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Re: ROTATION

On Thu, 7 Sep 2000, David Abineri wrote:

If a long thin rod is rotating about one end is it also rotating about
its center? If so, what is the relationship between its angular
velocity about its end and its rotational velocity about its center?

Others have commented on the equivalency. I'd like to point out that the
equivalency is not obvious to most students. Indeed, it may be biggest
conceptual block students have to understanding why air flows
counter-clockwise around a region of low pressure in the Northern
hemisphere. Let me explain...

Most students can understand that stationary air (stationary to the
earth's surface) at the north pole is actually rotating around the earth's
axis along with the earth (counter-clockwise looking down at the north
pole). Consequently, most students can understand that if the air is
forced toward the north pole (e.g., in response to low pressure there),
the air will conserve its angular momentum much like a skater and start
spinning more rapidly counter-clockwise (i.e., faster than the rotation of
the earth). The resulting wind pattern is then counter-clockwise
(relative to the earth) around the center of the low pressure.

Locations off axis (at, say, 40 N) are also rotating and thus experience
the same phenomenon, but students have difficulty seeing the rotation as a
rotation about an axis other than the axis going through the earth's
poles. Consequently, they cannot conceptualize the conservation of
angular momentum around a low pressure center that is not at the north
pole (or south pole).

A little off topic, perhaps, but I can't resist an opportunity to mention
a topic dear to my heart...

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
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