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

["John Clement" <clement@hal-pc.org>]

Actually the result you claim is right.  But the reason for the effect is
not the buoyant force.  The high deflection with just water is due to the
fact that the box has to supply a pressure force.  But when an empty box is
glued to the bottom, the pressure on that area is now the pressure on the
top of the box which is lower.  This deflection result is the same if you
consider that a buoyant force is added to the water pressure force on the
area below the box.  The pressure on the aquarium bottom is decreased.

So one can come to this conclusion by either analysis.  Unfortunately the
stresses in the glue will be different according to which analysis is used.
Doing a free body diagram and considering the water pressure it can easily
be seen that the glue is under compression.

You have to remember that buoyancy is the net fluid pressure on an object.
But if there is no fluid under it, then the net fluid pressure is down and
not up.  From the point of view of an early student model of buoyancy, the
case of no fluid under the object should be ignored.  A similar example to
glue would be a square suction cup covering the bottom of the box which is
compressed so as to squeeze out all of the fluid.  This example shows that
there has to be a normal force interaction between the bottom of the box and
the aquarium, or a compressive force.  The interior of the suction cup can
only have a vacuum and a vacuum exerts no forces.

John M. Clement
Houston, TX

> I understand what you (and everybody) else is saying.
>
> I'm disagreeing and waiting to be proven wrong.
>
> I maintain that attaching the empty box to the bottom of the aquarium
> will produce a measurable upward force (and deflection) of the glass
> bottom. The deflection should be easy to observe.
>
> Actually it would (will?) be seen as reduction in the downward
> deflection of the bottom.
>
>
> At 12:16 AM +0000 11/4/10, Philip Keller wrote:
> > 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.
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