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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.)