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

["Robert Cohen" <Robert.Cohen@po-box.esu.edu>]

Scott Orshan wrote:

> I don't know. I can't get this to happen, using a variety of 
> pot and jar lids and metal bowls held upside down as a pot of 
> water is filled. Everything wants to pop up when I let go.

:)

> Are you sure that 
> the air didn't escape as you were filling it? You saw no 
> bubbles? 

Saw none but I pressed down prior to filling the container.  

> Were you pushing on the center of the lid as you 
> held it down, deforming it a little?

It is a plastic lid and a plastic container.  Yes, as you suspected, I
pressed down in the middle.  

Pressing down likely decreased the pressure inside when I let go.  That
wouldn't change the weight of the water displaced (if anything it would
decrease it), which some have argued is important.  Yes, that would
likely increase the difference in pressure on the top surface of the
lid, which others have argued is important (suction effect).

By the way, the plastic lid floats when filled with water, which tells
me that the plastic is less dense than water.

> Is there a soft plastic rim on the lid that might be forming 
> a seal against the bottom?

It is as soft as the rest of the lid.  Keep in mind that there is a
horizontal compression force and the water will eventually seap into the
space under the lid, at which point the lid will float to the top.  It
takes a while for that to happen, though, so I had plenty of time to
move the setup from the sink to the lab table and then take a picture.

> Is your classroom in free-fall? 

Oh, I forgot to mention that point. :)

> Can you 
> try holding the lid down from the edges when filling the container?

Sure.  It doesn't stay down. :)

If you are using a metal lid, perhaps you can try filling the container
with mercury instead of water?  Or maybe you can fill it with 30 feet of
water instead of just a couple of inches?  Or maybe try it in a room
that is in free fall?

Or maybe it just isn't possible...(insert eerie music)

John Clement wrote:

> I think a closeup of the lid would be very helpful.  The oh 
> so light air should have compressed and lid deformed or bowed 
> downward.  This would be more confirmation of compressive 
> interaction between the lid and the aquarium.  To see the 
> compression an object like a soda straw across the lid could 
> be used as a reference.

This requires more work than I am willing to devote to this.

> Nice experiment, but as I already said it is just a suction cup.

No argument from me there.

I think enough has already been said regarding the physics -- I was just
carrying out the experiment.  However, it is interesting to note that
the difference in pressure (between inside the lid and the water
pressure on top) should be about 1000 Pa (1% of atm), based on it being
about 10 cm below the surface of the water.  If I push down on the lid
first, I can increase the difference to X% of atm for each X% I decrease
the volume enclosed by the lid, assuming it springs back to its original
volume when I let go.  That seems to be an easier way of increasing the
pressure difference than evacuating the air under the lid or increasing
the depth of my container (or using a more dense fluid, like mercury).

Then again, maybe there is a force I haven't considered...(eerie music
again)

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University 
570.422.3428   rcohen@po-box.esu.edu    http://www.esu.edu/~bbq