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B Cause of buoyant force

  1. Oct 2, 2016 #1
    how does pressure on a submerged body in water experience an upward thrust ,when the cause of pressure is gravity, which can only be downwards
     
  2. jcsd
  3. Oct 2, 2016 #2

    Dale

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    The premise of this question is wrong. Pressure is not caused by gravity. For example, the international space station is pressurized even though it is in microgravity. Pressure is a contact force between a fluid and a surface, it is not caused by gravity, and it acts in a direction perpendicular to the surface (which may be upwards).
     
  4. Oct 2, 2016 #3
    buoyant force is resultant of all of those forces* exerted by liquid on the floating body

    * force acting at a point (which varies with variation of liquid column above the point) can be calculated if pressure is known

    so you need to do ∫ P.dA , cyclic integral over whole surface of the body to get buoyant force
    where - P is pressure at a particular point on the body
    and dA is surface area of that infinitesimal region of body , where pressure is P
     
  5. Oct 2, 2016 #4
    !!

    you are wrong
    pressure is force / area, that force may gravitational or something else, whatever
     
  6. Oct 2, 2016 #5

    Dale

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    Consider a pressurized container of gas in 0 g. There is pressure and there is no gravity. Therefore, pressure is not caused by gravity.

    Conversely, in an evacuated container there is no pressure, even in the presence of gravity. Gravity is neither necessary nor sufficient for pressure.
     
  7. Oct 2, 2016 #6

    Orodruin

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    You would do well not to make rash assertions that someone else is wrong. In particular when you are wrong yourself. As Dale has already stated, gravity is neither required for or directly causing pressure. That a fluid in a gravitational field experiences a pressure gradient, leading to a buoyant force, is a different thing.
     
  8. Oct 3, 2016 #7
    That is the thing in this case !!
    and
    pressure is force / area, that force may gravitational or something else, whatever
     
  9. Oct 3, 2016 #8

    Orodruin

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    For the buoyant force, yes. This is just where Archimedes' principle comes from.

    Also, you are wrong in your statement that pressure is a result from gravity:
    Gravitational force is a force on a mass, not an area force. That this force leads to internal forces causing a pressure gradient is something else entirely. The pressure force itself is a contact force, it certainly is not a gravitational force.

    What you quoted from Dale and claimed was wrong was not a statement about the buoyant force, it was a statement about pressure. Pressure can be quite relevant even if the net buoyant force is zero - just go diving without equilibrating the pressure in your ears and you will see - or my favourite when flying, closing an empty PET bottle at maximum altitude and observing the effects as the flight descends.
     
  10. Oct 3, 2016 #9

    ???

    if we have pressure varying throughout the liquid column , buoyant force is non zero
     
  11. Oct 3, 2016 #10

    Orodruin

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    Poor choice of words, intended net buoyancy. This is the buoyant force minus the gravitational force.

    Edit: Actually, not bad choice of words. I was talking about a situation where there is no (significant) gravitational field leading to a pressure gradient - as was Dale. What you will feel in your ears when you dive or go flying is not the result of a pressure gradient on your body - it is a very local phenomenon based on the pressure difference between the outside and inside of your ears.
     
  12. Oct 3, 2016 #11
    Hey Orodruin
    what is this happening when i see your post #10 i can read this
    why am i not seeing the underlined thing while just viewing the thread
    i asked this to you cause you are a staff member of PF

    <<Mentor note: Edited for quotes>>
     
  13. Oct 3, 2016 #12

    Orodruin

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    Please use quotes properly. Otherwise it is impossible to know what you are saying and what you are quoting. I edited the post shortly after posting it. You must have loaded it in the meantime.
     
  14. Oct 3, 2016 #13
    this has nothing to do with buoyancy,
    buoyancy is resultant of all the 'local' forces (exerted by fluid on the body inside the fluid)
     
  15. Oct 3, 2016 #14
    Sorry
     
  16. Oct 3, 2016 #15
    murlidharn,
    have you got this all ??
     
  17. Oct 3, 2016 #16

    Orodruin

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    Of course he has not, he has not been logged in since before post #2 was made.
     
  18. Oct 3, 2016 #17

    Orodruin

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    Exactly, neither did Dale's quote that you claim is wrong. It never mentioned buoyancy. Please read the posts that you are going to claim are wrong.
     
  19. Oct 3, 2016 #18

    sophiecentaur

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    This thread has veered right away from the OP. The whole of the question involves a 'real' earthbound situation and its level of knowledge requires a suitable answer and not a high level PF response plus a squabble amongst the better informed.
    If the OP ever returns, I would like to reassure him / her that the first level answer to this question is fairly simple:
    Gravity causes the pressure under water (On Earth, of course). That pressure at any level is due to the weight of water above (forget about the atmosphere and its pressure, it is only an added sophistication). The lower you go, the greater the pressure. Fluid pressure acts in all directions (up/down/sideways) and will be the same at that level, all over a bath / lake / ocean. It's there whether or not there is an object for it to act on and a fluid will flow until the pressure balances out everywhere.
    Assume, for simplicity, that we are dealing with a cylinder with a vertical axis; you can extend the argument to any shape, later. The upwards pressure on the bottom face of the cylinder is greater than the downwards pressure on the top. The pressure against the sides is the same in all directions at any level, so it cancels out. The buoyant force ('upthrust' was our favourite word at school and still is, with students) will depend on the area of the cylinder faces (Pressure times area). The pressure depends on the height difference so the upthrust will be equal to the volume occupied by the cylinder (the volume of water that's 'displaced') times the density of the water that isn't there times g. This is Archimedes' Principle.
    Note, for cylinders of the same volume, the buoyant force will be the same. The distance from top to bottom may be less but the total area will be more so you get the same answer.
     
  20. Oct 3, 2016 #19

    Dale

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    For A to cause B there are several conditions. Most importantly, A must be a necessary and sufficient condition for B. As I have shown above, gravity is neither necessary nor sufficient for pressure, therefore gravity is not the cause of pressure.

    In this case there is gravity and there is pressure and buoyancy is related to both, but gravity is still not the cause of pressure. This is important because of the logic of the OP, which goes: gravity causes pressure, gravity points down, therefore pressure must point down. The recognition that gravity does not cause pressure allows the OP to then learn what does cause it, and then correct his chain of logic.

    Again, gravity does not cause pressure. It is neither necessary nor sufficient. Please do not post further misinformation to the effect that it is the cause.
     
  21. Oct 3, 2016 #20

    sophiecentaur

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    There is concrete thinking and there is formal thinking. All your points are quite correct (unarguable, in fact) but are they helping a questioner who wants a concrete view of the phenomenon? (It's a 'B' question)
     
  22. Oct 3, 2016 #21

    Dale

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    Not you too. No, gravity does not cause pressure! See above. If I take a scuba tank and fill it up, then the pressure is much higher than it was previously, but the gravity is not any different.

    Gravity is not the cause of pressure. Full stop.

    For a hydrostatic fluid gravity causes a pressure gradient. Gravity does not cause fluid pressure, gravity plus the hydrostatic condition together imply a pressure gradient.

    The pressure is the isotropic part of the internal stress within the fluid, it is the force per unit area normal to a surface, which means that the direction of the force from pressure depends on the direction of the surface, not the direction of gravity.
     
  23. Oct 3, 2016 #22

    Dale

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    The OP has presented a specific chain of logic which is based on a flawed premise. I don't know how you expect him to learn if the flawed premise is not addressed. If you (falsely) agree and say "gravity causes pressure" then his reasoning that "therefore it must point down" is much harder to address.

    It is not turning a B question into an A to point out a false premise. The part that is making it difficult is all of the other respondents who are arguing against the "unarguable".
     
  24. Oct 3, 2016 #23

    sophiecentaur

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    The same argument could have been made about the function of a see-saw. Gravity acts downwards so the lighter person should move down. Reductio ad absurdum can be a good argument.
    Hmm. The pressure at the depth of water in this particular context, is only there because of the local gravitational field. ("full stop" :wink:) That is the level of the question. Specific answer to specific questions are quite acceptable in many circumstances. In circuit questions, we quite happily talk about the "battery" in the circuit and that is as good as 'emf source'. I think you should give the thread some slack and acknowledge that it's horses for courses. If no one had brought up that issue in the first place we would have put it to bed with three or four posts. Someone should probably have included a bit of a caveat with the words 'in this case, the pressure is due to the gravity' near the top then we wouldn't be getting hot under the collar.
    Like I said, earlier, concrete and formal explanations both have their place. WE both know that, if you google with the term 'hydrostatic pressure' you will find
    P = ρhg in pretty much every hit. That's all the OP needed.
     
  25. Oct 3, 2016 #24

    Dale

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    That actually was exactly the other argument that I was considering.


    No. Consider an object submerged in a hydrostatic fluid inside a pressure vessel. The pressure can be substantially increased independently of the gravitational field. Only the pressure gradient is there because of the gravitational field.

    I do agree with this as long as the specific answer does not incorrectly teach a wrong general principle. Saying that pressure is caused by gravity is flat out wrong, both in general and in specific. It was stated by the OP (and now by two others including yourself), so clearly it is a common misconception.

    I don't think so. I don't see any indication that the OP is not familiar with the formula for hydrostatic pressure. He just didn't understand how it could point any direction other than downwards given the premise that it is caused by gravity. Citing the formula doesn't address the specific reasoning of the OP.
     
  26. Oct 3, 2016 #25

    Orodruin

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    (My emphasis)
    No, this is definitely false. Pressure is note dependent on the local gravitational field, even in hydrostatics. It is accumulated from the global matter distribution, the fluid state and distribution.
     
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