Buoyancy gedanken

wrote: in message news: snipped-for-privacy@p77g2000hsh.googlegroups.com...

^^^^^^^^^^^^^^^^^ It floats to a depth at which it displaces its own weight of water. Depending on its shape, that could be more or less than 1/8". If it is more, then it is resting on the piston--not floating. ^^^^^^^^^^^^^^^^^

^^^^^^^^^^^^^^^^^^^^ Visualize a piston shaped so it fits around the floating object nicely, It would then be possible for the object to float in a tiny amount of water, and still displace its own weight. ^^^^^^^^^^^^^^^^^^^^ Close the valve in the piston.

^^^^^^^^^^^^^^^^^^^^ No. The object still floats to a depth at which it displaces its own weight. ^^^^^^^^^^^^^^^^^^^^ Drain the water below the piston.

^^^^^^^^^^^^^^^^^^^^ No. Once the valve is closed, events below the piston have no effect on the water or what floats in it. The object still floats to a depth at which it displaces its own weight.

Why do you ask? Is this part of your high school physics homework?

snipped-for-privacy@yahoo.com wrote:>

Well, the object is resting on the piston at that point. The object has to displace an amount of water that has a weight equal to the object for it to float.

Not if the weight of the water above the piston is less than the weight of the object.

nope

nope

Pete

Dave wrote: Is this even possible?

Pete wrote: Not if the weight of the water above the piston is less than the weight of the object. ^^^^^^^^^^^^^^^^ Pete, do this gedanken experiment: There is enough water above the piston to float the object. A tiny diver goes down and starts adding clay to the top of the piston, and dipping out water, so the level does not change. He molds the clay so it almost touches the entire wetted surface of the object, with a very thin layer of water between the clay and the object. The object is still floating on the water--it does not know how deep the water is. The "displaced volume" is the volume of the floating object below the water's surface. So it IS possible for the object to float in very little water if conditions are just right.

(snip)

Neat thought experiment.

It seems that many are simply unable to get past the utter conviction (without proof) that an object must always displace it's weight in water to float. It must seem like heresy to them.

This may Make History.

Don , I apologise.!!! I have seen the errror of my ways and hope I can describe it so anybody else can see the truth.

Without too much formulas. When a body sinks in a constrained vessell it will sink till the imersed volume * the relative density of the liquid is equal to the mass of the inserted object. And since it is constrained the level rises but the imersed volume is what counts not the original volume of available liquid.

Again I am sorry for not understanding earlier.

-- John G

I think if the "thought experiment" concept from Einstein (didn't invent it, but it was his main method for exploring concepts), instead of the E=mc^2 concept, were popularized, the world would be a better place.

Dave

Yes of course. Thin disc of balsa wood would float in a very shallow pool of water. A steel washer would never float in water. but If the water in the pool weighs less than the object itself, it will not float even if it's balsa wood. If, in the above gedanken, the diver removed so much water that the remainder weighed less than the object, it wouldn't be floating either.

Pete

"John G" wrote: (clip) the imersed volume is what counts not the original volume of available liquid. ^^^^^^^^^^^^^^^^^^^ Click. The light is ON. ^^^^^^^^^^^^^^^^^^^ Again I am sorry for not understanding earlier. ^^^^^^^^^^^^^^^^^^^ Don't be sorry. Not understanding is no sin, if you continue to think about it.

There seems to be some disagreement here about what actually constitutes 'buoyancy' and 'floating'. I can dump a litre of oil on the floor, set a 4'X8' sheet a 1" steel on it, and slide it all over the place. You could say it's floating, but it sure ain't because of buoyancy. Same with driving a car at high speed across a deepish puddle, it will actually hydroplane, and be supported by the water, but it isn't buoyancy that's doing it.

Pete

Geez, don't apologize for considering another presentation! I've been told I was full of it many times, sometimes correctly.

Substituting gedunkin for gedanken, have a look at

"Pete Snell" wrote: (clip) dump a litre of oil on the

^^^^^^^^^^^^^^^^ You certainly could float a steel plate on a puddle of oil, and it wouldn't be buoyancy, since the oil is totally UNDER the plate, so no volume is being displaced. What's holding up the plate? A combination of viscosity and surface tension, which keeps the oil from flowing out the edges. But it's not going to happen with a liter of oil under a 4'x8' plate--that calculates out to about .001" film thickness. You will never find a floor and plate flat enough to achieve that theoretical result. Secondly, a 32 sq ft viscous liquid film .001 thick would offer quite a bit of resistance to sliding. Third, if you slid the plate very far, it would slide off of its oil film and get harder to push.

Maybe we can get Don Foreman to try it ;-).

That agrees with what I said in the first thread. The volume is that defined by the minscus of the liquid and the floating object. So if there is even a very thin layer of liquid sides and bottom it is "floating" and I suppose you could float a battle ship in a few gallons of watter if the container were the right shape. :-) ...lew...

Gosh, I'd love to -- but I'll bet Leo's floor is better. I think we should do it on Leo's floor! Tell ya what, though, I'd send another liter of oil so you'd have *two* liters!

Wouldn't need to be new oil, right?

Oh no, I did it again. I have a hell of a time with apostrophe's. I should know better, having once had to give a stand-up talk (in fifth grade) on the proper use of the apostrophe. Couldn't figure out why folks were grinning. I'd pronounced it "A (as in apple) pus trohf" with accent on the first syllabl'e. Perhaps I should stick with nickle-plated axels. Jeff Wisnia loves teasing me about that.

Question is, what is the quantity. Must that quantity be the same as the weight of the floated object? (Answer: not necessarily)

No extra charge for the chips? ;-)

In a thought experiment, you exclude neither the easy questions, nor the hard ones.

You form a habit of questioning every thing along the way, excluding nothing, and sometimes it leads to an insight. In this case, yes, it was a trivial question, but its more about the thought process.

And no, not homework- was just looking for an interesting way to examine the current 'buoyancy' vs. 'not buoyancy' question.

Dave

"Don Foreman" wrote: (clip) Question is, what is the quantity. Must that quantity be the same as the weight of the floated object? (Answer: not necessarily) ^^^^^^^^^^^^^^^^^^^^ Oh, yes. An object sinks until it displaces its (no a-pus-trofe) own weight of the supporting liquid. If it's denser than the liquid, it just keeps on sinking. And, of course, we already know it's (yes a-pus-trofe) the volume immersed, not the actual volume of the liquid.

Two experiments have shown cases where this is not true, and the results are consistent with theory -- though perhaps not with your theory.