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Pressure and gravity forces

  1. Oct 26, 2013 #1
    I would like study a theoretical problem. A system composed of 2 spheres can move at left of at right in water. One sphere has gas at low pressure and other sphere has water in it. The wall of spheres are very thin and has the same density of water. Sphere of gas don't has force because it has same water at left and at right. But sphere of water "see" a virtual sphere of water that attract it (because in other direction there is sphere of gas), this force is F1. The pressure must compensate F1 but I don't understand how because pressure is give all around sphere of water and density is not a linear law and angles are not the same (if you change angle, you change distance).

    And especially the layer around the sphere of water is not at R but at R more half thickness of molecule. Do you know if this calculation is done somewhere in the net ?

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  3. Oct 26, 2013 #2

    Simon Bridge

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    I don't understand your question.

    The water-filled sphere with walls that have the density of water is basically an isolated lump of water. Therefore it floats in water at any depth. The buoyancy force is equal to the weight provided the sphere is totally submerged.

    The air-filled sphere displaces it's own mass of water. Since this mass of water has a smaller volume than the sphere, the sphere will float with only part of it's volume submerged. If the sphere were totally submerged, then the buoyancy force on it would be the same as the buoyancy force on the water-sphere (provided they were the same volume)... i.e. equal to the weight of water displaced. The difference in behavior is because the buoyancy is not the only force at work here.

    I think you will see the situation more easily if you use vertically oriented cylinders instead of spheres. The pressure of the fluid increases with depth - this means the pressure on the bottom of the cylinder is bigger than the pressure on the top creating a net force upwards. This is the buoyancy force.

    I also suspect you'll benefit from Donald Simanek's treatment of buoyancy - which is in the context of the quest for perpetual motion (which helps explain the phenomena), making it more fun ;)
    pmm can be a useful way of demonstrating physics - by counter-example.
    Last edited: Oct 26, 2013
  4. Oct 27, 2013 #3
    I'm interesting on the difference between attraction and pressure forces. If you let my study on Earth, supposed that gravity is exactly perpendicular to the image, like that it's easy to understand.

    1/ Consider the sphere full of water: what are forces on it ? At right, gas attract near nothing in the contrary at left water attract sphere of water. For know the attraction, it's necessary to calculate all forces from one molecule of water at left on all molecules of water at right. Look at angles.

    2/ Consider the pressure on sphere of water: what forces on it ? The pressure is given by the sum of all external molecules of the sphere.

    Angles are not the same, so the distance not too, so the sum of force must be different (but it is equal), so I forget a force somewhere.

    It's easier to calculate with square object like image is showing.

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  5. Oct 27, 2013 #4
    you're right it's easier to calculate:

    On Earth, put a column of water on the ground. I put at top of the column a solid in water, with the same density of local water, the solid mustn't move. Pressure from Earth (gravity) give a weight to the solid and buoyant force from Earth attraction is the same force but at top (Archimedes). I'm agree with that.

    But, I think water in the column attract molecules of water at top, between solid and walls of the column. For me there is another buoyant force (very small) due to the presence of water in the column. And I don't find the other force that canceled it. Why water in the column don't attract water at top ?
  6. Oct 27, 2013 #5

    Simon Bridge

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    It is not useful to think of gravity as exerting a pressure. It is quite a different thing.

    In order for the situation described to occur - the solid must be completely submerged.

    And I meant that the solid should be considered to be a cylinder - not the body of water it is floating in.

    You mean the surface tension?
    Like you get a meniscus due to the attraction of the liquid to the sides of the container or whatever?

    I don't understand the question - all the water is attracted to all the water all the time but there is no special affinity of water for water.

    Perhaps you are wondering why water doesn't just all get squashed at the bottom of the container - compressed under it's own weight? The answer is that it does - that's what you see. The force holding the water up as a column is electrostatic. That is what causes the pressure difference with depth.
  7. Oct 27, 2013 #6
    it's easier, yes the solid is in water. I think it's possible to think in 2d.

    not at all.

    I think it's that I don't understand. Take a molecule at top between wall and solid. What forces on it ? From Earth and from water behind it, no ? For me the column add additional gravity, not ?

    Edit: don't forget gravity change with 1/d² in the column too, it's very important because a molecule at bottom of the column has more force to the center of Earth than a molecule at top. When a molecule at top attract molecule at bottom, it's not a mass with a mass, it's a weight with another weight. For me, the pressure at bottom decrease and the pressure increase around the solid. Another point, gravity force from Earth is very big compared than attraction of molecule/molecule in column, so this is not like in space. I lost something ?
    Last edited: Oct 27, 2013
  8. Oct 28, 2013 #7
    I understood, no additional force for solid due to the presence of water in column. The same for water on water (buoyancy), all forces cancel themselves on the last case.

    I have 2 anothers questions:

    1/ Study 1

    a/ Put a solid (height = 1 m, same density than liquid in column) at 11 m of altitude, the weight is P1
    b/ Now, put below the solid a column full of water (height of the column = 10m), the weight of solid is now P2 with P2>P1 because water attract more solid (solid is not in water)
    c/ Move down the solid of 1m (solid is in water), what is the weight ? I thought it is near P2 but if water don't attract solid when it is in water, the weight is P1 ?

    2/ Study 2

    Look at the image, it's the top of the column with a solid in it. Molecules of water attract each others. This don't increase Archimedes law ?

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  9. Oct 28, 2013 #8

    Simon Bridge

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    Somehow don't think you did, because you go on to ask:

    What makes you think the water has a special attraction for the solid that it does not have for other water? Where is this idea coming from?

    Are you thinking of gravitational attraction here?
    Please be clear.

    What attraction are you talking about?

    Note: the weight of the solid is defined to be "the strength of the gravitational force on it".
    Close to the surface of the Earth, with objects very small compared with the Earth, the weight is approximately constant. You do get small variations depending on the local distribution of mass though.

    The weight of an object sitting next to a swimming pool is technically going to change a bit when you put it in the swimming pool because there are different mass distributions around it - it's got closer to the Earth's core by the depth of the pool (if it sunk) for eg. But that difference is so miniscule it is safe to ignore it - you can have a go calculating it. None of your scales will be able to measure the difference.

    How are the molecules of water attracting each other? What are you talking about here?

    Any mysterious attraction forces would contribute to the pressure of the liquid - so I doubt there would be any need to alter the calculation. You do need to alter the calculation for surface tension - which is important for very small objects.

    I'm sorry - nobody can help you if you won't say what you are talking about.
  10. Oct 28, 2013 #9
    Yes, I speak about gravitationnal attraction. I know water attract solid or anything else but I don't understand the difference of weight when an object is in water or not, (it must be the same, ok) take this case:

    1/ On Earth, a solid is placed at 100,001 km of altitude, at this altitude the weight is P1 (I don't care about the precise, it's just P1 value and I ignore gas). Solid is 1 m of height.
    2/ I put a column of water (height 100 km) under solid, the water attract solid, true ? so the weight is not P1 but a little more: P2
    3/ I move down the solid of 1 meter in water, the weight must be a little below P2 (-1 m), true ? but when solid is in water how water can do for attrack solid in water ? Could you explain this point ?

    A molecule attract any other molecule in water, no ? when a solid is put in water, molecules at right attract molecules at bottom, and molecules at left attract molecules at bottom. The solid is between molecules, there is a force on it, no ? They can't attract at top, there is no water. Lateral forces cancel themselves. But I see a up force and I don't know how Archimedes law take in account this because it depend of the shape, number molecules at sides.
  11. Oct 28, 2013 #10


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    Archimedes Law does not take into account the gravitational force of the water molecules on each other because those forces are insignificantly small.
  12. Oct 28, 2013 #11

    Simon Bridge

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    In buoyancy, gravitational attraction of everything for everything else is completely accounted for by the weight of the object and the pressure of the water.

    The weight of the object includes the gravitational force exerted between the object and the water.
    The attraction of the water to itself as well as to the Earth, the walls of the container etc, is what gives rise to the difference in water pressure with depth.

    The difference in water pressure with depth is what gives you the buoyancy force.

    Archemedes Law - as you find in text books, would need to be altered slightly to allow for this sort of thing but, as russ_watters points out, the correction is extremely tiny. You will never encounter a situation where it matters.

    Similarly - water molecules will have electrostatic forces on each other (which are much much bigger than the gravitational forces btw) and those don't have much effect either. Water is also not usually pure H2O - and the impurities may change the effect. This is a bigger effect as well, but the biggest effect the impurities will have is in changing the average density of the water so Archemedes law will still hold.
    Last edited: Oct 28, 2013
  13. Oct 28, 2013 #12
    Ok, I don't knew ! thanks !!

    For my last message, #9 step3, the weight at step 3 is P1 or P2 ? If it's P2 could you explain how water can do for attract solid in it ? Because, solid attract molecules of water but they are in contact with solid and the force is canceled, no ?

    I know the force I speak is very small but I want to understand how this works in reality
  14. Oct 28, 2013 #13


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    Sorry, I still find your messages mostly incoherent.
  15. Oct 28, 2013 #14

    Simon Bridge

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    That's OK I got it. Gh778 is struggling to express himself in English - a matter exacerbated by being unfamiliar with the terms. I think I got it...

    I think you need to be more careful with your words - they don't actually mean what you appear to think they do.

    Per you last message #9 - lets take them in order:
    The weight of the object is, by definition, the force of gravity on the object.
    That value is given by Newton's Law of Gravitation thus:

    $$P_1=\frac{GMm}{(R+A)^2}$$... where R is the mean radius of the Earth at sea level, and A is the height above sea level (the usual meaning of "altitude"). M is the mass of the Earth, and G is the gravitational constant. You can look up all those values and do the math yourself.

    ... if we use distance in km then G=6.67x10-17 km3kg-1s-2.

    ... ME=5.972x1024kg

    ... lets make the test mass an even 1kg

    ... R=6371km

    ... A=100,001km is very high indeed - it's about a third of the way to the Moon's orbit.

    You probably mean that r=R+A=100001km but what you said was "altitude" so I will take you at your word, so the center of mass of the object is r = R+A = 106372km away from the center of mass of the Earth.

    Is this object in free-fall by any chance? i.e. what is keeping it there?

    OK - you take a bit of the Earth (where else would the water come from?) and move it closer.
    The column is h=100km high and has a mass Mw (you really needed the other dimensions) so it's center of mass is (r-h/2)km away from the center of the Earth, which is 50km away from our object... and the Earth mass has decreased by ##M_w## and the calculation becomes:
    $$P_2=\frac{G(M-M_w)m}{(R+A)^2}+\frac{4GM_wm}{h^2} > P_1$$

    There is a 4 in the second term because there was an (h/2)2 in the denominator.
    Note - we could imagine the column of water is a cylinder with end-surfaces of 1km2 areas - so the overall volume of the water is Vw=100km3 ... which sounds like a lot, but the Earth has 1385920460km3 available :)

    OK. What I want you to do is calculate how much bigger P2 is from P1 as a percentage.

    When the solid is a short distance d below the surface of the water, it's altitude is now A-d, and there is a column of water height d above it. The water above it attracts the mass upwards - so now we have three terms in the calculation.

    $$P_3=\frac{G(M-M_w)m}{(R+A-d)^2}+\frac{h-d}{h}\frac{4GM_wm}{(h-d)^2}-\frac{d}{h}\frac{4GM_wm}{d^2}$$ ... again, you can crunch the numbers: this means that ##P_3<P_2##

    Those (h-d)/h and d/h terms are because the mass of water below the object is (h-d)Mw/h and the rest is above it.

    I suspect you want to try to apply these calculations to Archimedes next.
    For that you have to be a lot more careful with your setup - if the water and the solid object are both in free fall, that will affect your calculation. Archimedes is kinda assuming that the container for the water is sitting on the ground.

    Archimedes also kinda assumes that things are happening on a smaller scale.
    With very big volumes - water is not really un-compressible, for example.
    Gravity is no longer uniform - there are tidal effects - stuff like that.

    Lesson: Archimedes is good on the scale of bathtubs and crowns.
    For other stuff, you may just have to add up all the little contributions... which means calculus.
    Last edited: Oct 28, 2013
  16. Oct 29, 2013 #15
    Sorry, it's not 100001 km, it's 100 km +1 m = 100.001 km :( it is altitude, like that when I put a column of water of 100 km height the solid is just above.

    The diameter (or square section) of the column is only 1 m, this could change the center of mass of Earth ? If yes, for simplify the study, it's possible to add another column of water at opposite diameter ? like that this don't change the center of mass of Earth. It's easier for me to understand if Earth don't move.

    For me, and if I take your equations:

    1/ at 100 km +1 m the solid has P1 weight
    2/ I add 2 columns of water diametrically opposed on Earth, height of column = 100 km, the solid is outside water, the solid has weight P2
    3/ I move down object of 1 m it is full in water, now, what is the weight, it is P1 or P2 ?

    and thanks to take your time and try to explain even my english is bad :)
    Last edited: Oct 29, 2013
  17. Oct 29, 2013 #16

    Simon Bridge

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    I urge you to do the calculations yourself.
  18. Oct 29, 2013 #17
    I don't know, for me it's P1 because additional water don't attract solid when it is in water, I don't want the exact value, I just want to understand what's happen when solid is move down of 1 meter, the weight must be the same P2 (at 1 meter near), but like molecules of water touch walls of the solid for me molecules of water can't attract solid so the weight is only the weight from Earth like if column of water is not there.
    Last edited: Oct 29, 2013
  19. Oct 29, 2013 #18

    Simon Bridge

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    I told you - it's neither.
    Gravity works all the time - it does not magically switch off when you are under water.
  20. Oct 30, 2013 #19
    Ok, you accept these forces. So with a shape like that in water, on Earth, red object move alone at left ? Even there is viscosity the lateral force must be 0 (you can replace water by helium).

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  21. Oct 30, 2013 #20


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    No, it does not move. We've discussed this exact issue many times.
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