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Re: [Phys-l] buoyancy on a submerged pole



I think I can have it both ways. Toothpaste is a shear-thinning pseudoplastic material, as the note in the Wikipedia article that I mentioned the other day says. (http://en.wikipedia.org/wiki/Thixotropy) Once you get it flowing, it thins out. It can act as a sealant as long as the external pressure does not start it flowing. At that point, the water would push its way under the object.

Am I wrong in observing that toothpaste will both hold its shape on a surface, yet be slippery underfoot?

This still does not address the question of whether toothpaste is an adhesive, which will have to be determined by another experiment in a vacuum, or by seeing if a layer of toothpaste could support more weight than air pressure would allow for. But it would be so hard to get a good uniform thickness free of air pockets that the vacuum experiment might be required.

I'm not sure what you mean by this comment:

>>The pieces of the pole will either be pushed apart or pushed together, depending on whether the fluid can weasel its way between the pieces.

Assuming a solid pole not made of pieces, ... never mind - let me generalize.

My contention is that any object with vertical sides (pole, block) that is under water, with neither the object nor the water in motion, will be in vertical (axial) compression, because the only forces that can act on it will be perpendicular and inward to one of the surfaces, and its weight will put it in compression. Whether it is stuck to the bottom or floating, there is no mechanism for the water to pull the ends of the object apart. If the object is rising or falling, there are fluid flow interactions with the sides of the object, so I'm not sure if there might be situations where it would be in tension. (I'm thinking of how a stream of water from the sink breaks up as it accelerates downward, but I'm also thinking of how a raindrop assumes a spherical shape as it falls through the air.)

An interesting experiment would be to stack cylindrical segments of metal or wood (a segmented pole) and see what happens when they are allowed to sink or rise in water. Do they separate, stay together in compression, or stay together neutrally (no significant force between the segments)?


* /From/: John Denker <jsd@av8n.com <mailto:jsd%40av8n.com>>
* /Date/: Sun, 07 Nov 2010 05:53:09 -0700

------------------------------------------------------------------------

On 11/04/2010 04:22 PM, Scott Orshan wrote:
Toothpaste is not glue, and it's not particularly sticky.

On 11/05/2010 08:02 AM, Scott Orshan wrote:
I think you are seeing an air pressure effect. Assume a six inch
diameter funnel with a wall thickness of 1/16 inch. The area of the rim
that you are sealing up is over two square inches, which amounts to 30
pounds of air pressure - quite enough to support a plate.

On 11/05/2010 04:07 PM, Scott Orshan wrote:
I claim that in thin layers, the toothpaste exhibits tremendous
shear viscosity.

I disagree. Put some on the floor. It's like a banana peel - slippery.
You wouldn't want to walk down steps coated in toothpaste.

Anyway, that's a side issue. We're trying to figure out if it is an
adhesive or just a sealant. Someone needs to hang one thing from another
thing using toothpaste as the sealant, and put them in a vacuum bell to
see how much weight can be supported. I don't think it's much of an
adhesive.

You can't have it both ways.
-- Either the paste is a fluid and flows (under the conditions
of the experiment in question) (*) ...
-- or it isn't and doesn't.

If it flows, there is no "air pressure effect" and it is not "a
sealant".

This point is absolutely central to the physics of the issue we
have been discussing for the last three weeks. The pieces of the
pole will either be pushed apart or pushed together, depending
on whether the fluid can weasel its way between the pieces.

==================

*) Note that a banana peel is considered a solid by any reasonable
definition. It acts as a solid until you exceed its yield strength.
As such, it is not different in principle from almost any solid;
the main difference is its relatively low yield strength.

*) Even for things that are flowing, whether the flow is relevant
to the experiment depends on details such as viscosity, time-scale,
and length-scale. So this problem that started out as a simple
statics problem winds up having a fluid dynamics component. In
particular, imagine two optical flats with nothing between them
but a thin layer of water, and imagine pushing them together to
make the layer thinner. Calculate the force involved. It's
not a small number.

This addresses a point raised near the very beginning of this
discussion: If something is /stuck/ to the bottom of the ocean,
you'd better plan on raising it very /slowly/ at first, and/or
taking direct action to get it unstuck (such as injecting fluid
underneath it).