Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: smoke-ring paradox



On Tue, 23 Jul 2002, Bob Sciamanda wrote:

Yes, all the streamlines are circular in the CM frame. But in the lab
frame the CM has a forward momentum.

In the lab frame, if forward flow must equal rearward flow in order to
satisfy continuity, isn't the net momentum zero? Changing the frame would
change the observed momentum of the entire "atmosphere", it wouldn't
affect the momentum of the moving pattern.


Compare with a forward travelling, spinning frisbee (or football).

The density of those objects is far greater than the density of air.
Compare a stone wich moves through vacuum. Better to imagine a blob of
gelatine moving through a water tank, or a blob of aerogel moving through
air, and then ignore the tiny extra mass of the molecules in the gel
network.


In fact, this trick question haunted me for ages before I solved it by
observing a very crude animation I made. It originally arose during
fights about (gasp!) Newton vs. Bernoulli regarding airfoil lifting force.
If all streamlines are closed loops, how can an airfoil deflect a net
amount of air downwards? Yes, circulation can exist, however the net
downward transport of air must be zero. The "Bernoulli" camp concludes
that no air moves downwards on average, therefore an airfoil flys because
of pressure difference alone, not because of F=MA of downards-accelerated
air. Sounds like an air-tight argument, yet it contains an unnoticed
"leak."

:)


(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci