Re: [Paddlewise] Water Motion Within a Wave (was: How a Kayak Turns)

I've given some thought to the "particles in motion within a wave" issue and
have come to the conclusion that Dave is (mostly) right. In addition, the
two url's I gave do agree with each other but you have to think about it
carefully.

First, if you view the wave motion from left to right then the water
particles do move in a clockwise circle; not just up and down.

Now the quote:  "As a wave arrives it lifts water particles. These travel
forward,
then down and back so that each particle completes a circle. "

Now look at the animation here: (The third animation, "water waves" is
pertinent)
http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html

The trick here is to focus on the blue particles and, at the same time,
focus on the wave motion. It's apparent that the quote is correct but not
very well written. The animation is clearly right, however. The confusion
(for me ) is that I was imagining the particles staying within the wave.
Instead the particles travel from the trough in front of the advancing wave,
up the face to the crest, then forward within the wave and finally down the
back of the wave to the trough again... where they'll repeat, rinse and
dr... oh, wait.

So the quote which says that the wave lifts the water particles is
correct... they are lifted as the face of the wave travels forward. This
would have to be along the face of the wave as the animation shows. When the
particles are at the crest of the wave they are now within the wave and
following the motion of the wave they move forward. If the wave slows and
steepens at this point then the crest of the wave collapses because the
particles within it carry their own momentum forward and move off the wave
and become unsupported.

But in a swell (or non breaking or cresting wave) the particle would be at
the top of the wave moving forward (in the direction of motion of the wave).
At this point the waves move forward and out from under the particle (in a
non-breaking wave) and the particle descends along the back of the wave (the
trailing edge) and into the trough completing the clockwise circle.

This means, of course, that the animation and the textual description both
describe the same things. And Dave, I submit, is mostly right. Water
particles in the face of the wave are rising, not falling.


Craig Jungers
Royal City, WA



On 1/22/07, Dave Kruger <kdruger_at_pacifier.com> wrote:
>
> Well, this topic has generated more uncertainty than I had expected.  Bob
> Myers and I have a discussion going on, back channel, in which each of us
> is hedging on which of the diagrams we each submitted might be "correct."
> I'll confess I'm at sea, so to speak.
>
> However, I'm pretty certain of this:  Craig, watch that animation in your
> second link closely and you will see that the _motion_ of the blue dots
> does not correspond to what the _language_ in the first link says, if "As
> a
> wave arrives" is taken to mean "beginning with the trough," and "forward"
> is taken to mean "in the same direction as the wave is traveling."
>
> In fact, the animation in the second link shows the water moving _back_,
> UP
> the face (like you said originally), until the crest arrives;  then the
> water moves _forward_ and down. (I took out the ":" so the link will come
> up without editing)
>
> Craig wrote:
>
> > Here is a quick quote from http://www.mos.org/oceans/motion/wind.html
> >
> > 'As a wave arrives it lifts water particles. These travel forward,
> > then down and back so that each particle completes a circle. "
> >
> > Of course, there could be some confusion as to what constitutes the
> > "face" of a wave and what is the "back". For me, the face is the
> > frontside of a moving wave and the water particles are moving down
> > that face. Even though the water rises, the particles in the water on
> > the face of that wave are falling.
>
> > Counter-intuitive, I know.
>
> And, at odds with the animation in the second site [below], I believe.
>
> > An excellent animation of various wave motions can be found at:
> > http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
> > The animation of water waves (3rd animation down) clearly shows how
> > the particles of water behave in a wave.
>
> --
> Dave Kruger
> Astoria, OR
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