Re: [Phys-l] Curious toy

[Roger Haar <haar@physics.arizona.edu>]

Hi,
	The first web site on which I saw  a picture of this toy and two 
sentences describing it, used a flat-topped dowel, but had a link site 
in my original post.  The quick and dirty test unit which I made, also 
had a flat topped dowel.  The dowel pops out just fine.  The only thing 
that I have thought of, but have not tried is getting a tighter fit 
between the dowel and the hole.

Thanks,
 Roger

John Denker wrote:
> 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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