If I have a pipe for which I know the height and velocity of the fluid at the left and right end, and I am asked to find the gauge pressure at the right end, how would I go about doing this if the internal pressure at both ends is unknown to me? I am aware that finding the difference in pressures will give me the correct answer, though I dont necessarily understand why this is the same thing as P(g) = P(abs) - P(atm). It just doesn't intuitively make sense to me. Any explanation would be much appreciated. Thanks!!