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Archimedes principle vs Atwood's principle

  1. Oct 8, 2013 #1
    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
     
    Last edited: Oct 8, 2013
  2. jcsd
  3. Oct 8, 2013 #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.
     
  4. Oct 8, 2013 #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?
     
  5. Oct 8, 2013 #4

    D H

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  6. Oct 8, 2013 #5

    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.
     
  7. Oct 8, 2013 #6
    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
     
  8. Oct 8, 2013 #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.
     
  9. Oct 8, 2013 #8
    can you tell me the difference please?
     
  10. Oct 8, 2013 #9
    i have edited the question and included more details
     
  11. Oct 8, 2013 #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.
     
  12. Oct 8, 2013 #11
    Just remembered, my math teacher is a naval engineer , i will ask him when i get back to school
     
  13. Oct 8, 2013 #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.
     
  14. Oct 8, 2013 #13

    arildno

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    That part of the article has now been removed. I'll keep removing that nonsense. :smile:
     
  15. Oct 8, 2013 #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.
     
  16. Oct 8, 2013 #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.
     
  17. Oct 8, 2013 #16

    D H

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    Let alone relativity. That really set him off.
     
  18. Oct 8, 2013 #17

    arildno

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    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?
     
  19. Oct 8, 2013 #18
    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
     
  20. Oct 8, 2013 #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.
     
  21. Oct 8, 2013 #20

    mfb

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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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