Archimedes principle vs Atwood's principle

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  • #1
abdo799
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If we have an upward moving sphere with 0.4 m3 volume and 100 kg mass , according to Archimedes buoyancy force = 4000 N so acceleration = 40 m/s^2
But according to Atwood's principle , buoyancy force ≈ 1600 N acceleration = 16 m/s^2
what makes such big difference . ( ignore drag force and downward weight force , i am talking about upwards force only liquid:water ) http://en.wikipedia.org/wiki/Buoyancy#Beyond_Archimedes.27_principle
 
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  • #2
SteamKing
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I'm afraid your going to have to explain in more detail about the buoyancy force derived from the Atwood principle. Are you sure you haven't overlooked or misinterpreted something? I can assure you that Archimedes' principle is alive and well and living at the heart of naval architecture.
 
  • #3
russ_watters
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For starters, what fluid are we talking about? Water?

Also, I've never heard of Atwood's principle and a Google doesn't find it. What is it?
 
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D H
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russ_watters
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Ah, I see. Whether a useful concept or not, the error/confusion arises from the conflagration of definitions: "buoyancy force" and "dynamic buoyancy force" are not the same thing.

A little surprising to find such a big mess in a subject so simple.
 
  • #6
abdo799
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Are you writing about that nonsense at the end of the wikipedia article on buoyancy (link: http://en.wikipedia.org/w/index.php?title=Buoyancy&oldid=575614101#Beyond_Archimedes.27_principle). From looking at the article's history, this section is the subject of an ongoing edit war at wikipedia. IMHO, that section needs to be deleted and the article needs to be locked. This nonsense is exactly why wikipedia can at times be a bad reference.
yes, exactly , i was so sure that Archimedes principle was right , then i read this , i am confused now , is this atwood guy right or wrong ?
worst part , i have 12 days off school and i cant contact my physics teacher
 
  • #7
D H
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George Atwood never authored a web page. He died in 1807. That section of the wikipedia buoyancy article is not true to Atwood's principle. The author of that section is either a crackpot or is someone who has been deceived by a crackpot. Reference #4 in that wikipedia article is a crackpot link.
 
  • #8
abdo799
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Ah, I see. Whether a useful concept or not, the error/confusion arises from the conflagration of definitions: "buoyancy force" and "dynamic buoyancy force" are not the same thing.

A little surprising to find such a big mess in a subject so simple.

can you tell me the difference please?
 
  • #9
abdo799
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I'm afraid your going to have to explain in more detail about the buoyancy force derived from the Atwood principle. Are you sure you haven't overlooked or misinterpreted something? I can assure you that Archimedes' principle is alive and well and living at the heart of naval architecture.

i have edited the question and included more details
 
  • #10
russ_watters
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You can see from the descriptions and equations that they are completely different. One is independent of the weight of the object and the other is not.

To be clear, I haven't done the work to figure out if the concept has any merit (maybe DH has): I'm just saying it isn't buoyancy.
 
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  • #11
abdo799
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Just remembered, my math teacher is a naval engineer , i will ask him when i get back to school
 
  • #12
D H
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That last part of that article is 100% nonsense, abdo. The upward buoyant force is the weight of the displaced water. Period. The net force, at least initially, is the vector sum of the upward buoyant force and the downward gravitational force. As soon as the object starts moving it will of course be subject to drag in addition to buoyancy and gravitation, but that's a different question. This nonsense equation does not address drag. It's just nonsense.
 
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  • #13
arildno
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That part of the article has now been removed. I'll keep removing that nonsense. :smile:
 
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  • #14
D H
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Thanks! And reference #4 went bye-bye as well! Excellent work.

I don't understand why that section remained for such a long time. That reference doesn't stand up to wikipedia requirements, and that entire section violated the no original research rule.
 
  • #15
arildno
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Thomas Smid was an oldie back here at PF some years ago, with his silly misconceptions of pressure, Bernoulii and aerodynamic lift.
 
  • #16
D H
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Let alone relativity. That really set him off.
 
  • #17
arildno
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Let alone relativity.

Wonder why..maybe he also has pet peeves about quantum mechanics as well.
It's a fairly standard repertoire among crackpots, but his views on pressure and lift had, at least, shall we say, a certain..originality to them?
 
  • #18
abdo799
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Thanks! And reference #4 went bye-bye as well! Excellent work.

I don't understand why that section remained for such a long time. That reference doesn't stand up to wikipedia requirements, and that entire section violated the no original research rule.
I use wikipedia a lot , and after this , i will be checking every thing twice , but can you tell me how to be sure that the article in wikipedia is correct ?
thanks
 
  • #19
D H
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That's the problem. You can't know if something in wikipedia is correct unless you already know the subject. That said, the vast majority of wikipedia articles are correct. Whether they're well written, that's a very different question.

I'm a bit shocked that something as basic as buoyancy attracted the attention of a crackpot. Usually you find crackpots focusing on more advanced topics, particularly those topics that have significant political/economic/religious ramifications. Buoyancy? That's settled science, and it's Newtonian mechanics to boot.
 
  • #20
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You can reduce the risk to get wrong information:
- if available, check the references in the article
- check the version history. Did the article get some significant edits recently? Multiple reverts, large +xxx or -xxx bytes edits and very recent edits (especially from IPs) are a good indication that some content can be dubious. Check how the article looked a few months ago, or in a phase where it was not edited for a long time.
- if you know multiple languages, look at the article in other wikis. It is extremely rare that the same wrong information is in multiple wikis at the same time.
 
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  • #21
Nugatory
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I use wikipedia a lot , and after this , i will be checking every thing twice , but can you tell me how to be sure that the article in wikipedia is correct ?
thanks

You can never be sure, but looking at the talk page for the article will usually tell you when something bad wrong is happening.
 
  • #22
D H
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Guess what? It's baaack!
 
  • #23
D H
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And the rationale for the revert: "Yes, it's physics, but dynamics, not statics."

Sorry, that's wrong. With dynamics you would bring the tools of fluid mechanics into play. Gravitation? Check. Buoyancy? Check. Drag? Check? All the fluid mechanics nastiness that tells you what drag is? Check.

Crackpot physics? Uncheck.
 
  • #24
AlephZero
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And the rationale for the revert: "Yes, it's physics, but dynamics, not statics."

Sorry, that's wrong.

I agree the analysis of the situation on the wiki page is nonsense, but there is a very faint glimmer of sense behind it - i.e the fact that if the buoyant object moves in one direction, some fluid has to move in the other direction somehow, otherwise you would leave a "hole" in the fluid.

But the "atwood assumption" that if the buoyant object moves in one direction, an equal volume of fluid must move an equal distance in the opposite direction is complete garbage. Fluid flow ain't that simple (even in incompressible and inviscid fluids).
 
  • #25
SteamKing
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I'm wondering if Smid is trying to account for the concept of added mass in the dynamic response of a submerged buoyant object which is suddenly released. This added mass of entrained fluid would serve to reduce the acceleration of the buoyant object from the value a simple static analysis would give. Mind you, once the object is released and starts to move, a more complicated problem is created due to fluid flow, as others have stated in this thread.
 
  • #26
arildno
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"Gone with the wind" again. or was it "Gone in 60 seconds"??
Anyway, the war has begun. :devil:
 
  • #27
abdo799
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I read the talk page about the atwood's machine , he is saying that this concept puts in consideration the inertia of the moving fluid that replaces the volume of the object . And saying that this equation will show that buoyant acceleration can not be greater than g , thus law of conservation is correct , anyways, i used his formula and the buoyant acceleration is greater than g (1600/100=16).
 
  • #28
boneh3ad
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I use wikipedia a lot , and after this , i will be checking every thing twice , but can you tell me how to be sure that the article in wikipedia is correct ?
thanks

Compare it to other sources. Anyone, and I mean anyone, can edit Wikipedia. There are standards for how you have to do it, but even that is kind of fishy because there are so many articles and it is impossible to truly police for accuracy. The bottom line is if you don't already know anything about the subject, you need to check it against other sources.
 
  • #29
arildno
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"And saying that this equation will show that buoyant acceleration can not be greater than g , thus law of conservation is correct"

Note the fallacy here:
That some correct result follows from an equation does NOT mean that equation as such is justified.

I
 
  • #30
cjl
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"And saying that this equation will show that buoyant acceleration can not be greater than g , thus law of conservation is correct"

Note the fallacy here:
That some correct result follows from an equation does NOT mean that equation as such is justified.

I

I'm not even convinced that that result is correct. I can't say for sure that it's wrong, but I can think of possible explanations for why it could either be correct or incorrect, and I'm not willing to make a definitive statement either way without either a good source, a good experiment, or a fairly significant amount of theoretical work. There's no inherent reason why gravity pulling something downwards limits accelerations to 1G - the thing moving downwards is limited to 1G, yes, but it can propel something else upwards at much greater than 1G. I'm not sure if that's the case here though.
 
  • #31
arildno
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I'm not even convinced that that result is correct.
I didn't say so. I said that you cannot justify an equation even if it has some correct partial result.
So, I agree with your withholding of judgment.
 
  • #32
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I read the talk page about the atwood's machine , he is saying that this concept puts in consideration the inertia of the moving fluid that replaces the volume of the object .
That is just the standard drag in a medium. It will not correspond to anything you can calculate with that Atwood approach.
The acceleration of objects in water can exceed g.
 
  • #33
abdo799
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That is just the standard drag in a medium. It will not correspond to anything you can calculate with that Atwood approach.
The acceleration of objects in water can exceed g.

I know, i searched about this atwood thing in a textbook, found nothing
 

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