# General > AquaTalk >  How to calculate water pressure on glass walls.

## XnSdVd

I've got something in the works, but need to know how to calculate the force 1kg of water exerts on the sides of a 10cm cube.

Is it simply a case of 1000g/5 = 200g per side? Since water is usually found flat on the ground when not in a container and in a state of equilibrium?

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## hwchoy

no lah, its the weight of the water column divided by the differential surface area at a given depth. the bottom surface don't count, only the four sides. the pressure falls as you go up the water column that's why its a differential equation. now you know why those hydroelectric dams are thicker at the bottom and thinner at the top?

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## ranmasatome

no lah...hahahaha.. so funny la you choy..

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## hwchoy

simi funny? not calculate this way meh? pressure on glass depends on the depth mah.

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## ranmasatome

yes.. correct.. but the way you say is funny lah..

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## hwchoy

> yes.. correct.. but the way you say is funny lah..


      

I'll wait at the airport and "help" you with all the fishes.

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## valice

Let's organise a Justin (fish to be exact) welcome party at the airport...  :Laughing:  

Damn... This is so out-of-topic...  :Flame:  

But seriously, I being a computer engineer, I have no idea how to do the calculation... Choy, care to elaborate more on the differential formula? Power 2 differential equation or just the simple one? It would be interesting...

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## hwchoy

hey I am also computer science like you, but heck this is A level physics right?

but I don't know what kind of differential equation leh, I meant to tell him that you have to slice the depth parameter to an "infinitely" small instance and calculate the pressure there, which is the definition of differential calculus right? note I am speak from memory as old as you are

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## XnSdVd

Well crap... i was hoping one of you would be able to give me the formula...  :Razz: 
Now it's up to Thomas to show us the light.

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## hwchoy

well if you know any guys who design hydro dams, I'm sure he'd know it by heart  :Smile:

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## XnSdVd

We're on an Island master Choy... i don't think we'll find any. But strange that no one's asked considering everyone's afraid of their tanks breaking.

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## ranmasatome

actually for such a small container.. why are you worried?? we arent building anything of epci proportions to require such calculations

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## hwchoy

any civil engineer should know how to calculate, any retaining wall would entail similar calculations.

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## XnSdVd

I'm asking because from the formula we can predict what will happen when i try to build a 5x1.25x1.25m tank.

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## Shadow

check out this web site:

How to Calculate the Glass Thickness for your Aquarium:
http://saltaquarium.about.com/gi/dyn...ickness.0.html

or maybe this one

DIY Custom Aquarium Glass Thickness Calculator :
http://saltaquarium.about.com/cs/aqu...customtank.htm

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## ranmasatome

you want to build that?? err..in your house?? err... by yourself?? err..ok.

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## hwchoy

no problem lah, it only weighs seven tons.  :Grin:

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## XnSdVd

Therefore, the question is really, how much force does 7812.5kg of water put on the sides of a container? 

And yes, it will be constructed one day... ONE DAY. After i've graduated and started working and bought my own place that's not a HDB. It might never happen. But hey, knowledge is always useful.

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## hwchoy

> bought my own place that's not a HDB.


not only it has to be not a HDB, but it has to be sitting on the ground floor, with no basement.  :Kiss:

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## andrewtyr

why would you want to make your own 5ft tank? IMO, the hassle isn't worth it unless it's a strong reason.

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## hwchoy

its 5 metres lah!

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## oblivion

wow!

thats well over 16 feet!

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## XnSdVd

> not only it has to be not a HDB, but it has to be sitting on the ground floor, with no basement.


Yes.. i've got my eye on old shophouses. 

1) Because no matter how old or run down they are, they still look good
2) High ceiling, How else am i going to accomodate the damn tank?
3) Airwell! Some natural light is always good for the tank, and i can collect/pipe rainwater too
4) Dimensions, they're usually 5x15m and 2 1/2 storeys high
5) Heritage site! garmen won't try to buy me out and tear it down... mah
6) A really really cheap one in little india will cost you $300,000 - 500, 000


Same price range as a 5 room HDB!  :Shocked:  Sure it's ugly, but i have a habit of tearing up anything premade and re-doing it anyway.  :Razz:  
Problem is i'll only be making like $1300 - 2000 when i start working  :Opps:  

And back to the question: What's the formula?

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## hwchoy

find a civil engineer lah.

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## Justikanz

Hmm... I am not a civil engineer, never studied fluid mechanics... Anyway, this thread is changing course to a chit chat thread on Shannon's proposed 5m tank...  :Opps:

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## XnSdVd

Unfortunately even my math lecturer doesn't know. He's asking HIS physics lecturer to see if there's actually a formula to calculate it.

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## hwchoy

you ask the wrong chap, this is engineering (physics also can) not maths  :Grin:

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## stephen chung

> Yes.. i've got my eye on old shophouses. 
> 
> 1) Because no matter how old or run down they are, they still look good
> 2) High ceiling, How else am i going to accomodate the damn tank?
> 3) Airwell! Some natural light is always good for the tank, and i can collect/pipe rainwater too
> 4) Dimensions, they're usually 5x15m and 2 1/2 storeys high
> 5) Heritage site! garmen won't try to buy me out and tear it down... mah
> 6) A really really cheap one in little india will cost you $300,000 - 500, 000
> 
> ...



Shophouse in Little India so cheap? $300k-$500k ??

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## andrewtyr

just go look see other ppl's 5m tank and measure the glass thickness and observe the construction. Even if u know the hydrostatic stress, you probably have to account for other factors like gravel, safety factor etc. Considering probable things like leak repair and transport of glass, it more worthwhile to spend that couple of hundreds more to get the pro to do it.

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## hwchoy

> just go look see other ppl's 5ft tank and measure the glass thickness and observe the construction. Even if u know the hydrostatic stress, you probably have to account for other factors like gravel, safety factor etc. Considering probable things like leak repair and transport of glass, it more worthwhile to spend that couple of hundreds more to get the pro to do it.




psst Andrew, its 5 METRES.

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## andrewtyr

> psst Andrew, its 5 METRES.


 :Grin:   :Grin:   :Grin:   :Grin:   :Grin:

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## valice

> just go look see other ppl's 5m tank and measure the glass thickness and observe the construction.


Think the only place he can go see that is either the Bird Park or the zoo enclosures... And the glass are freaking thick!  :Surprised:  
Superb distortions when viewed at an angle...  :Laughing:

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## XnSdVd

Haha, looks like we aren't used to the metric system in aquatics... Anyway, i need to calculate more than just the glass. I'm thinking to do 1 side glass 4 sides fibreglass.

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## andrewtyr

Anyways, you have to take note of bending moments at the middle of the tank which is most prone to failure. And probably the floor load. And you would probably need to save some 10 years before you can pay for a shophouse. And...And... :Shocked:

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## uklau

Everybody has a dream & it's good because it keeps your life going on in a meaningful way. :Angel:

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## XnSdVd

AH HA!! Finally have the formula!! 

(Atmosphearic pressure + Pressure at bottom of tank x height x ro?(some funny P looking sign) x surface area)/2 = force against glass

Now, can someone please make sense of it for me?

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## Shadow

:Laughing:  what is "ro?(some funny P looking sign)"? water molecules weight?

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## andrewtyr

density of water i think

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## Justikanz

'ro' is the sound of the mathematical symbol commonly used to represent the density of the subject of measurement. Yes, it looks like a 'p' tilted at an angle towards the front...

Good luck and have fun with the formula, Shannon!  :Razz:

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