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

[Hugh Haskell <haskellh@verizon.net>]

At 18:44 -0400 11/03/2010, chuck britton wrote:
> 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.
Chuck, it seems to me that you are missing something important here. 
If there is a buoyant force on something under water, it must come 
from the fact that there is a surface that faces downward where the 
pressure can exert its force upward. If the object is embedded in the 
bottom in such a way that there is no connection underneath the 
object with the water in which the rest of it is embedded, then the 
water can exert no upward pressure on the object. No upward pressure, 
no buoyant force. That's a complication that seems to me to require 
consideration.

Now that seems to me to be a testable proposition. I would suggest 
the following experiment. Take a plastic pole (plastic so it can be 
very smooth with no small areas where water can exert upward forces 
on it, and less dense than the water, so, under ordinary 
circumstances it would float)), and embed it in a water bath in such 
a way that the bottom of the pole is "dry" (i.e., has no contact with 
the water). Then affix a strain gauge to the pole, oriented 
vertically, so it can measure any vertical strain put on the pole by 
the water (obviously, the strain gauge needs to be installed before 
the pole is immersed in the water). I predict that the gauge will 
read zero or very near to it. Perhaps the pole could be slightly 
tapered upward to insure no possible surfaces where the water can 
exert an upward force.

What do you think?

Hugh
--

Hugh Haskell
mailto:hugh@ieer.org
mailto:haskellh@verizon.net

It isn't easy being green.

		--Kermit Lagrenouille