# Fluid Dynamics: Pressure / Opening a hatch underwater

1. Apr 29, 2007

### Farzan

Let's say that you're deep under the surface of the water in a submarine type thing.

There is a hatch on the top.

Let's say that the absolute pressure on the top of the hatch was calculated to be 2 atm.

Inside the vessel, the pressure is only 1 atm.

Someone is inside the vessel and the force needed to open the hatch must be calculated.

Do you need to take into account the pressure inside the vessel?

P = F/A

Do you plug in 2 atm, or 1 atm?

My physics teacher said that you only look at the pressure on the outside, but he does not remember why.

2. Apr 29, 2007

### Danger

Then why is he teaching physics?
I know pretty much nothing about anything, but I do know that the force required to open the hatch is based upon the ratio of interior/exterior pressure.

3. Apr 30, 2007

### lpfr

Not the ratio. The difference: F= (Po-Pi)S

4. Apr 30, 2007

### Danger

Good catch. I have to start being more accurate in my terminology. After a few beers I get sloppy.

5. Mar 29, 2008

### Deathwing

Wouldn't you take the difference in pressure? You have 2 atm pushing in and 1 atm pushing out, requiring you to push through 1 atm to open the hatch, right?

6. Mar 29, 2008

### Staff: Mentor

Realize that you are responding to a thread that's been dormant for almost a year. In any case, the point about needing the difference in pressure was made by lpfr in post #3.