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    Re: Coriolis and gyros
    From: John Huth
    Date: 2009 Aug 20, 09:54 -0400

    You have to be a bit careful in describing these.   The term "fictitious force" is intended to imply that it is an apparent force that arises in an accelerating frame of reference, whereas such a force does not exist in an inertial (i.e. non-accelerating) frame.   For this reason, I refer to this as the "Coriolis effect", as opposed to "Coriolis force", to avoid confusion.   

    It is certainly true that you can express the Coriolis effect as an effective force if you are in a rotating frame of reference.

    I would be careful in separating the frame dragging effect, which is a result of general relativity from the classic Coriolis effect.   Although they both result from the rotation of a mass, frame dragging only comes from general relativity and is a very small effect.

    I'm not sure about why you say that the Coriolis effect has a deep connection to magnetism. Torque causing precession is happens in many systems.   I don't think you're suggesting that magnetic fields are the result of non-inertial frames, or are you?



    On Thu, Aug 20, 2009 at 9:02 AM, <frankreed@historicalatlas.com> wrote:

    As Douglas Denny stated originally (later withdrawn), the Coriolis acceleration is most definitely responsible for the north-seeking capability of a gyro-compass (IF you work in coordinates rotating with the Earth --the natural coordinates for these problems). Just make sure you use the full three-dimensional vector cross-product when thinking about the strength of the Coriolis acceleration.

    The Coriolis acceleration is frame-dependent --it depends on the choice of a rotating frame of reference. It's intriguing to note that there is also an extremely tiny physical, non-frame-dependent, version of the Coriolis acceleration which is created by spinning masses. This is known as "frame-dragging" or "gravito-magnetism" and was measured (barely due to various problems with the ultra-sensitive gyroscopes) by a spacecraft known as "Gravity Probe B": http://einstein.stanford.edu/highlights/status1.html.

    There is a deep, non-accidental analogy between the Coriolis force and the common magnetic force. And just as a spinning current loop experiences a torque which causes it to precess in a constant magnetic field, a spinning "mass loop" (a.k.a. a gyroscope) experiences a torque which causes it to precess in a rotating frame of reference.

    Finally, for Gary, there is nothing "fictional" about the so-called "fictitious forces" in rotating frames of reference. They are every bit as real as the common acceleration of gravity, or, if you like, they are every bit as un-real. The complicated explanations for various phenomena which you have learned that avoid using the expression Coriolis are un-necessary, though they can serve as useful reminders that physics can be done in different frames of reference and the results have to be the same no matter what.

    -FER








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