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

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


On 7/12/06, Cliff Parker <> 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" <>
To: "Forum for Physics Educators" <>
Sent: Wednesday, July 12, 2006 7:58 AM
Subject: Re: [Phys-l] Curious toy (was: Bernoulli)

> Roger Haar wrote:
>> 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
>> of thought, I am worried that the Bernoulli effect is not the correct
>> 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
> for parcels of air provided they have _the same total mechanical
> 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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