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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
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_______________________________________________
Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l