# Pressure at the Bottom

talaroue

## Homework Statement

What volume of water has the same mass as 8.58 m3 of ethyl alcohol?

If this volume of water is in a cubic tank, what is the pressure at the bottom?

## Homework Equations

denisty= m/v
Pb=Pa+density*g*h

## The Attempt at a Solution

I found the first part by setting denisty=m/v solving for m=Vrho. Then plugged that in for water in the same equation and got 6.7782 m^3. For the second part though, I thought I could use Pb=Pa+density*g*h. and the height i found by taking the 6.7782^(1/3). But would the Pa be zero since its at the bottom if not what do I use for the Pa?

## Answers and Replies

Homework Helper
Atmospheric pressure.

Homework Helper

## Homework Statement

What volume of water has the same mass as 8.58 m3 of ethyl alcohol?

If this volume of water is in a cubic tank, what is the pressure at the bottom?

## Homework Equations

denisty= m/v
Pb=Pa+density*g*h

## The Attempt at a Solution

I found the first part by setting denisty=m/v solving for m=Vrho. Then plugged that in for water in the same equation and got 6.7782 m^3. For the second part though, I thought I could use Pb=Pa+density*g*h. and the height i found by taking the 6.7782^(1/3). But would the Pa be zero since its at the bottom if not what do I use for the Pa?

Well if the mass is the same, what is the pressure of ethyl alcohol at a depth of (8.53)1/3 ?

talaroue
pressure = F/A...F/8.53^2/3...and the force is equal to mg?

talaroue
I don't understand this step at all. So it wants the pressure at the bottom of the tank of water. Since its a cube you can find out that the sides are 1.89x1.89x1.89. since its at the bottom that gives the height 1.89, density of 1000, and gravity is 9.8. So that gives yous Pb=Pa+19404 correct?

But then Pa=Fa/A. The area is 1.89*1.89, but then is the force just mg?

Homework Helper
pressure = F/A...F/8.53^2/3...and the force is equal to mg?

What's wrong with

P = ρ*g*h

where ρ in this case is the density of ethyl alcohol.

talaroue
That woudl me that...

P=790kg/m^3*9.8 m/s^2*2.04 m
P=15818.65341 Pa, which is wrong according to the program.

Homework Helper
That woudl me that...

P=790kg/m^3*9.8 m/s^2*2.04 m
P=15818.65341 Pa, which is wrong according to the program.

I'd think that is a bit too much precision.

talaroue
another thing if the question is asking about water why would i be using the density of ethyl alcohol?

Homework Helper
another thing if the question is asking about water why would i be using the density of ethyl alcohol?

OK. Use water of a different dimension cube.
I think I was just cutting out the middleman.

Ooops. I wasn't I forgot about the cube root of the dimension. Sorry. Not a short cut at all.

Last edited:
talaroue
so use the volume of water to find the dimension? and then use density of water to find out the pressure?

I tried what you gave me and i rounded to the tenth and it still didn't work.

Homework Helper
so use the volume of water to find the dimension? and then use density of water to find out the pressure?

I tried what you gave me and i rounded to the tenth and it still didn't work.

You're right. I corrected myself. I forgot that the dimension is cube rooted not linear.

So your method of

P = ρgh

using water and the 1.8888 depth is the way to go.

talaroue
I did and it came up wrong this is in Pascals right?

Homework Helper
I did and it came up wrong this is in Pascals right?

Well of course check your units. Do they want the answer as Pa or kPa?

talaroue
It doesn't specifiy which one the want but we when work out this problem the way we did we get N/m^2 which is Pa.

talaroue
A 1.03 m diameter vat of liquid is 5.47 m deep. The pressure at the bottom of the vat is 1.79 atm. What is the mass of the liquid in the vat?

Pb=density*gravity*height
V*Pb/density*g*h
density can be put in m/v and Pb can be turned to Pa. then just solve for the mass right? Is there something wrong with the way I am rearranging this equation for both of these problems?

Homework Helper
It doesn't specifiy which one the want but we when work out this problem the way we did we get N/m^2 which is Pa.

That's correct. You should get something like 18,510 Pa = 1.8888*1000*9.8

Homework Helper
A 1.03 m diameter vat of liquid is 5.47 m deep. The pressure at the bottom of the vat is 1.79 atm. What is the mass of the liquid in the vat?

Pb=density*gravity*height
V*Pb/density*g*h
density can be put in m/v and Pb can be turned to Pa. then just solve for the mass right? Is there something wrong with the way I am rearranging this equation for both of these problems?

In this one you know the depth and gravity and pressure in atm that you convert to Pa, then solve directly for ρ don't you and then figure the mass from there?

Homework Helper
You can save a step by noting that it is a column so ...

F = m*g = P*A

So m = P * A / g

where A is the area of the bottom.

talaroue
That anwser isn't coming out right. and neither is this one... Why does it seem i am doing all the work right but the anwser is wrong?

talaroue
still having a bit of a problem

Homework Helper
still having a bit of a problem

Show how you are calculating it.

Mentor
A 1.03 m diameter vat of liquid is 5.47 m deep. The pressure at the bottom of the vat is 1.79 atm. What is the mass of the liquid in the vat?

Pb=density*gravity*height
V*Pb/density*g*h
density can be put in m/v and Pb can be turned to Pa. then just solve for the mass right? Is there something wrong with the way I am rearranging this equation for both of these problems?
What about atmospheric pressure?

talaroue
I attached it

EDIT: that answer i came up with is wrong

#### Attachments

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talaroue
What about atmospheric pressure?

P(below)= P(above)+density*gh

P(above) would that be considered atmospheric pressure?

talaroue
Am i wrong in assuming that volume is of cylinder?

Mentor
P(below)= P(above)+density*gh

P(above) would that be considered atmospheric pressure?
That's what I would assume.

Homework Helper
Sorry I confused things earlier by dropping the atmospheric pressure. It is needed here for a static liquid.

talaroue
How would I calculate that? Force=Pressure/Area

talaroue
Sorry I confused things earlier by dropping the atmospheric pressure. It is needed here for a static liquid.

No worries, you have helpped me a lot! I apperiate it.

Mentor
How would I calculate that? Force=Pressure/Area
Calculate what? You are given Pb and you know Pa.

talaroue
how do i know Pa already?<<<<<<This would solve all my problems?

Mentor
how do i know Pa already?<<<<<<This would solve all my problems?
See post #27. That's what we've been talking about.

talaroue
I understand the equation, but i don't know how to come up with a number for Pa, to plug into the equation.

Mentor
I understand the equation, but i don't know how to come up with a number for Pa, to plug into the equation.
Hint: What's atmospheric pressure in units of atm? :tongue:

(You can also express atmospheric pressure in standard units.)