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Homework Help: Finding the gauge pressure at the bottom of a barrel

  1. Dec 6, 2016 #1
    1. The problem statement, all variables and given/known data
    A barrel contains 0.130-m layer of oil floating on water that is 0.290 m deep. The density of the oil is 610 kg/m3
    What is the gauge pressure at the bottom of the barrel?

    2. Relevant equations
    = ρ(g)(h)

    3. The attempt at a solution
    I found the pressure of the oil which is 777 Pa by 610kg/m^3(9.8)(0.130m)
    for this one I know I have to add the 777 Pa, but I messed up on the height. I used 0.16 by taking the difference of 0.290m-0.130m = 0.16 m and used that height to multiply to the 1000kg/m^3(9.8)(0.130m)

    Can someone explain to me why it's not 0.16m? Does that part of the barrel only take into consideration the pressure of water?
  2. jcsd
  3. Dec 6, 2016 #2


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    There are two liquids contributing to the pressure at the bottom. You need to calculate the pressure from each separately and then add the two pressures.
  4. Dec 6, 2016 #3


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    Because the water isn't 0.16m deep, it's 0.290m deep.
  5. Dec 6, 2016 #4
    Ohhh! I get it! You thought they said the total depth was 0.29m and you subtracted to get the depth of the water. Reread the problem. The depths of the oil and the water are given separately. 0.29m is the depth of the water alone.
  6. Dec 10, 2016 #5
    Oh, I see now! Thanks all!
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