Re: phase vs group vel.

[rueckner@fas.harvard.edu (Wolfgang Rueckner)]

> 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.

Actually, you can also demonstrate this in a ripple tank.  Float a wooden
ruler (or any rectangular piece of wood) in the tank and give it a downward
tap with your finger.  Do this a few times to get a good sense of what's
going on, otherwise the student just sees the water being disturbed.
Wolfgang Rueckner