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Re: [Phys-l] NYT article: Centrifugal force



John D said:
"As a more modern application of the same idea, if a certain star is
supposed to be straight overhead, and you aim your telescope "straight
up" neglecting the bulge in the earth's figure, you will miss your
target by about 1/3rd of a degree. That's 1.5 times the width of the
full moon. That seems like a lot to me."

But the moon subtends 0.5 degrees at the earth!
Please explain.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

On Jul 3, 2009, John Denker <jsd@av8n.com> wrote:

On 07/02/2009 08:11 PM, Anthony Lapinski wrote:
I always thought that the Earth bulges due to inertia, not forces.
Particles on the equator naturally want to go on a tangent. I tell my
students that every force must have a source!

When you go on the "Gravitron" (rotor) ride at the amusement park, you do
feel an outward force. It's simply the reaction force (you on wall) to the
centripetal force (wall on you). It's called a centrifugal force but it
acts on the wall, not you. Thus, it's not part of the free-body diagram
(on you).


That's all true relative to a non-rotating reference frame,
_and not otherwise_.

You are free to choose whatever reference frame you like, but
you should respect other folks' freedom to choose differently.

++ If you do your own analysis in a non-rotating frame, that's
fine.
++ If you tell your students that rotating frames are beyond
the scope of the course, that's within your prerogative.
-- If you tell them that rotating frames do not exist, that's
going way too far.
-- If you contradict Dr. Gordon (or anybody else) who has
analyzed the shape of the earth in terms of centrifugal fields,
that is going way too far.

The physics is simple: The centrifugal field exists in a
rotating frame and not otherwise.

The centrifugal field is as real as the gravitational field.

Do not equate "laboratory frame" with "nonrotating frame",
especially when discussing the earth's equatorial bulge. Most
laboratories I've seen partake of the earth's rotation.

On 07/02/2009 11:08 PM, M. Horton wrote:
"It makes the earth bulge."

Yes, it makes the earth bulge.

That is barely hardly somwhat slightly sort of true.

Actually that is significantly really in practice completely
and readily observably true.

Making such a
statement gives students a highly exaggerated picture of the truth.

In temperate latitudes, the direction "up/down" differs from the
direction "straight toward the center of the earth" by about 1/3rd
of a degree, due to the centrifugal contribution to the universally-
accepted practical notion of "up/down".

This is readily observable: If you neglect the bulge in the earth's
figure, your architects are going to have big problems, because
obelisks will be observably non-vertical and pond surfaces will be
observably non-horizontal.

This does not depend on fancy notions of general relativity; it just
depends on operational definitions of vertical and horizontal that
have been in use for thousands of years.

As a more modern application of the same idea, if a certain star is
supposed to be straight overhead, and you aim your telescope "straight
up" neglecting the bulge in the earth's figure, you will miss your
target by about 1/3rd of a degree. That's 1.5 times the width of the
full moon. That seems like a lot to me.

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