Re: [Phys-l] Curious toy (was: Bernoulli)

["Matt Coia" <matt.coia@gmail.com>]

I'm just going to build one when I get a chance.  If it works then I'll
consider making more for others.

Matt


On 7/12/06, Cliff Parker <cparker@charter.net> wrote:
> Bernoulli or not I want one of those.  I have Googled with no
> luck.  Anyone
> know where I can get one of these toys?
>
> Cliff Parker
>
>
> ----- Original Message -----
> From: "John Denker" <jsd@av8n.com>
> To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
> Sent: Wednesday, July 12, 2006 7:58 AM
> Subject: Re: [Phys-l] Curious toy (was: Bernoulli)
>
>
> > Roger Haar wrote:
> >
> >> http://www.grand-illusions.com/toycollection/raketti/
> >
> >> The top of the dowel is flush with the top face of the block.
> >
> > In the picture, it's not flush.
> >
> > Also note that it's far from being a generic chunk of dowel;
> > the top has a distinctive cone shape.
> >
> >>      The claim is that this is an example of the Bernoulli effect.
> >> Initially I thought this might make an interesting demo, but after a
> bit
> >> of thought, I am worried that the Bernoulli effect is not the correct
> or
> >> at the total explanation.
> >
> > Such skepticism is very appropriate.  Also the experiments RH did are
> > very appropriate.
> >
> > Any demo involving blowing a jet of air (from your lips, or from a tube)
> > is probably *never* a demonstration of Bernoulli's principle.  The
> > principle allows us to describe the pressure-versus-velocity
> relationship
> > for parcels of air provided they have _the same total mechanical
> energy_.
> > This proviso is emphatically not valid for a jet of air.
> >
> > I hypothesize that the toy involves some funny entrainment effects,
> > possibly
> > the Coanda effect (by which I mean curvature-enhanced turbulent mixing).
> >
> > To test such hypotheses, try different-shaped dowels.  For starters, I
> > predict that if the top of the dowel is flat and flush with the top of
> > the block, it will be verrry difficult (probably impossible) to get it
> > out by blowing across the top.  (This would make a fine demonstration
> > unto itself, by way of counterexample, debunking a common misconception
> > about the Bernoulli effect.)
> >
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