Re: [Phys-L] Another Fluid/Density 'Problem'

["Anthony Lapinski" <Anthony_Lapinski@pds.org>]

Great, tough question! Actual "modern" as I saw it in a book in recent
years. I forget the name, but I think I have it somewhere at school. If
anyone wants to know, I can find out. The answer has to do with the forces
coming from the slanted bottle neck.

In my opinion, this is very difficult for kids. Best as a demo or bonus
question. Remember that kids don't think like us. Most never will unless
they become teachers. And even then, it takes years to fully grasp physics
concepts. We must appropriately challenge students, and this comes with
experience.

Phys-L@Phys-L.org writes:
> I’m not sure my students would have accepted this as an explanation of
> how there can now be greater fluid force acting on the bottom of the
> bottle.
>
> Where did the extra force ‘come from’?
>
>
> On Jan 29, 2014, at 8:41 PM, Brian Whatcott <betwys1@sbcglobal.net> wrote:
>
> > Pressure is proportional to head in a homogeneous liquid, where
> horizontal extent is immaterial.
> > Pressure changes where the less dense component occupies a greater
> proportion of the head because it is  confined to a  reduced horizontal
> extent. Which is to say - horizontal extents of components of differing
> density are material!
> > Brian Whatcott    Altus OK
> >
> > On 1/29/2014 4:51 PM, Chuck Britton wrote:
> > >    |  |
> > >    |  |
> > >   /    \
> > >  /      \
> > > |        |
> > > |        |
> > > |        |
> > > |        |
> > > |        |
> > > |        |
> > > |        |
> > > |________|
> > >
> > > An Old Fashioned Glass Milk Bottle with a narrow neck is delivered to
> your doorstep in the morning and because it is no(t) homogenized, the
> Cream has risen up into the narrow(er) region.
> > > The fluid pressure exerted on the bottom of the bottle is (rho) g h.
> > > Where (rho) is the average density of the column of milk/cream that
> extends from the top surface to the bottom.
> > > Now we will thoroughly MIX the bottle of milk and we note that the
> average density of this same column of milk is GREATER than (rho).
> Let’s call this new density (RHO).
> > > How can the pressure have increased while the area of the bottom has
> stayed constant?
> > > How can we accept a greater amount of downward fluid force caused by
> ‘just’ mixing the milk?
> > > (This is reprised from back in the last century.)
> > > Probably presented better back then.
> > > ;-)
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