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

It occurs to me that the glue between the bottom of the box and the bottom of the aquarium might be treated as a viscous fluid. Even if the glue does seal the box to the aquarium so that no water can seep between them, as the aquarium is filled with water the pressure at the bottom of the aquarium rises, and this pressure might then be expected to squish the glue around the edges, in turn raising the internal pressure inside the thin layer of glue itself, and being communicated to the bottom of the box, despite our best efforts to prevent this from happening. If this were to occur, the box would effectively again have a buoyancy force.

Of course, if you insist on "ideal glue", like massless strings and perfectly rigid rods, .... :-)

Easy to do the experiment: just put a hook in the top of the box (or pole, as some have said) and tie it to a force sensor overhead, arranging things so there is some tension in the string (which is recorded by the force sensor). Now add water. If the glue is communicating a buoyancy force to the box/pole, the tension in the string should decrease.

KC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller
Sent: Wednesday, 03 November 2010 8:17 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] buoyancy on a submerged pole

I don't understand: are you saying that you believe the glue is in tension? I don't think that can be true. I think the glue would be in compression. Start with an empty aquarium with the box already glued to the bottom. Now add water. There is no point when you can convince me that the water begins pulling upward on the box.
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of Chuck Britton [cvbritton@embarqmail.com]
Sent: Wednesday, November 03, 2010 8:16 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] buoyancy on a submerged pole

So you are saying that you would NOT expect an empty box glued to the bottom to affect the flexion of the bottom?

I'm ready for the experimental results to prove me wrong.
(But the theoretical considerations are falling a bit short for me, personally.)


At 4:56 PM -0700 11/3/10, John Mallinckrodt wrote:
This is a great way to see that "the buoyant force" IS not, and isn't
even EQUAL TO, "the weight of the displaced fluid" in general.

Indeed, in this case there is no "buoyant force." The water clearly
exerts a downward force on the box. Assuming that the box is a
rectangular parallelepiped and that the hole in the bottom of the
aquarium fits the cross section of the box tightly enough not to leak,
but loosely enough not to bind, the box would accelerate downward, when
released from rest, with an initial acceleration that would be greater
than g by a factor of 1 + (rho g h/M) where rho is the density of the
water, h the distance from the top of the box to the surface of the
water, and M the mass of the box.

John Mallinckrodt
Cal Poly Pomona

On Nov 3, 2010, at 3:50 PM, curtis osterhoudt wrote:

If no water can seep under the box, then why not extend the
experiment to
drilling a hole in the glass bottom of the aquarium under the box
(or simply
cutting out a big rectangle of glass to the edge of the box). The
box is now
effectively part of the floor of the aquarium. What's the buoyant
force on the
box?

/**************************************
"The four points of the compass be logic, knowledge, wisdom and the unknown.
Some do bow in that final direction. Others advance upon it. To bow
before the
one is to lose sight of the three. I may submit to the unknown, but
never to the
unknowable." ~~Roger Zelazny, in "Lord of Light"
***************************************/




________________________________
From: chuck britton <cvbritton@mac.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>;
betwys1@sbcglobal.net
Sent: Wed, November 3, 2010 4:44:39 PM
Subject: Re: [Phys-l] buoyancy on a submerged pole

I propose an experiment to help me organize my thoughts.

An aquarium, filled to it's rim, has an empty box glued to it's bottom.
Enough SuperGlue to prevent any water from seeping under the box.

I would (naively) construct a Free Body Diagram for the box as
having two opposing forces. One (upward) labeled Buoyant Force and
the other
(downward) labeled Contact Force or Glue Force.

The bottom surface of the aquarium is experiencing an upward 'Normal
Force' that is N3 paired with the Glue Force of the boxes FBD. From
equilibrium we can say that the upward force on the bottom of the
aquarium is equal to the Buoyant Force acting on the box.

I'll wager that a careful measurement of the flexion of the glass
will verify that the upward force on the glass is dang close to the
weight of water displaced by the box.

What is wrong with this simplistic analysis of an everyday phenomena.

I readily admit that we're sweeping a LOT of complications under the
rug of simplicity.

What complications need to be retained for intro students.

At 3:36 PM -0400 11/3/10, bennett bennett wrote:

On 11/3/2010 9:33 AM, Chuck Britton wrote:
(an off-list exchange - posted without permission - hope
it's ok)

At 10:14 AM -0400 11/3/10, bennett bennett wrote:
The way I see it, the force of fluid pressure on solid is
normal to the surface at all points, so the anchored pole, (with
no water pushing on the bottom surface), is not lifted by the
water on its side, unless the water is viscous and moving upward.

If there is a notch, the pressure on the non-vertical surfaces
of the notch will stretch only the thin part of the pole, but
the up and down forces will be equal, since the vertical
components of the
>>>>> forces on the surfaces of the notch are equal.

And the way _*I*_ see it is that the top of the (totally)
submerged pole doesn't give a flying-flip what's going on at the
bottom of the pole.
The top of the pole 'wants' to float, and it WILL if given a chance.

How does the complicated contact force at the bottom change
what's going on with the rest of the object??
(Still scratching my head vigorously)


By preventing the water from contacting the bottom surface.

--
Clarence Bennett
Oakland University
Dept. of Physics, (retired)
111 Hannah
Rochester MI 48309
248 370 3418
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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