[Phys-L] Re: Buoyancy question

[anngeorg at PACBELL.NET (Bernard Cleyet)]

This reminds me of a very famous teaching Dr.'s instructing a class on
the very crude method of diagnosing diabetes, i.e. tasting the urine for
sugar.  He had a sample in which he dipped a finger, licked a different
finger, and handed the phial to the nearest student.

bc

p.s. Another famous physician (BBC TV series about him) did it by
observing the white dust on the patient's shoes, obviously doesn't work
for women except those from Santa Cruz.

p.p.s.  I suspect a very clean needle also won't float; handling it
ensures it's oily.  [Are metals generally hydrophobic(philic)?]

Scott Goelzer wrote:

> If you are feeling evil, surreptitiously coat your forefinger with soap
> before the doing the demo in a large shallow pan (cookie sheet). Reach
> in and retrieve the needle with the soapy finger. Let the class try and
> duplicate the demo...
>
> Sooner or later someone will want you to float the needle again. A
> teachable moment!
>
> Scott
>
>
>
> *******************************************
> Scott Goelzer
> Physics Teacher
> Coe-Brown Northwood Academy
> Northwood NH 03261
> s.goelzer@comcast.net
> *******************************************
>
> On Dec 8, 2004, at 3:41 PM, John M Clement wrote:
>
>
>
> > If you gently place an ordinary needle sideways on the surface of the
> > water it will float.  I did this back in elementary school as I
> > recall.  I think you have to lower it with a fork, and be extremely
> > gentle.  The surface tension will hold it up.  But if you put a drop
> > of detergent into the water, it sinks.  This is obviously not a
> > buoyancy effect, as it will sink if you just drop it in or put it in
> > point first.  It is a fun little amazing experiment.
> >
> > John M. Clement
> > Houston, TX
> >
> >
> >
> >
> > > I'm having trouble understanding Clement's response.
> > >
> > > On Tue, 07 Dec 2004 23:30:53 -0600 John Clement <clement@HAL-PC.ORG>
> > > writes:
> > >
> > >
> > > > I believe he is referring to the fact that a needle if gently placed
> > > > on the surface floats.  This is a surface tension effect and not a
> > > buoyancy
> > >
> > >
> > > > effect. Of course if you add a drop of detergent it doesn't work.
> > > >
> > > > John M. Clement
> > > > Houston, TX
> > > >
> > > >
> > > >
> > > > > Greetings everyone!
> > > > >
> > > > > I have a question pertaining to buoyancy phenomenon:
> > > > >
> > > > > What's the condition that a piece of iron released gently on the
> > > > surface of water sink? Ignore surface tension.
> > > >
> > > >
> > > > > Thanks,
> > > > >
> > > > > Hasan Fakhruddin
> > > > > Instructor of Physics
> > > > > The Indiana Academy for Science, Mathematics, and Humanities
> > > > > BSU
> > > > > Muncie, IN 47306
> > > > > E-mail: hfakhrud@bsu.edu
> > > Herb Gottlieb from New York City
> > > A friendly  place to live and visit