TPRH SW Physics Fluids Question #232

[KevinGnapoor]

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This is how I tried to solve it:

SG oil = density of oil / density of water

density of oil / density of water = total depth / total depth - depth oil = 0.25

This is the solution that tprh provides:

1. I don't understand their explanation of the interface thing
2. Why is the depth of water y instead of the bottom of the tube?

 

[Cawolf]

I can give you a trick for these types of questions.

The math will always work out that the densities are a ratio of how much of the lighter liquid is submerged.

Here we see that there is 5 cm of oil above the water and 20 cm below - a total of 25cm of oil.

20cm/25cm = 0.8

 

[KevinGnapoor]

I understand that the depth of the oil is 25cm. I guess I'm mostly confused about why the depth of water is 20cm.

 

[Cawolf]

At the point where the two fluids touch (interface) their pressures must be equal, or else it would not be in equilibrium.

The water is exerting force to reach equilibrium in the tube - so we only look at the difference between where the water is and where the top of it's column is, given as y = 20cm.