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Re: phase vs group vel.



Or a canoe. Another misconception which is rampant in the physics teaching
community is the idea that the Kelvin wake left by a ferry (or a canoe or
a duck!) has an opening angle that is somehow related to its speed and the
"speed of waves on the surface of water" in the same way that the opening
angle of the shock wave due to a supersonic object (a bullet or an airplane
or a car!) is. The phenomena are utterly unrelated, but still problems in
physics books are based upon this erroneous idea. It is easily demonstrated
that the Kelvin wake angle is independent of the speed of a canoe.

It's a good thing that the Allied Military didn't know how wrong this was
when they plotted the motion of convoys during WWII. And NASA is STILL
laboring under this misconception by charting ships motion from satellite
images! (The ship is too small to see but the wake is clear and is SAID to
indicate the speed.)

(But maybe your Kelvin wake isn't the same as these ship's bow waves???)

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Chuck Britton Education is what is left when
britton@odie.ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936