View Full Version : Does Gas defy Gravity?


Zardozi
05-31-07, 02:50 PM
How is it posible that based opon lightness it can defy gravity? Doesnt gravity pull down everything?

ie- helium is lighter than air but how does it escape the force of gravity?


also,
water does not act as a wall in a bong to air?

TruthSeeker
05-31-07, 03:00 PM
Gravity is a weak force.

mikenostic
05-31-07, 03:13 PM
How is it posible that based opon lightness it can defy gravity? Doesnt gravity pull down everything?

ie- helium is lighter than air but how does it escape the force of gravity?


also,
water does not act as a wall in a bong to air?
Yes, gravity (if it's strong enough...big enough planet) does pull down gases. If it didn't, do you think Earth would have an atmosphere? Do you think Jupiter/Saturn/Neptune and Youranus would be gas giants if not? No, they would just be gas in space.

Oli
05-31-07, 03:15 PM
Why does a boat not get pulled to the bottom of the sea?
Think on that and apply to gases.

Read-Only
05-31-07, 03:15 PM
How is it posible that based opon lightness it can defy gravity? Doesnt gravity pull down everything?

ie- helium is lighter than air but how does it escape the force of gravity?


also,
water does not act as a wall in a bong to air?

You answered your first question yourself - it's lighter than air and therefore it floats upwards in it - just like an air bubble in water. It's all a matter of density and specific gravity. (You might want to look up those two terms and learn some VERY basic stuff.)

And the answer to the last question is that you apply a partial vacuum to the bong so air pressure pushes the smoke through the water. More very basic stuff.

spidergoat
05-31-07, 03:21 PM
Gravity still works on it, but it works more on denser gasses, thus bringing the denser gasses closer to the center of the planet, thus displacing the lighter gasses, forcing them up.

darksidZz
05-31-07, 04:39 PM
Yes... what of the gyroscopes?

Read-Only
05-31-07, 05:36 PM
Yes... what of the gyroscopes?

What about them? Get one spinning and drop it - it falls.

I do realize you're talking about it's ability to resist having it's orientation changed - like balancing it on a string. But that's nothing but a matter of inertia and nothing to do with gravity. Remove the string and it will fall as quickly as anything else.

darksidZz
05-31-07, 05:51 PM
:(~~~~~~~~~~

James R
05-31-07, 09:50 PM
ie- helium is lighter than air but how does it escape the force of gravity?

It doesn't. It's just that there is an extra force on it - buoyancy.

Helium floats upwards in air for exactly the same reason that a block of wood floats upwards in water. The wood doesn't defy gravity; the water buoys it up.

The upward force is a result of the fluid density increasing with depth. Just as water pressure increases with depth, so air pressure increases lower down in the atmosphere. The pressure differences create net buoyant forces on objects.

Read-Only
05-31-07, 10:02 PM
It doesn't. It's just that there is an extra force on it - buoyancy.

Helium floats upwards in air for exactly the same reason that a block of wood floats upwards in water. The wood doesn't defy gravity; the water buoys it up.

The upward force is a result of the fluid density increasing with depth. Just as water pressure increases with depth, so air pressure increases lower down in the atmosphere. The pressure differences create net buoyant forces on objects.

You were doing just fine until you made this misleading statement: "The upward force is a result of the fluid density increasing with depth. :D

While both that statement and this one: "The pressure differences create net buoyant forces on objects. are both technically correct, they lend a slight misrepresentation in answering the direct question that was asked.

The fact is that no increase in fluid density is required to make helium bouyant - the other gasses present already have a higher specific gravity than helium does. Meaning they are already more dense and heavier. ;)

James R
05-31-07, 10:26 PM
Sorry - The word "density" was incorrect. My statement ought to have said: "The upward force is a result of fluid pressure increasing with depth."

Obviously, buoyancy still occurs even in fluids of (approximately) constant density.

Read-Only
05-31-07, 10:33 PM
Sorry - The word "density" was incorrect. My statement ought to have said: "The upward force is a result of fluid pressure increasing with depth."

Obviously, buoyancy still occurs even in fluids of (approximately) constant density.

But my main point was that even if the other gasses were only a single atom/molecule thick on the body of a perfect sphere, the helium would still float on top of them because it has a lower specific gravity. No "increase" in depth is even needed.

And it's not the fact that pressure increases with depth that keeps wood floating, it's the fact that right at the surface of the water, the wood is less massive than an equal volume of water (specific gravity once again). :)

Fraggle Rocker
05-31-07, 11:01 PM
also, water does not act as a wall in a bong to air?Let's answer this question first. You can push your face into a sink full of water and blow bubbles through it, right? A bong works the same way. When you suck on the inhaler, the air pressure on top of the bowl blows air and smoke bubbles through the water. It's exactly the same thing.

Water is not a solid, so there's no way it could possible act as a "wall" under ordinary circumstances. Its molecules happily move aside to let the air molecules pass through. That's how liquids and gases behave, totally unlike solids.
How is it posible that based opon lightness it can defy gravity? Doesnt gravity pull down everything? ie- helium is lighter than air but how does it escape the force of gravity?Most of the other answers are correct but Mike gets the prize in the following post, because he simply does a better job of explaining it, without using complicated scientific terminology, and without defining "buoyancy" as a force, which is a bit of a problem.
Yes, gravity (if it's strong enough...big enough planet) does pull down gases. If it didn't, do you think Earth would have an atmosphere? Do you think Jupiter/Saturn/Neptune and Youranus would be gas giants if not? No, they would just be gas in space.Good job Mike. I hope you are a teacher. Everyone else's explanations are a bit too complicated for a layman to understand. You point out the obvious: Helium obviously does not escape the force of gravity because it is pulled toward the earth.

The fact that helium is on top of the stack of gases is a separate and interesting question, but it doesn't contradict the fact that the helium is nonetheless firmly captured by the earth's gravity and does not float off freely into space. (Well actually if there's any free hydrogen it would be topmost because it's even lighter than helium--but all those rocket ship exhausts have probably burned up what little free hydrogen might have been there. :))
It doesn't. It's just that there is an extra force on it - buoyancy.Uh... "buoyancy" is a bogus force like "suction." I would avoid using those terms to explain science to laymen because it can actually make the explanation more convoluted. And some day they'll find out there really is no such force as buoyancy and it will be like Santa Claus: Why didn't you just tell us the truth so the universe would not seem so complicated? :) There is no force pushing the helium up. There is a force pulling it down, and that force is gravity. The problem is that the force of gravity pulls more strongly on all the other gases because they are denser: oxygen, nitrogen, carbon dioxide, etc. So they push their way past the helium and leave it sitting on top of them. But it is still being firmly pulled by gravity just like smoke, water, humans, rocks, and all other matter.
Helium floats upwards in air for exactly the same reason that a block of wood floats upwards in water. The wood doesn't defy gravity; the water buoys it up.I have had more success getting people to understand gravity by simply explaining that water is denser than wood so it pushes its way past the wood and sinks beneath it. This way you're only explaining one force rather than two.

There are only four elementary forces in the entire universe: electromagnetism, gravity, the strong nuclear force and the weak nuclear force. I believe that postulating buoyancy as a force, when it is merely a local relative negative gravity, does not prepare people for their science classes. I vote for giving Mike the Science Fair Award. :) And I hope to the goddess that he is a teacher.

James R
05-31-07, 11:03 PM
But my main point was that even if the other gasses were only a single atom/molecule thick on the body of a perfect sphere, the helium would still float on top of them because it has a lower specific gravity. No "increase" in depth is even needed.

I disagree. I'm not convinced that a single molecular layer would result in the helium bubbling to the top. What happened in that case would depend in a very specific way on the surface chemistry.


And it's not the fact that pressure increases with depth that keeps wood floating, it's the fact that right at the surface of the water, the wood is less massive than an equal volume of water (specific gravity once again).

Unless we're talking about the same thing in different ways, I think you are wrong. What keeps wood floating is that there is a larger pressure on its bottom surface than on the top surface, producing an upward force. That force exists when the block is totally immersed.

Your "specific gravity" explanation is not wrong, but it is a secondary explanation. The relative densities of fluid and object only become relevant because of the differential pressure in the fluid.

Also, there are no buoyant forces in the absence of gravity, so buoyancy is not solely a result of differing densities, as you seem to be implying.

Fraggle Rocker
05-31-07, 11:05 PM
Moderator: This should probably be in Math & Physics.

James R
06-01-07, 12:17 AM
Fraggle:


And some day they'll find out there really is no such force as buoyancy and it will be like Santa Claus: Why didn't you just tell us the truth so the universe would not seem so complicated?

I have no problem at all talking about "bouyant forces". Immerse an object in water and the net force on it consists of two separate effects: (1) gravity pulling it down + (2) pressure forces (buoyant forces) pushing it up.

In terms of fundamental forces, we're dealing with two forces here - gravity and electromagnetism - not just gravity. Buoyancy cannot be explained in terms of gravity alone, if you want to get "fundamental" about it.


There is no force pushing the helium up. There is a force pulling it down, and that force is gravity. The problem is that the force of gravity pulls more strongly on all the other gases because they are denser: oxygen, nitrogen, carbon dioxide, etc. So they push their way past the helium and leave it sitting on top of them. But it is still being firmly pulled by gravity just like smoke, water, humans, rocks, and all other matter.

This does not explain anything.

Take a rock immersed in water. It "weighs" less than it does in air. Thus, we conclude that there is, in addition to its weight, a separate upward force on it, which we call the "buoyant force". This force exists even when the rock is held stationary in the water, so it cannot have anything to do with denser or less dense water pushing past it.


I have had more success getting people to understand gravity by simply explaining that water is denser than wood so it pushes its way past the wood and sinks beneath it. This way you're only explaining one force rather than two.

Hopefully, you can appreciate why that is wrong, and you'll explain it better in future.

Facial
06-01-07, 01:05 AM
Certainly fluids try to displace every volume under gravity. This is because there is always more hydrostatic pressure as depth increases, resulting in uneven forces, leaving an upward resultant. The only vertical force left to consider is weight - if it is heavier, it sinks. But the buoyant force is still there. Even something as dense as a rock will weight heavier when inside a vacuum because of the atmospheric buoyant force.

Pete
06-01-07, 01:23 AM
A misunderstanding of buoyancy lies behind a number of supposed perpetual machines.

For example:
http://www.lhup.edu/~dsimanek/museum/torus.gif
The material of the ring floats is less dense than the blue fluid and more dense than the red fluid, so it will spin on its own... or will it?

Another:
http://www.lhup.edu/~dsimanek/museum/tubes2c.gif
The ball floats in the fluid on the right and falls through the air on the left. The fluid stays up on the right side because of the gates G1 and G2, which are timed to quickly open and close just long enough for the ball to pass . Assuming perfect (or even "good enough") engineering, the ball will cycle endlessly... or will it?

Here's a similar one:
http://users.on.net/~peterbyrnes/Pete/Sciforums/BuoyantPMM.GIF
This time, there are a series of buoyant balls connected to a chain, passing through a cleverly sealed container of fluid.
Will it continually turn?

BenTheMan
06-01-07, 07:22 AM
James---there is a cruicial paper written by Archimedes 2300 years ago which you could refer people to, in which all the details are explained.

BenTheMan
06-01-07, 07:23 AM
The point being---these ideas have been well understood for quite some time.

Read-Only
06-01-07, 07:27 AM
I disagree. I'm not convinced that a single molecular layer would result in the helium bubbling to the top. What happened in that case would depend in a very specific way on the surface chemistry.



Unless we're talking about the same thing in different ways, I think you are wrong. What keeps wood floating is that there is a larger pressure on its bottom surface than on the top surface, producing an upward force. That force exists when the block is totally immersed.

Your "specific gravity" explanation is not wrong, but it is a secondary explanation. The relative densities of fluid and object only become relevant because of the differential pressure in the fluid.

Also, there are no buoyant forces in the absence of gravity, so buoyancy is not solely a result of differing densities, as you seem to be implying.

OK, allow me to cut directly to the chase. It was clearly obvious by the fact the OP asked the question that he/she didn't understand even the most basic physical principles involved. Therefore, I was making an attemp to answer as simply as possible so as to not confuse him/her by going off into much deeper aspects of the problem as you did.

And it's clearly obvious to anyone who's made it through even a rudimentary class in physics that there's no buoyant effects without gravity. So that particular statement of yours - saying I seem to be implying such - was completely absurd and uncalled for.

To me, the most straightforward way to give the OP a handle on the answer was the suggestion to do some reading about specific gravitiy and I did so in my very first response. After that, however, I think that the vast majority of what was said - including even your most initial statements - went directly over the OP's head and only created confusion for him/her.

if you wish to get down into all the deeper aspects of the question, fine. Open another thread and we'll discuss it all you want. I'm more than prepared to match wits and facts with you on that level but this thread was NOT the place for it. Someone wanted a simple answer and you carried it much too far.

iceaura
06-01-07, 10:03 AM
Immerse an object in water and the net force on it consists of two separate effects: (1) gravity pulling it down + (2) pressure forces (buoyant forces) pushing it up.

This is really interesting (if you're my kind of fool). It's the opposite of the "common sense illusion" of a car hitting a tree, where the important error is to overlook the fact that the tree hits back. Here the illusion is that of the water doing something - pushing up (we do ignore surface effects) - when the only actual force involved is the boat and water both being pulled down.

Now, which way does a helium ballon move in a suddenly accellerated car, again ? Memory fails me - - -

The point being---these ideas have been well understood for quite some time. But the best ways of explaining them are not nearly as well understood eh? We know the world by the stories we tell ourselves about it, not just the pictures we see, and when the two conflict - - - .

Pete
06-01-07, 10:18 AM
Here the illusion is that of the water doing something - pushing up (we do ignore surface effects) - when the only actual force involved is the boat and water both being pulled down.

It's no illusion... the boat pushes on the water and the water pushes on the boat, just like the car pushes on the tree and the tree pushes on the car.

iceaura
06-01-07, 12:13 PM
It's no illusion... the boat pushes on the water and the water pushes on the boat, just like the car pushes on the tree and the tree pushes on the car. So you see no problem,intuitively, with ascribing active agency to the water, similar to the elastic structural response of the tree?

BenTheMan
06-01-07, 01:15 PM
But the best ways of explaining them are not nearly as well understood eh?

I don't know---they're perfectly clear to me.

iceaura
06-01-07, 03:17 PM
I don't know---they're perfectly clear to me. Have they proven perfectly clear to others, when you explained to them?

There seem to be a few apparently quite intelligent people on this forum to whom they are not perfectly clear.

Read-Only
06-01-07, 03:24 PM
Have they proven perfectly clear to others, when you explained to them?

There seem to be a few apparently quite intelligent people on this forum to whom they are not perfectly clear.

It's quite clear to me. Simply put, one of the principles of specific gravity means that a lighter material (fluid or gas) will be suspended by a heavier one. Which is actually all this thread was about in the beginning. It's everything beyond that that introduced the confusing elements that were totally unnecessary in addressing the simple question.

iceaura
06-01-07, 04:36 PM
It's everything beyond that that introduced the confusing elements that were totally unnecessary in addressing the simple question. To those who don't already know the answer, it's not that simple a question.

To get a simple, clear, and not misleading answer - one that produces in someone likely to ask such a question an intuitive understanding in agreement with fundamental physical principles - seems to me a challenge.

People who can explain stuff only to themselves don't seem all that solid or reliable in their own understanding, to me. Granted explanation is difficult, but it ought to be possible, in everyday physical matters anyway.

Oli
06-01-07, 04:38 PM
So you see no problem,intuitively, with ascribing active agency to the water, similar to the elastic structural response of the tree?

Even if it was a brick wall there'd be a push back onto the car. It's not about the elastic structure of the tree.

Read-Only
06-01-07, 04:42 PM
To those who don't already know the answer, it's not that simple a question.

To get a simple, clear, and not misleading answer - one that produces in someone likely to ask such a question an intuitive understanding in agreement with fundamental physical principles - seems to me a challenge.

People who can explain stuff only to themselves don't seem all that solid or reliable in their own understanding, to me. Granted explanation is difficult, but it ought to be possible, in everyday physical matters anyway.

No, this particular question was NOT all that difficult. You seem to be draging out your point for no real constructive reason that I can see.

Now - if someone with no knowledge of electronics asked how a TV worked, THAT would be difficult to answer!!!

iceaura
06-01-07, 05:28 PM
Even if it was a brick wall there'd be a push back onto the car. It's not about the elastic structure of the tree. I didn't mean "rubbery" by "elastic" - I meant resistant to deformation, so that the force is not absorbed but rather bounced back.

And that is beside the point, here: the question involves the unlearned person's intuitive handling of the "passive" agent when imagining a combination of forces.

No, this particular question was NOT all that difficult. You seem to be draging out your point for no real constructive reason that I can see. So everyone who has trouble with any technically correct explanation of it is a real dummy ? They don't all seem that way to me.

Oli
06-01-07, 05:38 PM
And that is beside the point, here: the question involves the unlearned person's intuitive handling of the "passive" agent when imagining a combination of forces.
My bad. Apologies.
You're right, the intuitive response was pushed out so long ago I couldn't see what you were getting at.

Read-Only
06-01-07, 06:13 PM
So everyone who has trouble with any technically correct explanation of it is a real dummy ? They don't all seem that way to me.

Now THAT was totally absurd and completely uncalled for!!!!!!!!!!

I never said anything like that or even implied such a preposterous thing!! How dare you try to put such an insulting thought into anything I or anyone else said. One more completely unjustified crack like that will earn you an infraction if I see it.

Every single person is at some stage of the learning process and obviously some are ahead of others - but that doesn't make the one not yet quite as advanced "a real dummy."

Pete
06-01-07, 06:26 PM
So you see no problem,intuitively, with ascribing active agency to the water, similar to the elastic structural response of the tree?

Resistance to deformation is only part of the picture. Resistance to acceleration is another, and resistance to being lifted is yet another.

The water resists being lifted. This resistance is what pushes up on the boat and stops gravity from pulling it to the bottom of the ocean. Just like your chair's resistance to deformation stops gravity from pulling you to the floor.

iceaura
06-01-07, 06:46 PM
The water resists being lifted. So you are happy with the intuitive sense of the water being an active agent here - lifting and pushing and so forth, generating force ?

Now THAT was totally absurd and completely uncalled for!!!!!!!!!! Hey, you're the guy who claimed the question was a simple one to answer, even from someone likely to ask it. I'm the one who is claiming it to see evidence of difficulties and complexities - right in this thread.

Read-Only
06-01-07, 07:12 PM
Hey, you're the guy who claimed the question was a simple one to answer, even from someone likely to ask it. I'm the one who is claiming it to see evidence of difficulties and complexities - right in this thread.

I'm not even going to comment on that. What I want YOU to address and apologize for is this:

Originally Posted by iceaura
So everyone who has trouble with any technically correct explanation of it is a real dummy ? They don't all seem that way to me.

Now THAT was totally absurd and completely uncalled for!!!!!!!!!!

I never said anything like that or even implied such a preposterous thing!! How dare you try to put such an insulting thought into anything I or anyone else said. One more completely unjustified crack like that will earn you an infraction if I see it.

Every single person is at some stage of the learning process and obviously some are ahead of others - but that doesn't make the one not yet quite as advanced "a real dummy."

Pete
06-01-07, 11:41 PM
So you are happy with the intuitive sense of the water being an active agent here - lifting and pushing and so forth, generating force?
I don't know what your idea of "intuitive" is, but the physics is as I said.There is a force acting between the boat and the water.

BenTheMan
06-02-07, 12:00 AM
Have they proven perfectly clear to others, when you explained to them?

Come to Ohio State and take Physics 131.

James R
06-02-07, 02:33 AM
Read-Only:


OK, allow me to cut directly to the chase. It was clearly obvious by the fact the OP asked the question that he/she didn't understand even the most basic physical principles involved. Therefore, I was making an attemp to answer as simply as possible so as to not confuse him/her by going off into much deeper aspects of the problem as you did.

We can argue this, but I think my fundamental answer is quite simple. Here it is in one sentence:

Pressure increases as you go downwards in a fluid, and it is that difference in pressure that buoys up immersed objects.

Do you agree or disagree with that?

Moreover, I argue that this is ultimately a better explanation than:

Objects are buoyed up in a fluid because they have a different specific gravity than the fluid has.

My explanation explains where the buoyancy effect ultimately comes from. Your explanation does not.


And it's clearly obvious to anyone who's made it through even a rudimentary class in physics that there's no buoyant effects without gravity. So that particular statement of yours - saying I seem to be implying such - was completely absurd and uncalled for.

I think saying it was uncalled is a bit precious. In fact, no explicit mention of the role of gravity had been made by either of us up to that point, if I recall correctly.


To me, the most straightforward way to give the OP a handle on the answer was the suggestion to do some reading about specific gravitiy and I did so in my very first response. After that, however, I think that the vast majority of what was said - including even your most initial statements - went directly over the OP's head and only created confusion for him/her.

I'd be interested to know what the original poster found most helpful. If you're still around, opening poster, please tell us!


if you wish to get down into all the deeper aspects of the question, fine. Open another thread and we'll discuss it all you want. I'm more than prepared to match wits and facts with you on that level but this thread was NOT the place for it. Someone wanted a simple answer and you carried it much too far.

The question was answered - two or three times in two or three ways. Then the rest of the discussion started. I see no problem with that.


iceaura:


This is really interesting (if you're my kind of fool). It's the opposite of the "common sense illusion" of a car hitting a tree, where the important error is to overlook the fact that the tree hits back. Here the illusion is that of the water doing something - pushing up (we do ignore surface effects) - when the only actual force involved is the boat and water both being pulled down.

No. As Pete said, the boat pushes down on the water below it, and the water below it pushes back upwards on the boat.

ALL forces are interactions, as Newton's third law makes clear.


Now, which way does a helium ballon move in a suddenly accellerated car, again ? Memory fails me - - -

If the car accelerates forward, the balloon moves forward.


To get a simple, clear, and not misleading answer - one that produces in someone likely to ask such a question an intuitive understanding in agreement with fundamental physical principles - seems to me a challenge.

Perhaps. But the questioner can always ask follow-up questions. I'd encourage them to do so, if they are still confused.

iceaura
06-04-07, 03:45 PM
No. As Pete said, the boat pushes down on the water below it, and the water below it pushes back upwards on the boat. So you, also, regard the ascription of active agency to the water - the water generating a force, presumably in some way analoguous to the tree generating force by the relaxation of elastic bonds in its internal structure - as carrying no risk of misleading the questioner ?

Perhaps. But the questioner can always ask follow-up questions. I'd encourage them to do so, if they are still confused. But what if they think they are not confused, adn simply accept the answer as given - and walk away believing that water is the sort of stuff that pushes up on things, and that's why things float on it?

And then, of course, later, when you are not around, they would understand this situation:
Now, which way does a helium ballon move in a suddenly accellerated car, again ? Memory fails me - - -

If the car accelerates forward, the balloon moves forward. as the air in the car pushing forward on the balloon.

BenTheMan
06-04-07, 04:39 PM
But what if they think they are not confused, adn simply accept the answer as given - and walk away believing that water is the sort of stuff that pushes up on things, and that's why things float on it?

Then they have learned nothing, and attempted to learn nothing. The intelligent person would quickly see that this analogy breaks down when things are denser than water, for example. Learning physics can't be reduced to memorizing what happens in each physical situation. It's about learning a set of principles (in this case, the Principle of Archimedes) and how to apply those principles to the natural world.

Read-Only
06-04-07, 04:55 PM
Then they have learned nothing, and attempted to learn nothing. The intelligent person would quickly see that this analogy breaks down when things are denser than water, for example. Learning physics can't be reduced to memorizing what happens in each physical situation. It's about learning a set of principles (in this case, the Principle of Archimedes) and how to apply those principles to the natural world.

Precisely! ANd that's exactly why I kept harping on specific gravity. Principles, as opposed to simple isolated facts, are more important because they have BROAD applications.

Pete
06-04-07, 07:12 PM
Then they have learned nothing, and attempted to learn nothing. The intelligent person would quickly see that this analogy breaks down when things are denser than water, for example.
What analogy?
Water still pushes up on denser-than-water objects, just not enough to counteract gravity.


they would understand this situation:

If the car accelerates forward, the balloon moves forward.
as the air in the car pushing forward on the balloon.
That seems to me to be a perfectly correct way of understanding it... What am I missing?

BenTheMan
06-04-07, 08:07 PM
Water still pushes up on denser-than-water objects, just not enough to counteract gravity.

Of course---how could I forget:) This is one of the coolest experiments that I remember doing, to easure the bouyant force on an object which sinks!

iceaura
06-04-07, 09:20 PM
Water still pushes up on denser-than-water objects, just not enough to counteract gravity.
- -
as the air in the car pushing forward on the balloon.

That seems to me to be a perfectly correct way of understanding it... What am I missing?

How does the water, or air, know which way to push ?

Pete
06-04-07, 09:28 PM
How does the water, or air, know which way to push ?

Gravity or acceleration, which produces a pressure gradient.

P = P_0 + \rho gh
For water, g is the acceleration due to gravity and h is the depth of the water.
For the air in the car, g is the car's acceleration and h is displacement parallel to the acceleration.

The force comes from the pressure:
P = FA

Because of the pressure gradient, Since the pressure is higher on one side than the other, the force is higher on one side than the other, and the object feels a net push.

iceaura
06-05-07, 05:17 PM
Because of the pressure gradient, Since the pressure is higher on one side than the other, the force is higher on one side than the other, and the object feels a net push. But this pressure gradient is not something the water does, but something done to the water, no?

Do you still, after your explanation, see no danger in assigning force production, active agency, to the water ?

Pete
06-05-07, 06:24 PM
Oh crap! Sorry iceaura, I edited your post by accident. :(
I think I've restored it to the way it was. Anyway...


But this pressure gradient is not something the water does, but something done to the water, no?
The pressure gradient exists as a result of both the water and the gravity field.
Its existence requires both the water and the gravity field.
Not that it matters where the pressure comes from - the question is what its result is.
And the result is that the water pushes on its surroundings, according to the pressure at that point.
You don't have a problem with saying that water pushes on the bottom of a dam wall, do you? It's the same thing.


Do you still, after your explanation, see no danger in assigning force production, active agency, to the water ?
I don't understand what you're getting at by the term "active agency".
The simple fact is that there is an upward force on the boat that stops it from falling to the bottom, and that this force is between the boat and the water.
Just like there is an outward force on a dam wall that the wall must resist, and that this force is between the wall and the water.

I'm not sure that it's helpful to call the water an "active agent" in either case.

Perhaps if you explain what danger you see, I might understand you better?

James R
06-05-07, 11:58 PM
iceaura:


So you, also, regard the ascription of active agency to the water - the water generating a force, presumably in some way analoguous to the tree generating force by the relaxation of elastic bonds in its internal structure - as carrying no risk of misleading the questioner ?

Forces in physics don't have to be produced by "active agents". If I push against a brick wall, then the wall pushes back on me with an equal and opposite force. Does the fact that the wall exerts a force on my hand make it an "active agent"? I don't think so. Well, it's all a matter of definition, isn't it?


But what if they think they are not confused, adn simply accept the answer as given - and walk away believing that water is the sort of stuff that pushes up on things, and that's why things float on it?

That's a correct explanation.


But this pressure gradient is not something the water does, but something done to the water, no?

The pressure gradient is a direct consequence of the water having weight. Whether you consider weight as being something "done to the water" or not is irrelevant, as far as I can see.


Do you still, after your explanation, see no danger in assigning force production, active agency, to the water ?

I don't agree that force needs active agency. Gravity exists regardless of "active agents".