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Re: [Phys-L] the latest in science



On 01/03/2016 06:00 PM, Bernard Cleyet wrote:

Published on Feb 17, 2015
Bandar Al-Khaybari, a preacher with the Saudi Ministry of Islamic
Affairs in Al-Madina, claimed that the Earth is fixed and does not
revolve around itself, during a series of lectures held in the
Sharjah emirate, between January 28 and 31. The lectures were
posted on the Internet.

https://www.youtube.com/watch?v=-9Jp_XCvVto

1) That guy is an idiot ... not because he says the sun goes
around the earth, but because of everything /else/ he says
in that video clip.

2a) In fact there *is* a reference frame in which the sun goes
around the earth. It is a rotating reference frame. Anyone
who wishes to use this frame is welcome to do so ... keeping
in mind that others are free to choose differently.

The physics of rotating frames has been understood for more
than 180 years.

2b) Rotating frames are important. Everybody on this list --
and I do mean everybody -- routinely uses rotating frames.
The usual terrestrial lab frame is a rotating frame. If
you measure the acceleration of gravity in the lab frame,
using a pendulum or whatever, the result is different from
what you would get in a nearby nonrotating frame. It's a
small percentage difference, but huuuge compared to the
uncertainty of the measurement, even using high-school grade
equipment and procedures.

In the lab frame, the earth is stationary and the sun
goes around it once per day.

SUGGESTIONS:

++ In the high-school course, if you want to say that a
formal analysis of rotating frames is beyond the scope of
the course, that's OK with me.
++ I recommend going one step further, and saying that
the centrifugal field exists in the rotating frame and not
otherwise. That lets students see the boundary between
what they can handle and what they can't.
-- In contrast, it would *not* be OK to say that rotating
frames do not exist, or that the centrifugal field does
not exist.
++ It would be nice to point out that the lab frame is a
rotating frame. There is no need to analyze it formally
... just patch it up by fudging the magnitude and direction
of g, and leave it at that. The rotation can still be
detected, e.g. using a Foucault pendulum or a decent
gyroscope ... but the introductory class mostly refrains
from measuring things (other than g) that are sensitive
to the rotation.

Also beware that the web is oozing with grossly incorrect
discussions of rotating frames. Qualitatively as well as
quantitatively incorrect.

A reasonably competent discussion can be found at
https://www.av8n.com/physics/rotating-frame.htm