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



Curtis's objection rests on a definition of buoyant force:
He sees the box has weight; he acknowledges that if the box is light and hollow, it may not so much press down on the (remaining ) base, as attempt to lift the base, as it would rise to the surface if not glued down,
with or without a glass bottom surface under it: its lift off would just be accompanied by a rush of
escaping water in the latter case.

Brian W

On 11/3/2010 6:38 PM, curtis osterhoudt wrote:
But *what* removed the buoyant force? I think there wasn't one to start with,
personally.


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________________________________
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 5:30:49 PM
Subject: Re: [Phys-l] buoyancy on a submerged pole

Excellent observation!

You've effectively removed the buoyant force - at the expense of
creating a much more complex surface. I'll wager that the flexion of
the remaining flat section will very closely mimic the previous
flexion. i.e. that the remaining bottom surface will still deviate
from that of an identical aquarium that hasn't been affected by the
box.

Internal forces and contact forces are seldom trivial.
I'll hearken back to the earlier 'paradox' of the milk bottle with
cream that I mentioned previously.


At 3:50 PM -0700 11/3/10, 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?

________________________________
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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