Is physics always right? Water pressure dilema
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    Default Is physics always right? Water pressure dilema

    I've been told that when logical mental reasoning and physics clash, go with physics, so here is my dilemma. When I built my home I wanted a large aquarium, 1,000 gallon or more. I talked to a public aquarium operator in Dallas TX and was told 3/4" glass was the thickest glass publicly available. I asked if the glass would hold for a 48 x 96 front glass and was told it wouldn't. I knew about an aquarium that was 38 x 96 x 18 with 3/4 glass that was fine. I decided to go with 38 x 96 x 72 (1,138 gallon, 9,445 pounds of water). After a bit of research I found front to back depth didn't effect the pressure on the front glass, only height. I calculated the water pressure on the front glass to be 2,626 pounds of force. I built the tank and as I started filling it a straight edge showed it was bulging in the middle after it was only 1/4 full. I kept going for the entire 1,138 gallons and it held up with a significant bulging.
    Lets say that the front glass was simply held against a rubber seal by the 2,626 pounds of pressure alone, it would take 26 men pushing with over 100 lbs each the break the glass loose. We're probably all in agreement so far but here is where the laws of physics and logical thinking go separate ways. Now lets move the back glass forward until the 72" is .002" between front and back glasses, it will only take a 1/4 pint now to fill it. Will the 4 ounces of water actually create a force of 2,626 pounds on the front glass? Will it take 26 men, each pushing with a push of 100 pounds, to break the seal? Will the tiny weight of water bend the 3/4" glass the same amount as 1,138 gallons? A sight level tube placed anywhere on the front glass will show the pressure in the area it is installed in to have exactly the pressure to force the column of water in the tube to water level, I can accept that, but 4 oz of water creating 2,626 pounds of force somehow gives me trouble. aqur.jpg

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    [QUOTE=Bluechipx;3575080]I've been told that when logical mental reasoning and physics clash, go with physics, so here is my dilemma. When I built my home I wanted a large aquarium, 1,000 gallon or more. I talked to a public aquarium operator in Dallas TX and was told 3/4" glass was the thickest glass publicly available. I asked if the glass would hold for a 48 x 96 front glass and was told it wouldn't. I knew about an aquarium that was 38 x 96 x 18 with 3/4 glass that was fine. I decided to go with 38 x 96 x 72 (1,138 gallon, 9,445 pounds of water). After a bit of research I found front to back depth didn't effect the pressure on the front glass, only height. I calculated the water pressure on the front glass to be 2,626 pounds of force. I built the tank and as I started filling it a straight edge showed it was bulging in the middle after it was only 1/4 full. I kept going for the entire 1,138 gallons and it held up with a significant bulging.
    Lets say that the front glass was simply held against a rubber seal by the 2,626 pounds of pressure alone, it would take 26 men pushing with over 100 lbs each the break the glass loose. We're probably all in agreement so far but here is where the laws of physics and logical thinking go separate ways. Now lets move the back glass forward until the 72" is .002" between front and back glasses, it will only take a 1/4 pint now to fill it. Will the 4 ounces of water actually create a force of 2,626 pounds on the front glass? Will it take 26 men, each pushing with a push of 100 pounds, to break the seal? Will the tiny weight of water bend the 3/4" glass the same amount as 1,138 gallons? A sight level tube placed anywhere on the front glass will show the pressure in the area it is installed in to have exactly the pressure to force the column of water in the tube to water level, I can accept that, but 4 oz of water creating 2,626 pounds of force somehow gives me trouble. aqur.jpg[/QUOTE

    No way that tiny amount of water will put 2600 lbs of force on the glass. I think more research is required.

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    Interesting point. I’d never considered this before. The total force that is exerted on the glass can’t be anymore than the weight of the water in the gap, unless you increased the pressure on the water above atmospheric. The classical physics equations for pressure distributions assume an ‘infinitely large’ reservoir. In a case such as this, I think you would use a simple sum of the forces calculation, not a pressure distribution.

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    Really skinny fish in a .002 wide fish bowl.

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    The pressure only comes from the depth of the water, not the amount of it. Fresh water creates ~.43 psi per foot of depth. A 38 inch tall container will have a pressure at the bottom of about 1.37 psi whether it is an 8' x 4' fish tank with 6300 pounds of water or a 1/4" diameter soda straw with a little over an ounce.

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    Quote Originally Posted by Bluechipx View Post
    I've been told that when logical mental reasoning and physics clash, go with physics, so here is my dilemma. When I built my home I wanted a large aquarium, 1,000 gallon or more. I talked to a public aquarium operator in Dallas TX and was told 3/4" glass was the thickest glass publicly available. I asked if the glass would hold for a 48 x 96 front glass and was told it wouldn't. I knew about an aquarium that was 38 x 96 x 18 with 3/4 glass that was fine. I decided to go with 38 x 96 x 72 (1,138 gallon, 9,445 pounds of water). After a bit of research I found front to back depth didn't effect the pressure on the front glass, only height. I calculated the water pressure on the front glass to be 2,626 pounds of force. I built the tank and as I started filling it a straight edge showed it was bulging in the middle after it was only 1/4 full. I kept going for the entire 1,138 gallons and it held up with a significant bulging.
    Lets say that the front glass was simply held against a rubber seal by the 2,626 pounds of pressure alone, it would take 26 men pushing with over 100 lbs each the break the glass loose. We're probably all in agreement so far but here is where the laws of physics and logical thinking go separate ways. Now lets move the back glass forward until the 72" is .002" between front and back glasses, it will only take a 1/4 pint now to fill it. Will the 4 ounces of water actually create a force of 2,626 pounds on the front glass? Will it take 26 men, each pushing with a push of 100 pounds, to break the seal? Will the tiny weight of water bend the 3/4" glass the same amount as 1,138 gallons? A sight level tube placed anywhere on the front glass will show the pressure in the area it is installed in to have exactly the pressure to force the column of water in the tube to water level, I can accept that, but 4 oz of water creating 2,626 pounds of force somehow gives me trouble. aqur.jpg
    So distracting ,, all these strong men pushing and shoving.

    From your description, 2,626 pounds of FORCE,(not pressure) are being resisted by the elastomeric bond between the end and bottom tank elements.

    roughly... 38 +38 +96 X 3/4 = 129 in^2
    divide into 2626 =20.3 psi at the bond line AVERAGE.

    YES! a small amount of liquid can result in loading like that.

    No Matter how vast the ocean, It is the DEPTH that crushes the sub!

    eta

    Don't forget the 14.7 psi atmospheric pressure!

    Get the two plates of glass close enough, and you won't be able to separate them wet or dry!

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    The water load on the glass,deflection and ultimate /breaking strength of the glass sheet are varied by the dimensions of the glass.... the load ,which is pressureXarea......And very thin films of water have other forces than gravity acting on them......Its quite likely that by introducing some tie bars from either side of the tank ,a much larger sheet will be OK ......Depending again on the effect of cutting holes for the anchors.

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    Speaking as a person who has a degree in physics I can tell you with a lot of certainty that science is always dead wrong. The word "law" when used in science and even in physics is a tremendous misnomer.

    Science consists of a number of observations and theories that we poor mortals try to conceive to explain those observations. When a theory seems to accurately explain a lot of observations, then we like to call it a "law" of science. Sir Isaac Newton is famous for his laws of motion and gravitation. Yet, Einstein and others have since shown those "laws" to be flawed. And Einstein's theories are again being shown to be lacking by others.

    The "laws" of science are only theories that seem to correctly explain certain observations and which seem to be useful to predict the results of future experiments (observations). They are absolutely nothing more than that. When additional observations are no longer explained by the current laws or theories, then we must abandon those laws/theories and formulate new ones that do a better job. So science is never final. It is never settled. It is never completely accurate. It is always in a state of CHANGE. And we poor mortals, who use science, must always be aware that there are limits to the phenomena that any law or theory of science is used to explain or predict. Science is never, ever completely correct.

    Also, in any given situation, there may easily be more than one law or theory that needs to be applied to a situation to allow a fairly accurate assessment of what is going on and what will happen.

    OK, your aquarium and water pressure. The general rule for the pressure of the water or any fluid in an enclosed container (or lake or river) is that it increases with the depth of the water. This is fairly easy to understand if you consider a rectangular tank that is 12" wide by 12" from front to back by X inches tall and completely filled to the top with water. For each foot of it's height (X" / 12) there will be one cubic foot of water in the tank and all of that water will be resting on the 12" x 12", flat bottom (one square foot). That flat bottom will be supporting the weight of all the water in the tank - 100% of it. Now the density of water is 62 lb/cu ft. So a 12" x 12" column of water one foot high will weigh 62 pounds. A 12" x 12" column of water two feet high will weigh 124 pounds. One three feet high will weigh 186 pounds and so forth. And each of these weights of water would have to be supported by the bottom of that tank, which we said is one square foot (144 sq inches).

    It is a property of pressure in a liquid that it is the same amount in all directions. My high school physics teacher had a very effective teaching aid to demonstrate this. It was a metal ball about 6 or 8 inches in diameter attached to a bicycle pump type cylinder and with a multitude of equally spaced holes in it. In the middle of a lab class he gathered the entire class close to him, on all sides so we could see better. Then he drew this sphere out of the water bucket and held it over his head and quickly rammed the pump handle home. We all got wet. His point was that the water came out of all those holes equally well because the water pressure was distributed equally in all directions. He was a fun teacher.

    Anyway, back to my water tank. The same pressure that is exerted against the bottom of that tank is exactly the pressure that is exerted against the front, back, and two sides at the same level as the bottom. So the bottom edge of that front in a three foot tall tank is also seeing 186 pounds of pressure PER SQUARE FOOT. I leave it to the reader to divide by 144 if the pressure in PSI (pounds per square inch) is desired.

    And, as you should be able to see, the pressure in the tank depends on the depth of the water and ONLY on the depth of the water.

    Well, almost ONLY. And I am sure that as long as you keep the walls of the tank a fairly large distance apart, the pressure against the sides of the tank will remain the same as it is with a 12" x 12" tank or even a body as large as the Pacific Ocean.

    But I said above that there could be other theories, other things going on and in any real world tank, indeed there are. The one that comes to my mind first is surface tension. The surface of a liquid will have molecules that are only partially in contact with their fellow molecules, unlike their blood brothers who lie below the surface and which are completely surrounded by identical molecules. So the forces acting on those surface molecules will be different and, at the surface, there is a difference from the zero pressure which you would expect on the sides of the tank. That is only at the surface, but the effects of this surface tension can be felt for a small distance below the surface.

    Now consider the situation where the front and rear of that tank are brought into very close proximity to each other. Here comes your 0.002". Again we have the chance that there will be some unbalanced attraction between the molecules that are against the tank's walls and those that are in the interior. And again, that unbalance maybe felt at a small distance from those walls. So, there may very well be a difference in the pressure on that front side of the tank when the rear side is brought that close to it.

    Is this the only possible additional effect? I am not an expert on liquids, but I doubt it.

    My best guess is that, with any reasonably sized tank or with a large or even infinitely large body of water, the pressure on the sides of that tank or whatever is holding it in will indeed be determined only by the depth of the water FOR ALL PRACTICAL PURPOSES. A tank with a one square foot cross section will have the same pressure at each and every increment of depth as the Pacific Ocean has at that same depth and the strength of a vertical, horizontal, or any angled bulkhead that must hold that pressure back will also be exactly the same in any such instance. But if you start to create tanks that are only thousandths of an inch across, then other factors will start to take effect and other theories (laws) must be invoked.

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    if i recall correctly it was RHO x G x H

    Rho was density of liquid
    G was 9.81 a constant
    H was the variable for height.

    as density does not change ( water is not compressible ) g is a constant then H is the variable.

    Its been a while so check the expression for yourself.

    Usually they apply the pressure at the midpoint of the glass, the average pressure.

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    work with mass 1 cc of water can only generate 1 gram worth of force
    no mater what the shape of it
    as with most things formulas start not working when taken to
    extremes

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    Quote Originally Posted by CalG View Post
    Really skinny fish in a .002 wide fish bowl.
    I just knew this was coming! Actually, I made an giant ant farm once and I thought the common store bought 'uncle Milton's ones were way too wide at around 5/8" so I made mine about 1/4" so I wouldn't get cheated. In the store bought ones you had to keep turning it to see both sides to figure out what was happening.

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    force absorebed = force applied - friction

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    What's missing is depth (Y) [as defined by X being left-right, Y being forward-back, and Z being up-down]. When you have a very narrow tank, the unit plan area follows less closely that deeper Y sections due to factors already mentioned. But with simple approximations and ignoring "skin" effects and other electrochemical factors when you decrease Y, the outward forces you're worrying about will increase essentially linearly with that increase in Y.

    So you have your .002" Y depth tank. At three feet in Z, the unit force per plan area (X-Y) is the same as if the tank were infinite in Y, but the actual outward force (in X-Z) is just the weight of that water column itself, pressing outward at whatever Z height you measure it at. Let's call it "Q" at .002" Y for S's and G's.

    So increase the Y depth to .004". Again, ignoring the other effects the force outward (not column area but in X-Z at constant Z) is now 2Q. Go to 4" Y tank depth, it's now 2KQ. 40"? 20KQ.

    So yes, physics wins. That's why I consider Wile E. Coyote to be apostate, because he's constantly flaunting his lack of adherence to my religion.

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    EPA, my all time favorite graffito was written above an elevator door in the parking garage at the baseball park. Due to the limitations of this keyboard, I cannot exactly reproduce it here, so mentally change it to all symbols-

    E=MC squared +-3Db

    If the glass and water are really clean, the pressure will be less because at a .002" spacing capillarity will have a strong effect. Back when I was working on drafting pen development, I could have done the calculation in my head, but that was a long time ago and my head is crowded up with a lot of other garbage. Suffice it to say that the vertical column will be effectively shortened by a number of inches.

    Now, if we wipe a thin film of oil on the glass, capillarity will be shut off and the pressure at he bottom will be that exerted by a column that high.

    Suppose we wipe a several inch wide stripe of oil from top to bottom in the middle of the pane. Will the top of the water be in a zig zag, lower in the oiled area? What will the pressure at the bottom be now? Inquiring minds want to know.

    Bill

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    Quote Originally Posted by Bluechipx View Post
    I've been told that when logical mental reasoning and physics clash, go with physics, so here is my dilemma. When I built my home I wanted a large aquarium, 1,000 gallon or more. I talked to a public aquarium operator in Dallas TX and was told 3/4" glass was the thickest glass publicly available. I asked if the glass would hold for a 48 x 96 front glass and was told it wouldn't. I knew about an aquarium that was 38 x 96 x 18 with 3/4 glass that was fine. I decided to go with 38 x 96 x 72 (1,138 gallon, 9,445 pounds of water). After a bit of research I found front to back depth didn't effect the pressure on the front glass, only height. I calculated the water pressure on the front glass to be 2,626 pounds of force. I built the tank and as I started filling it a straight edge showed it was bulging in the middle after it was only 1/4 full. I kept going for the entire 1,138 gallons and it held up with a significant bulging.
    Lets say that the front glass was simply held against a rubber seal by the 2,626 pounds of pressure alone, it would take 26 men pushing with over 100 lbs each the break the glass loose. We're probably all in agreement so far but here is where the laws of physics and logical thinking go separate ways. Now lets move the back glass forward until the 72" is .002" between front and back glasses, it will only take a 1/4 pint now to fill it. Will the 4 ounces of water actually create a force of 2,626 pounds on the front glass? Will it take 26 men, each pushing with a push of 100 pounds, to break the seal? Will the tiny weight of water bend the 3/4" glass the same amount as 1,138 gallons? A sight level tube placed anywhere on the front glass will show the pressure in the area it is installed in to have exactly the pressure to force the column of water in the tube to water level, I can accept that, but 4 oz of water creating 2,626 pounds of force somehow gives me trouble. aqur.jpg
    Don't quit yer "Day job" just yet. Force applies to ALL of front, back, and ends. "Equally" even. Very DAMNED "equally". Nature of a fluid.

    The "power" to it?

    Well.. a contractor grouting a troublesome water-seepage source in a rock formation "Hill District", Northside Pittburgh, 1950's let the small but well-fed pump run too long.

    Thin seam a mere few thou thick filled? Pump kept on going, split MORE rock. Gravity was bored spitless waitin' a few million years for lack of any other amusement, took an interest. "Floating" rock, and the massive patch of hillside above it moved. Downhill of course. It were ignorant Northside, Pittsburgh. Not the Twilight Zone.

    Water and gas lines ruptured, residence foundations failed, roadways heaved.

    Hydraulics? Copied from nature, as it happens.

    And have become mankind's oldest and most high-ratio "lever", given they don't need no "fulcrum"... nor any place at all to "stand" on.

    "JFDWT" They will surely deal with YOU! Go off and calulate if you will, just how much stored energy is awaiting VIOLENT release in your DIY "glass seige engine bow".

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    Quote Originally Posted by Bluechipx View Post
    Will the 4 ounces of water actually create a force of 2,626 pounds on the front glass? Will it take 26 men, each pushing with a push of 100 pounds, to break the seal? Will the tiny weight of water bend the 3/4" glass the same amount as 1,138 gallons? A sight level tube placed anywhere on the front glass will show the pressure in the area it is installed in to have exactly the pressure to force the column of water in the tube to water level, I can accept that, but 4 oz of water creating 2,626 pounds of force somehow gives me trouble. aqur.jpg
    Google hydrostatic pressure.

    What you are not conceptualizing is the force vector directions as the water depth increases in relation to tank volume, you understand all the mass from from the water above is pushing down due to gravity and atmospheric pressure. Force vectors are exerting toward the center of the earth however because of the matter is in fluid state, hydrostatics come into play and force vectors push in a downward hemispherical fashion ultimately exerting direct force on the glass causing it to bow.

    You will find a solution. I recommend researching polymethylmethacrylate (PMMA) as it will be your most likely solution. Make sure the family is trained on how to clean it.

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    You are calculating a large force by multiplying a small pressure times a large area.
    The narrow aquarium filled to the same height, still sees the same pressure acting on the same area.
    Ignoring surface tension, the force is the same.
    Also, filling the narrow aquarium will take more water than you calculated because the deflection you observed increases the internal volume.

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    Remember too, the pressure is only the calculated value at the bottom of the tank and follows a linear gradient to 0 at the top of the water, so the average will be half the pressure at the bottom.

    Bill

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    Quote Originally Posted by Street View Post
    if i recall correctly it was RHO x G x H

    Rho was density of liquid
    G was 9.81 a constant
    H was the variable for height.

    as density does not change ( water is not compressible ) g is a constant then H is the variable.

    Its been a while so check the expression for yourself.

    Usually they apply the pressure at the midpoint of the glass, the average pressure.
    Sorry, but no. Because the force on the glass increases as the depth increases, the force diagram is a triangle. The resultant force vector is applied at 1/3 of the height above the bottom of the tank, or 12.67 inches in this case.

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    Quote Originally Posted by NRDock View Post
    You are calculating a large force by multiplying a small pressure times a large area.
    The narrow aquarium filled to the same height, still sees the same pressure acting on the same area.
    Ignoring surface tension, the force is the same.
    Also, filling the narrow aquarium will take more water than you calculated because the deflection you observed increases the internal volume.
    As long as our discussion is theoretical we can forget the gap widening and assume there is no deflection and the gap is constant at .002". When this whole pressure issue first hit me, I posted the question on the "physics forum" and there was no clear answer from the so called experts, just a lot of side tracked replies and seeming dodging of the real issue. I've wrestled with this for some time now and I'll reluctantly say I have to agree with the fact that the same pressure is still there. I visualize a hundred tiny glass elbows with long tubes pointing up past the surface of the water and randomly spaced on the outside of the glass and the pressure inside, in the .002" gap, is exerting exactly enough pressure to push the water in the tubes to the surface in every one of them. I'm not happy and completely satisfied with my conclusion but I guess I have to concede. If the glass was able to slide like a piston and there was a scale attached to it I still can't visualize it reading 2,626 pounds with 4 ounces of water poured in though.


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