# Pressure in a submarine

The pressure in a submarine inside water
P=h*ρ*g+Pa
Okay,
Since there's Atmospheric pressure inside the submarine (Pa),we calculate the total pressure on the submarine from the relation=h*density*g
we just neglect the atmospheric pressure
why?I don't understand?
Why don't we calculate the total pressure from the relation P=hρg+2Pa?
does the atmospheric pressure in the submarine have a different direction?

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russ_watters
Mentor
Where did you get those equations? For a rigid hull sub, the pressure inside can be whatever the designer and operator choose. You can't calculate it.

The pressure in a submarine inside water
P=h*ρ*g+Pa
Okay,
Since there's Atmospheric pressure inside the submarine (Pa),we calculate the total pressure on the submarine from the relation=h*density*g
we just neglect the atmospheric pressure
why?I don't understand?
Why don't we calculate the total pressure from the relation P=hρg+2Pa?
does the atmospheric pressure in the submarine have a different direction?
Why not ?
Do you know how less in the atmospheric pressure as in comparison to water exerting pressure on submarine below water ? Moreover atmospheric pressure is exerted maximum on surface of water. Do you know how below is the submarine from surface level ?

Counter question having same logic : Why do you think air friction is neglected during free fall of body ?

Pressure inside is independent of outside , yes Halls.
These are opposite too. You just can't add them Misr.
:)

@ HallsofIvy and Russ Watters
[I thought he was only referring to atmospheric pressure on surface of water.]

Last edited:
HallsofIvy
Homework Helper
Even "general physics" questions have to make sense. A submarine is made of rigid metal. The pressure on the outside (as long as it is not great enough to fracture the metal) is completely independent of the pressure on the inside.

mathman
The formula (which would be clearer if all symbols were defined) appears to be that on the outside of the submarine. Since almost all the outside pressure is from the water, neglecting the air pressure doesn't make much difference. Also the air pressure on the inside may be kept (for comfort) at roughly that of the surface air pressure, so the pressure formula would give the net pressure on the hull.

rcgldr
Homework Helper
Pressure outside the submarine hull = h*ρ*g+Pa
Pressure inside the submarine hull = Pa

Pressure differential on the hull = outside pressure - inside pressure = h*ρ*g + Pa - Pa = h*ρ*g

Resolved
Thanks very much