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

[Bill Nettles <bnettles@uu.edu>]

Do you have to consider the surface tension effects of the water and the glue? Yesterday I did a demonstration of lowering a cube of bronze into a beaker of water. The beaker was resting on a digital scale and the cube was suspended from a PASCO force sensor. As soon as the cube touched the water surface there was an increase in the downward force measured by the force sensor and a decrease in the digital scale reading. Continuing to lower the cube decreased the pull of the cube on the water to zero nonlinearly, then the linear (with height) change in the forces as the cube was lowered until the top of the cube reached the surface of the water, at which point the force gradient again because highly non-linear and a huge meniscus was produced as the cube top went lower than the water surface while remaining dry.  All this to ask: what type of surface tension effects are present at the bottom? I suspect they are real and important, but I don't have the equipment to measure them.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Chuck Britton
Sent: Wednesday, November 03, 2010 7:33 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] buoyancy on a submerged pole

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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