[Phys-L] Re: fluid

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

> Skip, the force of gravity on the water enclosed by the straw makes the
> water level in the straw to move slightly downward as the straw is lifted
> thus creating a pressure less than atmospheric pressure inside the straw.
> The pressure difference between the outside of straw and the inside
> pressure where your finger is keeps the finger "attached" to the straw. If
> the force of gravity on the water in the straw becomes greater than the
> pressure difference times the area where the finger and straw meet, then
> the straw should drop off.

I don't think this answers Skip's question.

Furthermore, in my experience (which includes a few moments ago when
I checked again to see if I was misremembering) the answer is much
simpler: The force that keeps the straw up is due to friction between
it and two fingers that are diametrically applied to the outside of
the straw just below its top.  Remove those fingers and the straw
falls.  The only way I can imagine that not happening is if one jams
the straw into the surface of the finger on top hard enough that it
is, at least temporarily, captured by the circular crease created in
the process.

Am I wrong?  Is it the understanding of others that the straw is
somehow supported against falling by its interaction with the water
or the surrounding air?

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>
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