Which way is up?

[Catharsis]

Hi...

 

I was thinking how fascinating “buoyancy” is; and I came to the conclusion that instead of thinking of something floating to the surface “in water” it’s better to think of it being squeezed to the surface. (and was wondering if I would be wrong?)

 

Now I’ll walk you through my thinking behind this (and of course keeping in mind - that I’m just guessing, based on what I know).

 

So let’s get the ball rolling: First of all - I know - “they don’t know” exactly what gravity is They say that it’s a warping of space and time (meaning, it’s not electric, chemical, magnetic and or “whatever”)... It’s this - what they call - a warping of space and time ... Now what that means: is that as long as you have something (any kind of matter taking up space) it will have gravity. In other words - if I were to press with my finger down on a bed sheet that was tightly pulled over a bed making an impression down into it and then rolled a marble around - it would circle around and around and make it’s way down toward the center “my finger” (like a toilet bowl full of water). Now if you were to take that example and apply it to a thee dimensional example - then you would get what it means to “warp time and space” and to get gravity. (No?)

 

So with that said and done - what makes air float and not only float but what makes it float up? Meaning how dose it know what up is? Also what makes it so powerful?

 

So I asked this question: what are the properties of buoyancy - when you take a handful of air and apply so much metal that it must sink? Meaning, what is it when you have a handful of air wrapped with just aluminum foil that will float without any problem to something that has so much metal (thick around it) that it will sink?

 

So hopefully, I know what makes an Atom of air different from a solid or liquid. And that is - the Amount of protons and neutrons at the center (no?). Meaning - if you were to take a bucket of rocks and place at the bottom, a rock made out of plastic - it would wanna rise to the top because it wasn’t as heavy. And what I’m thinking is that - Another way of looking at it would be, that (maybe) it gets squeezed to the top by all the heavier rocks (is this wrong in thinking of it like that?).

 

So with that said and done - the difference between the handful of air with all the metal around it just ends up being heavier than the water is able to squeeze “up” against it - and the air isn’t helping because there isn’t enough to counter balance the meatal’s weight (like a Bank account) not enough “credit” to balance off the debits...

 

So in closing: Am I correct in thinking of it like this? And any insight into the concept of “up” would also be helpful... Although, I guess it has to do with moving away from the center of a mass.... (No?)

 

Any suggestions, comments and opinions will be greatly appreciated.

 

Thank you in advance....

Edited September 26, 2008 by swansont
turned volume down

[Bignose]

Buoyancy is a consequence of the pressure of the fluid the object is immersed in. The direction of "up" is because the deeper you go into a body of fluid, the higher the pressure is. (i.e. the highest pressure in the Ocean is at the bottom of the Marianas Trench).

[Sisyphus]

That's incorrect. Pressure has nothing to do with it. Also, do not think of the air as having an "upward" force, because that is not correct, either. Everything has a downward force due to gravity, it's just a matter of how much.

 

In other words, it's just a simple of density. Water has more mass per unit volume than a bubble of air, hence there is a stronger force of gravity acting on it, hence it is forced down and pushes the air up and out of the way. The net result is the bubble of air being forced upwards. The amount of force with which is moves upwards is dependent on the difference in mass between the bubble of air and an equal volume of water.

 

This happens whenever you have anything in any fluid, including air. Helium is less dense than air, so a helium balloon is pushed up and out of the way as gravity pulls the air around it down more. In a vacuum, the helium balloon would fall exactly as fast as a big ball of lead.

 

Another good example is a human being, which has a density quite close to that of water. If you're swimming and you fill your lungs with air, you become slightly less dense than water and you float. If you empty your lungs, you become slightly more dense and sink. Submarines do something similar by filling and emptying internal tanks with water, making the overall ship more or less dense than water in order to sink or rise. Also, fat is less dense than water, and muscle is denser. That's why fat people have an easier time floating than thin people.

[insane_alien]

Sisyphus, it is the pressure differential that causes the force.

 

If it were density then the bigger the difference in density then the bigger or smaller the buoyancy force, but the buoyancy force is THE SAME for objects of identical shape no matter what their density. IFF this force is greater than its weight then it will be buoyant, if not, it will sink.

 

density differences will only tell you if it will be buoyant or not and the average acceleration but that is a trick of the principles behind it, much like how special relativity is a special case of general relativity,

[Klaynos]

You derive the buoyancy equations by considering the pressure difference on an object... So yes, pressure IS the fundamental underlying principle here.

 

To the OP,

 

Consider Ice on water, it's the same material but there is clearly an upwards buoyancy force acting on it... Ice is actually unusual as most materials when they freeze sink...

[CaptainPanic]

Many people (including me until 5 seconds ago) were thinking that "buoyancy" is the equilibrium between the actual buoyancy and gravity. But buoyancy is the force of the water pushing on the object's outer surface (force per area = pressure). Gravity is pulling it down.

 

And the equilibrium between the two is what keeps it from either sinking or taking off. - It's the common Archimedes principle, which does take density, volume into account.

 

The picture on wikipedia (as so often) is quite good: http://en.wikipedia.org/wiki/Buoyancy

[Bignose]

Sisyphus,

 

You can integrate over the pressure force over the entire surface of the object, or you can use Archimedes' Law. It's the same thing. Virtually any introduction to fluid mechanics book will go over this. To be specific, please see any of the recent editions of Munson, Young, and Okiishi's Fundamentals of Fluid Mechanics. A good course will have the students do both calculations for some simple shapes (blocks, cylinders, or spheres, cones, etc.) to prove that either formulation is equivalent.

[Catharsis]

Yea, (you know) I don’t know why I didn’t think of going to Wikipedia in the fist place .... But yup, right there was the answer.... And it looks like I was right - it kinda get’s squeezed up to the top....

 

But I certainly must say - “WOW” how POWERFUL some trapped air is - inside something like metal even. To be able to keep ton’s and ton’s of metal afloat (like in a battleship) REALLY sends my mind for a loop.

 

I mean so much so - that it makes me question the explanation....

 

So let me get this straight: I have some trapped air in between some gigantic slabs of steal - and in by doing that - the “air” molecules weigh “{so much more less}” than water molecules that the air will not let the “slabs of steal” sink (to a point)?

 

I think what amazes me - is that the air molecules are so “removed” from the surrounding water (because of the meatal) that you would think there could be no more connection between the two.

 

I mean to be so powerful, you would think you could put an air bubble in you hand and use it as a quick rescue to the surface (type of thing).

 

You know - come to think of it.....

 

Why is trapped air inside something so Powerful - but having it trapped under my shirt or something of the sort doesn’t seem to be so powerful?

 

I think that’s a very interesting question....

 

Any suggestions, comments and opinions are greatly appreciated.

 

Thank you in advance....

Edited September 27, 2008 by Catharsis

[swansont]

Yea, (you know) I don’t know why I didn’t think of going to Wikipedia in the fist place .... But yup, right there was the answer.... And it looks like I was right - it kinda get’s squeezed up to the top....

 

But I certainly must say - “WOW” how POWERFUL some trapped air is - inside something like metal even. To be able to keep ton’s and ton’s of metal afloat (like in a battleship) REALLY sends my mind for a loop.

 

I mean so much so - that it makes me question the explanation....

 

So let me get this straight: I have some trapped air in between some gigantic slabs of steal - and in by doing that - the “air” molecules weigh “{so much more less}” than water molecules that the air will not let the “slabs of steal” sink (to a point)?

 

It's not really the air, it's the water and steel. The steel has pushed the water out of the way, and it's the water pushing back that keeps the ship afloat. The air is at one atmosphere of pressure. It's there, but not really doing anything. All of the strength to withstand the forces is in the steel. The ship floats because the water displaced weighs more than the steel+whatever is inside the ship (were it to be completely submerged), so it exerts a greater force than the weight.

 

Contrast this with a balloon inflated underwater. Then it's the air restricting the collapse, but the air would have to be above atmospheric pressure.

[Dudde]

Why is trapped air inside something so Powerful - but having it trapped under my shirt or something of the sort doesn’t seem to be so powerful?

 

Also note, the water cannot go through the steel, as the steel's atomic structure is much more dense - whereas with a shirt, you have interwoven fibers that are designed for airflow (those are the comfy-est), and aren't very water-tight. A ship made with the same principles would probably not get nearly as far!

[Catharsis]

I can understand the principle as Wikipedia leads me to believe (how heavy things are “mass”). But then I start thinking about trapped air in a really thick container and then; how dose the water know it’s in there - which makes me think of (who said it?) the atmospheric pressure - the air wanting to make it back to “it’s own atmospheric pressure; also the air is not heavy (again - like before) so you really got two things going on. But then there’s all this about “displacement” - unless all this business about displacement is about the waters “Water Tension” then it all comes together.

 

So in closing: if your talking about floating on top of the water your talking about “displacement”. If it’s going under - then your talking about mass, like the formula on Wikipedia. If you’re talking about air then it’s this atmospheric pressure stuff and or mass if the air is in something..

 

But that’s it for me - I’m going to grab an aspirin...

[Bignose]

You are missing the concept of density -- mass divided by volume. The water "knows" that the air is trapped because the object weighs a certain amount and takes up a certain volume. And, the object isn't solid metal, because it weighs too little for that. It is the difference in densities that is important, not the absolute weight. A 1 inch by 1 inch by 1 inch block of wood floats the same as a 1 foot by 1 foot by 1 foot block of wood. Because the density of the wood is the same, even though a 1 foot by 1 foot by 1 foot block of wood is much, much heavier than the 1 inch cubed size.

[Catharsis]

Yea - my bust - I meant to say density.... But was everything else OK?

[dirtyamerica]

The weight of displaced water by the battleship is equal to the weight of the battleship.

 

If it is more, it will sink. If it is less the battleship will float higher. The ship will go up or down until an "equilibrium" is reached (although I don't think I should use that term with buoyancy?).

 

If you submerge a balloon and let it go it will quickly rise to the surface. Then look closely. It will float on the surface and displace a very small amount of water (which weighs as much as the balloon).

 

This discussion reminds me of a lesson I once did for my class back when I was scraping by as a science teacher....it's the old "Partially filled dropper inside a 2 liter bottle filled nearly to the top with water." Put the cap on the bottle. Squeeze the bottle and the pressure causes the air bubble in the dropper to decrease in size...thereby displacing less water. The amount of water displaced weighs less than the dropper and it sinks. Some specific amount of squeezing will cause the dropper to become neutrally buoyant and you can keep it in the middle of the bottle (vertically).

[am-tep]

The original question was beautiful. The answer is just as beautiful. Air gets it's 'power' from the electromagnetic force. In fact, gravity is an extremely weak force. Any ordinary table can stop a book from falling countering the gravity due to the entire earth in the process!

[Flashman]

Any ordinary table can stop a book from falling countering the gravity due to the entire earth in the process!

 

Something in the perky confidence of that statement makes me want to get a 2kg book and a walmart dining table, and drop it on it from 100m or so....

[iNow]

You would not be testing the question asked if you did that. You'd be adding a new force... the acceleration... instead of just comparing flatly gravity against electromagnetism and nuclear interaction.

[am-tep]

Really? Where does the pressure of a liquid or a gas come from? Electromagnetism.

 

And to go a hundred metres up, the electromagnetic force needs to work against gravity. So the energy needs to be stored up. So you aren't comparing the actual field strengths.

[Catharsis]

Just to re-iterate: because I know someone can get lost by my original post “example” of rocks: (what’s he talking about - rocks and one being plastic?)....

 

I was saying that - if you were to take an atom of water and an atom of air - that the difference between the two would be the amount of protons and neutrons at the center of their atoms...

 

So what I did was compare “that” - meaning one atom having more proton and neutron “make up” to a rock and the air atom having less proton and neutron “make up” to the plastic rock....

 

So if you have a rock (water atom) and a plastic rock (air atom) sitting next to each other..... Are you with me? What will make that air atom go up and the water atom not go up?

 

And what I was assuming was that - it’s the “accumulation” of more rocks (water atoms) being (denser, heavier “more mass” - this “all” being a big part of the debate “answer”) forcing the plastic rock (air) up.... Like sand filling up under something and lifting it up to the top...

 

******************

 

So I just wanted to clarify that - for people who couldn’t quite get the picture - of my rock and plastic rock analogy....

 

And you can see why I even questioned the idea of “up” too, also at the time...

 

So in closing I just wanted to make clear my “rock and plastic rock statement” (in the beginning of this thread) - it might have been a little brief, to be really clear - as now I hope it is.....

[am-tep]

Molecules in a gas move randomly because they undergo 'collisions' with each other.These collisions are nothing but repulsion of like charges. As the colliding molecules move further apart there is no force in gases which counters this repulsion.In liquids the cohesive forces(which are also em in nature) counter the repulsions to some extent.So gases try to occupy as much space as possible.But in spite of this spread, gravity does ultimately manage to hold the molecules together to some extent. If it was'nt so, the atmosphere would not stick to the Earth and niether would the oceans!

[Catharsis]

Yea... Very interesting the repulsion of like charges (good point).... But I’m still thinking of the concept of “up”... Because up seems to wanna be quite on the fast side of the fence VS. spreading out.... Example: air in water (fast going up)...

 

Why fast going up?

 

It’s this concept of sand filling up underneath - pushing up on the “lighter atom” above (no?)...

 

I would like to say “every force has a counter force of equal and opposite etc..” but if gravity has such an affect on a ball of air under water (fast rising up) - why all of a sudden dose it stop and float on top of water (yet while gravity is still having the same force on that ball of air as it did under the water)?

 

Well, the atoms now in the air - putting force on the air that was in the water (lets say it was a different form of gas of sorts) is not so dense now to have that much of a force to “quickly” propel that gas further up into the atmosphere “the sand effect” (unless it dose - depending on the gas were talking about)....

 

So what I’m saying: is that I have a hand full of sand (both hands, cupping it, like I’m making a snowball) and I’m squeezing the sand - and all the different layers of “density of atoms” are sorting themselves out “departmentalizing” to take there part in this layering affect to apply their force on the next lighter “mass atoms” above...

 

This is why we have up.... No?

 

************

 

But - the whole concept of having trapped air in between slabs of steal that weigh tons - that have so much force to keep a battleship a float (kinda blows my mind). Because how is “sand” getting underneath the air (in between the slabs of steal) to force it up so powerfully to keep tons and tons and tons afloat?

 

I guess you gotta think of it as a unit (and all that kind of stuff) - but still - to have that much power - you would think you could blow some air in your hand (while you’re under water) and shoot right up to the top.... But I’m just joking...

 

And then the whole concept of compressed air in tanks.... I’m just opening up barrels of worms (or is that monkeys)....

[npts2020]

For a steel ship, it is not just the hull that is floating. It is the average weight (or density) of all of the space occupied (or water displaced) by the hull including the air inside of it. It is the fact that most of the inside of the hull is air that makes the whole thing light enough to float. Replace the air with something heavier like water......well you know what happens. The air doesnt need to be "between slabs of steel" only kept from spilling out into the water below the waterline and water kept from spilling in.

[Catharsis]

Is that right? “The air doesn't need to be between slabs of steel”....

 

I thought it did? You mean if I had a single sided steal dish (with no honeycombing for sides) it would be just as buoyant as if it did have “air cavities”?

 

By all means - I don’t know - but it doesn't sound right.... Because I can’t just go out in the water with a dish shaped (boat shaped) piece of steal and have it float (just as long as no water get’s into it).... I mean if I’m not mistaken - that bad boy is going straight down without so much as “I’ll see ya latter”...

[insane_alien]

well you could have a spherical shell of steel and pump all the air out of it and it will float.

 

and you could go out in the ocean with an empty hull and it'll float. you can try this with a glass with a heavyish base.

 

if you need air trapped for it to float then the glass should not be able to float whn you sit it in water the right way up. same thing happens with steel or any other material denser than water.

[npts2020]

A dish like a plate will not work as well because it has no volume to contain air but if you could get a bowl to stay upright it would likely float. The difference being the amount of fluid like soup (air in previous discussion) you could pour into it, a plate not so much because whatever gets poured on flows over the side, whereas a bowl will hold much more because of its depth. Now change that soup to air and you can see why there are not steel ships shaped like a plate, they wouldn't hold a large enough volume of air to float.