# Which equation to use for hydrostatic pressure?

• morty92222
In summary: This is the standard acceleration due to gravity. So with that in mind, how would you convert the pressure in psi to the standard unit of pounds per square foot?Don't despair. You just got to get used to manipulating the units. Are you familiar with the use of ##g_c=32.2\ \frac{lb_m\ ft}{lb_f\ sec^2}##? This is the standard acceleration due to gravity. So with that in mind, how would you convert the pressure in psi to the standard unit of pounds per square foot?
morty92222

## Homework Statement

h = 10 ft
RHO = 10 ppg (or pounds/gallon)

## Homework Equations

P = 0.052 * RHO * h
P = RHO * G * h

I am very confused right now as why each equation gives a different answer! What are the correct units for the 2nd equation and how do I convert from second equation to the first one? And where does the g goes?! I looked it up online and but they just say "RHO" takes care of it! What does that mean?!

Thank you!

morty92222 said:

## Homework Statement

h = 10 ft
RHO = 10 ppg (or pounds/gallon)

## Homework Equations

P = 0.052 * RHO * h
P = RHO * G * h

I am very confused right now as why each equation gives a different answer! What are the correct units for the 2nd equation and how do I convert from second equation to the first one? And where does the g goes?! I looked it up online and but they just say "RHO" takes care of it! What does that mean?!

Thank you!
Let's see your attempt at doing some of the units conversions, starting with the 2nd equation. If RHO is given as 10 ppg, what is density in ##lb_m/ft^3##? What is it in slugs/ft^3?

So 10 ppg equals 74.8 lb/ft3. What should I use for g in the second equation (in terms of units)?

Last edited:
morty92222 said:
So 10 ppg equals 74.8 lb/ft3. What should I use for g in the second equation (in terms of units)?
I asked for the density in slugs/ft^3.

Chestermiller said:
I asked for the density in slugs/ft^3.
I never heard about "slug" in my entire life. After googling it, it appears to be 2.33 slug/ft3. But I don't have any feel for the unit or what it represents.

morty92222 said:
I never heard about "slug" in my entire life. After googling it, it appears to be 2.33 slug/ft3. But I don't have any feel for the unit or what it represents.
A slug is what you use if you are using English units, and you want to write F = ma (without a correction factor), with F in pounds force, mass in slugs, and acceleration in ft/sec^2. So, with g = 32.2 ft/sec^2, what do you get for the pressure using ##P=\rho g h##?

Chestermiller said:
A slug is what you use if you are using English units, and you want to write F = ma (without a correction factor), with F in pounds force, mass in slugs, and acceleration in ft/sec^2. So, with g = 32.2 ft/sec^2, what do you get for the pressure using ##P=\rho g h##?

I am not sure how to use the slug in P=Rho g h as there is no mass in there!

morty92222 said:
I am not sure how to use the slug in P=Rho g h as there is no mass in there!
You just use rho g h to get the pressure. Slugs IS mass.

Chestermiller said:
You just use rho g h to get the pressure. Slugs IS mass.

So 2.33 * 32.2 *10 = 750 [slug/(ft. sec2)]
What kind of units is this?

morty92222 said:
So 2.33 * 32.2 *10 = 750 [slug/(ft. sec2)]
What kind of units is this?
It is the same as slug-ft/(ft^2 sec^2). 1 slug-ft/(sec^2) = 1 lb_f. So the unit are lb-f/ft^2=psf. What units did you want the answer in?

Chestermiller said:
It is the same as slug-ft/(ft^2 sec^2). 1 slug-ft/(sec^2) = 1 lb_f. So the unit are lb-f/ft^2=psf. What units did you want the answer in?

I actually wanted the answer in psi so I can compare it to P = 0.052 * ppg * h = 0.052 * 10 * 10 = 5.2 psi

Edit: So 750/144 = 5.2 psi! Thank you so much.

I still do not understand how we reached this conclusion! :D You kinda took me through it that I don't even understand what was wrong in the first place.

morty92222 said:
I actually wanted the answer in psi so I can compare it to P = 0.052 * ppg * h = 0.052 * 10 * 10 = 5.2 psi

Edit: So 750/144 = 5.2 psi! Thank you so much.

I still do not understand how we reached this conclusion! :D You kinda took me through it that I don't even understand what was wrong in the first place.
Don't despair. You just got to get used to manipulating the units. Are you familiar with the use of ##g_c=32.2\ \frac{lb_m\ ft}{lb_f\ sec^2}##?

## 1. What is the formula for calculating hydrostatic pressure?

The formula for calculating hydrostatic pressure is P = ρgh, where P is the pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the height of the fluid column.

## 2. How do you determine the density of the fluid in the hydrostatic pressure equation?

The density of the fluid can be determined by using a hydrometer or by looking up the density of the fluid in a table. It is typically measured in units of kilograms per cubic meter (kg/m³).

## 3. Can the hydrostatic pressure equation be used for any type of fluid?

Yes, the hydrostatic pressure equation can be used for any type of fluid, as long as the density of the fluid and the acceleration due to gravity are known.

## 4. What is the unit of measurement for hydrostatic pressure?

The unit of measurement for hydrostatic pressure is typically Pascal (Pa), which is equivalent to Newtons per square meter (N/m²). However, other units such as bar or psi may also be used.

## 5. How does the height of the fluid column affect the hydrostatic pressure?

The hydrostatic pressure is directly proportional to the height of the fluid column. This means that as the height increases, the pressure also increases. For example, doubling the height of the fluid column will result in double the hydrostatic pressure.

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