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Gauge pressure at the bottom of a barrel

1. Homework Statement
A barrel contains a 0.110 m layer of oil and a density of 690 kg/m3 floating on water that is 0.300 m deep. What is the pressure at the bottom of the barrel?

2. Homework Equations


3. The Attempt at a Solution

Poil + Pwater = Ptotal

My question is why am I using this formula? I got the correct answer, but initially I wanted to treat the pressure of the water as a negation of the pressure of the oil, causing the barrel to float. So, Poil - Pwater. Is the reason for the addition because I am treating the bottom of the barrel as a point and the pressure acting on it in every direction equally and taking my gauge pressure from that exact point?

Thanks
 

CWatters

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My question is why am I using this formula? I got the correct answer, but initially I wanted to treat the pressure of the water as a negation of the pressure of the oil, causing the barrel to float.
My reading of the question is that both the oil and the water are in the barrel.
 
My reading of the question is that both the oil and the water are in the barrel.
Okay, thank you. I mis-read that. I was assuming the barrel was floating on the water. Paying attention to sentence structure is useful. ?:)
 

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