Re: Tides and tidal bulge

[David Dockstader <DRDOCK00@UKCC.UKY.EDU>]

On Sun, 30 Jun 1996 19:00:57 -0600 Jim Green said:
> Margaret, I don't know of an undergraduate book that gives a reasonably good
> description of the ocean tides.  All talk about Newton's explanation for two
> "tidal bulges"  None that I know of approach the tides as complicated damped
> driven harmonic waves.  Further there are only Ari and me who know the
> truth. (:-)
Margaret, like Jim I don't have a book to suggest, but if you really do want
to see how all this stuff Jim talks about comes together the article I
mentioned in a previous post:  "Modeling Tidal power" by David A Greenberg
Scientific American, November 1987, vol 257, No 5 does a good job and is
readable.

As far as teaching goes:  I assume that when Jim tells us that he and Arie
have the truth he's having a little fun with us.  But isn't this part of the
problem.  Students think we're going to tell them the truth, the whole truth,
and nothing but the truth and we let them think this.  We may even on occasion
encourage them to think this.  I think what we need to be telling students is
that no one will ever know the whole truth and even a lot of the partial truths
that we think we know may turn out at best to be only approximations and at
worst may be totally false.  However, people have been observing the earth for
a long time and over the years a number of models have been developed that
that have turned out to be useful.  Then we tell them we will fill them in on
some of the more interesting observations, show them how models were developed
from these observations, how subsequent observations led to revisions or
refinements of these observations, and how some of these models can be used.
Then we can tell them that if they keep their eyes open and their heads on
straight they too may be able to contribute to this body of knowledge.

As far as how to teach tides, I think Arie is on the right track.
Let them imagine a rigid sphere the size of the earth, throw on some water and
add a mass the size of the moon.  This is a good problem in its own right.
Once they catch on to the buldge on the back side they can understand
why comets break up when they get too close to a planet and why no planets
have moons in a close orbit, etc.  How far to go with this should be up to you,
but I like to spin my model earth.  Now the buldges are ahead of the moon.
This makes the earth spin slower and the moon orbit faster.  Now since our
students haven't talked to Jim and don't know that the buldges are a myth, they
might wonder if days are really longer than they used to be.  Contrary to my
personal experiences which suggest that they are shorter, geologists seem
pretty convinced that they have evidence that the earth really has had more
days in the year.  (sorry, I can't remember the evidence)  What else do we
do with the model.  Add the sun and see what it does.  Add Jupiter and see what
it does.  Mention that according to Jim folks with a big computer have
considered the effects of 63 bodies.
               Look at the effects of planets on the force equation and see if
Iben Browning was justified in predicting earthquakes in 1990 or if it was all
just a scam.  See what we can do to make a more realistic earth.  Put in all
the stuff Jim and Arie mention.  Then do we have the truth?  No, just a more
refined model.  When do you quit?  Whenever you want.  You'll never get it
perfect anyway so quit when you  or the students are tired of it or whenever
you'd rather do something else.