Re: phase vs group vel.

[Leigh Palmer <palmer@sfu.ca>]

> The question:
>
> > > Can anyone suggest a good source for a computer demo which depicts the
> > > difference between group velocity and phase velocity?
>
> To which Leigh responded:
>
> > Rather than a computer demo, have you a smooth-surfaced pond somewhere
> > into which you might drop some rocks? Lacking that, you might just as
> > well write a factoid on the board as depict the difference. They've
> > seen utterly unphysical things depicted by computers on TV; why do you
> > think they will put another depiction into a different class?
>
> If all else fails (absence of ponds, for example) consider a film or video
> of a boat moving on smooth water.  Better still, take them for a ride on a
> ferry so they can see for themselves the group and phase velocities of the
> wake.

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.