Pressure at the bottom of a cube immersed in two liquids

  • Thread starter songoku
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  • #1
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Homework Statement:
Please see the diagram below

A cube with side a = 0.1 m is immersed in 2 liquids as shown (upper liquid is oil and lower liquid is water). The value of h is 0.02 m and the value of d is 0.2 m

Find the hydrostatic pressure at the bottom of the cube
Relevant Equations:
Hydrostatic pressure = ρ . g . h
1602848978219.png


I am not sure about value of depth I need to use. What I did:

Hydrostatic pressure at bottom of cube = hydrostatic pressure by oil + hydrostatic pressure by water

= ρoil . g . d + ρwater . g . h

Is it correct I use value d = 0.2 m for depth of oil and value of h = 0.02 m for depth of water to calculate the pressure?

Thanks
 

Answers and Replies

  • #2
jbriggs444
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Is it correct I use value d = 0.2 m for depth of oil and value of h = 0.02 m for depth of water to calculate the pressure?
Yes. That is the correct approach and the correct values to use.
 
  • #3
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Yes. That is the correct approach and the correct values to use.

Just now the teacher said my answer is wrong, h = 0.02 m is correct but it is wrong to use d = 0.2 m as the depth of oil. Instead, I should use a - h = 0.1 - 0.02 = 0.08 m as the depth of oil because that is the height of the cube in the oil.

I don't understand why I should use a - h , I think all height of oil should be included in calculating the pressure at bottom of the cube because all height of oil exerts pressure?

Thanks
 
  • #5
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Thank you very much jbriggs444 and Chestermiller
 
  • #6
gmax137
Science Advisor
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And that calculated pressure is the "gauge" pressure, that is, the pressure relative to the ambient pressure in the air located above the layer of oil.
 
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