend on the orientation of the element. i.e. the normal n

I would expect the force magnitude to change if we change the orientation of the elment centered x

Why would they define an ideal fluid like that?