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the Kelvin Wake again



I've been abstaining from Phys-L recently, while I plough through
examination marking. I've just looked at the recent microwave flurry - an
interesting source of things to be investigated. However...

On Sunday afternoon, I was marking physics exams on the terrace next to
Duino Castle, overlooking the Gulf of Trieste. The terrace is some 75
metres above the sea below, so the view can be spectacular, and on Sunday
the sea was dotted with small craft of all kinds.

Looking up from my marking from time to time, as one does, I observed the
wakes left by some of these little boats, and was reminded of the thread,
which I've just checked in the phys-l archive, from January 1998, where I
learnt that I'd been propagating a misconception telling my students that
the v-shaped wake left by a boat had an angle which depended on the speed
of the boat, just like the shockwave cone of a supersonic aircraft. In this
thread I learnt that the angle is always about 39 degrees, and "is
determined by the principle that the group velocity of waves in deep water
is equal to one-half the phase velocity".

It was further emphasized that this "kelvin wake" *is* the v-shaped wave
that one sees.

Well, there were plenty of v-shaped wakes on view on Sunday afternoon. Let
me tell you that they came in all angles. Fast speedboats leave long narrow
wakes. Slow-moving craft, typically adorned with sunbathing bodies (la
bronzatura must go on, regardless of skin cancer risks) leave wide wakes. I
watched a long time, making sure to compare wakes at similar viewing
angles, since I was not looking down vertically at the sea. It was much
more interesting than marking.

I'll try to get some pictures. Of the wakes that is, not the bodies.

Mark



_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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