Has An Important Property Of Fluids Been Overlooked ?

[jim_mich]

The main determination in this situation is the condition of the bottom of the plinth. If the plinth is a solid walled container, then all discussion of hydraulic pressure is moot. It will act like a solid concrete or stone plinth. But if the plinth is flexible on the underside, then it, in effect, floats on top of the sand. And it become an hydraulic system. I would assume the top and side walls of the plinth are solid. But for it to 'float' there must be at least a small section of flexible side wall just above the sand. So, in effect, the plinth becomes a large diameter piston, with the post acting as a small diameter piston. When the post is pressed into the fluid, its pressure per square unit will be transmitted to every other square unit. This will cause the whole plinth to rise upward as the pressure of its bottom pushes downward.

I suggest an experiment. Find a shallow container, say an empty cat food tin can or tuna fish can, and put a hole in its bottom. Smooth any sharp edges. Put a balloon in the can with the neck of the balloon sticking through the bottom hole. Locate the hole off-center. Put the tin can, large opening side down on a table. Fill the balloon with water until the tin can begins to lift up from the table. Pinch the balloon closed, maybe with one of those squeeze type paper clamps. Now you can press different places on the bottom of the tin can to simulate different location of the post. And possibly you can even press against the balloon through the small hole.

I think you will find that the whole assembly tips, wobbles, wallows around on top of the balloon. And if you press into the balloon though the hole in the can (simulating the pressure of the post), then the whole tin can will rise upward.

[barksalot]

I think the plinths will behave the same as a solid one and tilt over time that being for the first example but have not thought much about the second one with the snorkel yet.

What would happen if you filled a air mattress with water and set it on some soft sand and sat down on one end. Are you saying it would leave a even depression all across it's length in the sand?

I know the air mattress has flexible container wall but even with a hard shell type walls it would still behave the same I think.

[rlortie]

Ralph .. thanks for the input but I tend to disagree - the fluid can't find it's own level because it can't move & the volume & density remains the same - pressure increase is always linear from what you started from.

"Fluid seeks its own level" a bad statement to make on my part. The self contained plinths will retain the same volume and density.

Since fluids are isotropic I can't yet see why there should be a tilting force on the bottom of the symmetrical plinth with load because forces are in equilibrium & spread evenly thru the fluid internally within the plinth.

"Symmetrical" is the key word here! I thought your question was asking for an answer regarding unsymmetrical loading, where I still believe your plinths will tilt and apply more pressure in the sand directly below the legs the fluid plinth is supporting. forces are not in equilibrium with earth although they are within the contained plinth, more so than a retaining wall filled with sand as per Jim's proposal.

Ralph

[eccentrically1]

10000 liters of water weighs over 22000 pounds. Plinths, solid or fluid filled, wouldn't make sense in this case. I'd build a simple table over the rocks. Make sure the posts drain well underneath.
The fluid idea would work if the whole silo and its contents floated in the plinths, similar to earthquake proof skyscraper design.

[Fletcher]

Ok & thanks ..

I created my own diversion to plinths ;7) - they were really an analogy to a hydraulic press - I think I can sum up by saying that the piston or what ever being in contact with the container is an issue & we'd like to see pistons etc that actually move to negate that.

See below for Pascal's barrel experiment - it is similar to text book experiments to prove that pressure is equal in all directions at different vertical depths & shows the Hydrostatic Paradox.

http://en.wikipedia.org/wiki/Pascal%27s_barrel

BTW .. IMO floating a battleship in a bathtub [Archimedes Paradox] i.e. less fluid fill volume than the displacement volume, is a variation on the Hydrostatic Paradox.

http://en.wikipedia.org/wiki/Archimedes_paradox

When I come back I'll show the work-around that allows the pistons [fluid filled bellows - membranes etc] to move independent of the structure providing a little f x d into the relationships.

[Fletcher]

I added & modified a concept from force pumps.

I've effectively introduced a symmetrical bell housing compression chamber - now the piston is free to move [albeit a very small distance] & in so doing the internal gas bags [or spring loaded small pistons etc] are compressed - this means that the main acting piston on the ascending side is free to move in relation to the fluid filled structure.


Here Is My Concept & Theory In Short:

1. That buoyant force is a result of pressure differentials of which Archimedes volume displacement & uniform fluid density is a sub-set [see Archimedes Paradox re Hydrostatic Paradox].

2. That floatation is a function of buoyancy where buoyant force is in equilibrium with mass weight force.

3. That a mass buoyed by a contained fluid will create pressure in said fluid - the pressure will be transmitted undiminished to all parts of the fluid [as per Pascal's Principle] - the pressure distribution is effectively a redirection of the mass weight force evenly thru the fluid medium even though the weight is the sum of the parts [hydrostatic paradox] - that the greater the mass the greater will be the pressure increase & the buoyancy force [synchronicity].

4. That the torque on the ascending side of such a symmetrical structure will be less than the descending side.

5. That only the vertical component of gravity force is displaced in this way & the horizontal component has a normal effect on the structure, so the proposed Virtual Displacement of Mass effect is maximum at the horizontal orientation & diminishes accordingly.

[Tarsier79]

There seems to be a lot of theorys popping up, such as this one, and Daan's theory that all relate in a way back to Chris's thread, "The Missing Word is 'Balance' ?". IE maintaining balance in the system, but finding a way to shift the mass, here theoretically, in Chris's case, actually.

I threw together an experiment to prove to myself what was possible with an airbag supporting a weight on an arm. Here are my findings.

OK, the diagrams below should be self explanitory, but I have added descriptions. Feel free to comment, or ask questions.

Cheers

Kaine


PS Sorry the pictures aren't in order. Please look at them from bottom to top for the best understanding.

[Trevor Lyn Whatford]

Hi kaine,

that is a bad example try Daxwc experiment and add a weight to the under side of the opposite side of the pan!

Regards Trevor

[Tarsier79]

It is only a bad example to you, if you do not see why it is a good example.

The bottom "lever" is an analogue for a wheel, the "Wheels" CCW Torque at 9:00 measured by the scales. The upper lever, supported by the airbag is merely testing if you can spread the weight over a portion of the wheel radius to virtually move its COM. The pressure inside the bag was increased, but since the airbag did not extend the entire distance from the weight to the axle, I would expect only a small movement of the COM, if this theory was correct. I saw no such movement....I do not feel it is a bad example. I wonder what Fletcher will think?


Also, the scales measure in 1 gram increments. So any variation would have been noticed.

[Trevor Lyn Whatford]

Hi Kaine,

just trying to be helpful in a positive way, Sorry!

Regards Trevor

Edit

that is a bad example try Daxwc experiment and add a weight to the under side of the opposite side of the pan!

[Tarsier79]

No need to be sorry Trevor. I am also defensive of my babies ;)

[daanopperman]

Fletcher
I apologise for contaminating your thread .

Tarsier 79,
Please look at fig1 , if you still have your testing rig , rig it up as in the drawing with the bag on top of the scale instead of between the bars , as the bag can now deform , it lifts the whole rig onto the scale whereas in between the bars it just pressurise the inside between the 2 bars .

[bobriddle]

daanopperman,
that is the basic idea I started working from a couple of years ago. It is highly suggested in Bessler's drawings.
Hope to route my board warping fixture this coming weekend.

[Fletcher]

Hi kaine,

that is a bad example try Daxwc experiment and add a weight to the under side of the opposite side of the pan!

Regards Trevor

Tarsier .. I think Trevor's suggestion from dax's experiment is closer to the mark regarding my proposal.

In the double levered drawing [your interpretation of Chris's ideas] with a compressed weight bag between you have proven one of the tenets of the hydrostatic paradox - that the weight force on the scales equals the summed masses of the components - that is what I would expect.

However, I am proposing that the applied mass increases the energy density of a fluid [in my case a liquid because I can increase PSI easily without adding compressed air for example] in the form of pressure - the critical difference to your experiment is that the 'container/structure' doesn't change shape & therefore the continuous distribution of fluid mass remains constant [density is the same] & there is no lowering of the system CoM [of individual parts] as per your drawing - this energy density is evenly distributed thru the liquid therefore there is a virtual displacement of mass for want of a better description at this time.

....................................

I would like to build up a picture of understanding of the fundamental principle with your help & others if they wish to contribute, as some have done already - in this way the hole in my logic may be found, perhaps sooner than later.

I'd like to start by asking you to do a small experiment with your scales - place a beaker on them almost filled with water - take a reading on the scales - now slowly push your finger into the liquid & then stop [don't touch the beaker] - what does the scales read ?

It should read higher by 10-20 gms or so - this experiment is to prove two things.

1. Newton's Law of for every Action there is an equal & opposite reaction.

2. That the weight reading on the scales is dependent on the internal pressure [force] on the bottom of the beaker - clearly your finger supported by you did not add any mass to the system to record a higher weight - so your finger displaced a volume of water which raised the water level & this created a higher pressure inside the beaker on the bottom surface.

N.B. number 2 goes towards proving that buoyancy force is a pressure differential.

If you find the same results is that how you would interpret the results ?

[daanopperman]

Hi Fletcher ,
This is my opinion ,

1) I don't think it has anything to do with Newtons first law .
2) You really don't look so skinny on the screen .