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Re: Earth's rotational speed



I looked at the site, and it seems to be fairly complete in terms of giving
experimental information, but I have a few objections to some of the use of
physics explanations. I know that this topic has been around before, but I
feel that it is important enough in terms of trying to teach good physics
that it bears repeating.

The explanation involving "centrifugal" force is very counterproductive to
trying to teach students to correctly apply Newton's laws. The fact that
this sort of explanation is used in all kinds of "popular" accounts does not
make this any easier. Students must first learn to analyze things in non
accelerating frames of reference. This means that they must learn to
associate forces with the objects that produce the forces. Gravity is
caused by the earth, but friction and normal forces are caused by contact
with a surface. Although in a microscopic sense there is no contact, in a
macroscopic sense this helps students identify when there are forces, and
what is producing them. Once students give up the idea of centrifugal force
and start analyzing situations according to this method, they will make
progress. When a student refers to any force, I always ask "what thing" is
producing it. I will not accept gravity, they must say the earth. Tension
is caused by strings... Similarly when they refer to a centrifugal force I
ask them to name the thing that produces it. Where is the centrifugal
producing the force????

Once students can handle analysis in inertial reference frames, it may be
possible to then go to accelerated frames, including rotating frames.
However, I would not do this with my physics classes. I am really satisfied
when most of my students can analyze simple situations and not throw in
fictitious normal forces at random directions.

A better explanation would be to appeal to the idea that the gravitational
acceleration toward the moon is greater for the water near the moon, less
for the earth as a whole and even less for the water on the far side. The
net result theoretically produces the 2 bulges. This explanation is also
more unified, as it does not depend on 2 different mechanisms. Yes, I know
that reality in the form of landmasses intrudes to produce all kinds of
weird tides.

While I know that others may disagree with me, most of them are not dealing
with more than 20% concrete thinkers. When students do not make the
connections between forces and what produces them they tend to think that
balls on the end of strings fly outward when released due to the centrifugal
force.

Whether or not the site contributes to good teaching of earth science I can
not say. I have not seen much research on that topic.

John M. Clement
Houston, TX


Check out http://www.ldeo.columbia.edu/users/jcm/Topic2/Topic2.html which
is the source for the following:

2.5 Length of Day Variations.

While it might seem fairly incredible the length of the day is
changing all
the time -- not by an amount that anyone would notice, but by amoints that
are easy to detect instrumentally. The changes are only a few milliseconds
(a millisecond is one thousandth of a second) but they are very easy to
measure. The record below shows that there are very regular variations and
also very irregular variations. All of the changes must in some way be
caused by the re-distribution of mass over the surface and with in the
Earth. Seasonal changes (curve d) are caused by the change in ice
volume and
relative amounts of water in the atmosphere from summer to winter. Longer
period changes and shorter period changes (curves c and e
respectively) are
more difficult to explain. Some are caused by the phase of the El Nino --
the Earth rotates differently in an El Nino year than in a La Nina year
because these phenomena change the distribution of wet and dry
parts of the
world. Even longer period changes must be caused by processes in the deep
Earth that move masses around such as subduction of large slabs of
lithosphere. The cause of many of the longer period signals is not well
understood.



----- Original Message -----
From: "E.C. Muehleisen" <docmule@HOTMAIL.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, December 31, 2001 3:15 PM
Subject: Earth's rotational speed


I have been aware that the earth's speed about the sun varies
as we travel
on our elliptical path. However, I have read that the earth's rotational
speed varies though the year. Would someone be so kind as to
explain this
change in omega and also explain the implications on L and the
conservation
of angular momentum.
Perhaps this will prompt a return to things of physics rather than
government.
Then a last barb on the much ballyhooed tax "rebate." A
quote:"Sometimes I
wonder if the world is being run by smart people putting us on or by
imbeciles who really mean it."

Too much of a good thing is wonderful!




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