# Pressure and height

## Homework Statement

question is in the diagram.
basically im not sure how to use the specific gravity and convert it into pressure.

P=p(rho)gh

## The Attempt at a Solution

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#### Attachments

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tiny-tim
Homework Helper
Hi TyErd! (have a rho: ρ )

Use the specific gravity to find ρ, the density …

ρ-of-oil = ρ-of-water times specific-gravity-of-oil (so specific gravity of water = 1)

ohhh! so the density of any liquid is density of water x specific gravity of the liquid?

in knowing that my working out is where density of water is 1000...: 80 + 1000(0.3)(9.81) - 1000(13.6)(9.81)(h) - 1000(0.72)(9.81)(0.75) = P_atmospheric. now what, i have two unknowns or do i assume P_atm is 101.325?

Last edited:
even if i do use 101.325 the answer is still wrong... i don't know why.

tiny-tim
Homework Helper
Hi TyErd! (just got up :zzz: …)
ohhh! so the density of any liquid is density of water x specific gravity of the liquid?
Yup! in knowing that my working out is where density of water is 1000...: 80 + 1000(0.3)(9.81) - 1000(13.6)(9.81)(h) - 1000(0.72)(9.81)(0.75) = P_atmospheric. now what, i have two unknowns or do i assume P_atm is 101.325?
ah, the question says the gage pressure (or gauge pressure) is 80 KPa …

gauge pressure is defined as actual pressure minus atmospheric pressure …

so you can ignore the atmospheric pressure! oh yeah so then what does all of that equal? it should equal the pressure at the top of the oil column so would that just be the 0 because we can assume the mass of air negligible?

tiny-tim
Homework Helper
oh yeah so then what does all of that equal? it should equal the pressure at the top of the oil column so would that just be the 0 …
yes the sum of the mgh's has to equal the difference between the outside air and the measurer (80 KPa) …

you can think of them as being an initial and final pressure, much like initial and final energy in a "solids" equation … because we can assume the mass of air negligible?
no, the mass of air is very heavy, it's because the 80 KPa is given as the difference from that okay so i did this: 80 + 1000(0.3)(9.81) - 1000(13.6)(9.81)(h) - 1000(0.72)(9.81)(0.75) =0, h=-0.017047 which is incorrect. where have i gone wrong?

tiny-tim
Homework Helper
80,000 ? even then its incorrect. the answer is h=0.582m

tiny-tim
I do get 0.582 