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.