1. The problem statement, all variables and given/known data Water stands at a depth D behind the vertical upstream face of a dam. Let W be the width of the dam. Find the resultant horizontal force exerted on the dam by the water. 3. The attempt at a solution So I know how to solve the problem, I was just unsure of whether we should be considering the force caused by the absolute pressure (pressure of the liquid & the pressure of the air above it) or just simply the gauge pressure (pressure of the liquid). My intuition tells me the latter (since a force from the pressure of the air would act vertical), but I would just like to verify.