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zero frame of rotation?

zero frame of rotation? - Physics Forum

zero frame of rotation? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 01-01-2005, 07:52 PM
Rocket Scientist
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Default zero frame of rotation?



I'm a physics graduate, but have not used my skills for 15 years, so I've
forgotton most of it.

Just some idle questions here if anyone has nothing better to do at this
hour...

I think it is generally recognised that gravity is not a force as such, but
is something that changes the zero acceleration frame (the accelleration
where no force is required to produce that acceleration aka newtons law
a=f/m). So for example in deep space the acceleration that requires no force
is zero (except for the forces/gravity from the rest of the universe), but
on the surface of the earth the acceleration that requires no force is
10m/s/s downwards, and an upwards force is required to produce an
acceleration of zero.

How does this work with a zero frame of rotation? This is the rate of
rotation where no centripetal force is required to keep the body from flying
apart.
In deep space the zero frame of rotation is zero rotation relative to the
distant stars, but what is it if you are in orbit around the earth? Is it
the same as deep space (characterised by the non-rotating object always
pointing to a distant star) or is it the same as the orbiting object (so
that when you're on the orbiting object, it will appear that it is not
rotating).
I think the answer is the former (take the example of the faucoults pendulum
(sic)) so that there is a true "absolute zero" frame of rotation.
I wonder whether there are any gravitational situations where this zero
frame is changed.


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  #2  
Old 01-02-2005, 01:22 AM
tadchem
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Default zero frame of rotation?


"Rocket Scientist" <[Only registered users see links. ]> wrote in message
news:cr6v1a$6mg$[Only registered users see links. ].pol.co.uk...

<snip>

flying
pendulum

The "zero frame" as you call it would be the condition in which the
apparatus exhibits zero angular momentum about any axis.

The moon keep one face pointed towards the earth. It revolves around the
earth in an average sidereal period (period relative to the so-called
'fixed' stars) of 27.32166 days. The earth is *not* the reference point for
an axis, however. The moon does not rotate relative to the earth, but is
does rotate relative to the stars, again with an average rotational period
of 27.32166 days. The axis of this rotation (the pseudo-axis of the angular
momentum) is perpendicular to the mean orbital plane, and coincides with the
north pole of the ecliptic.

Your instinct was correct, the distant stars will not 'see' any rotation of
the object of it is not rotating.


I cannot think of any, but I am not fluent in the peculiarities of General
Relativity. I am not sure how "frame dragging" would affect the apparent
angular momentum of a system. in the vicinity of a massive, rotating system.


Tom Davidson
Richmond, VA


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  #3  
Old 01-02-2005, 12:07 PM
Morituri-|-Max
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Default zero frame of rotation?

Rocket Scientist wrote:

Wow, what a waste of money.
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  #4  
Old 01-02-2005, 08:03 PM
Rocket Scientist
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Default zero frame of rotation?

> Wow, what a waste of money.

I agree. But when you're a student making choices of what to study, in the
UK at least, the set of subjects available to be studied is almost
completely different from the set of career opportunities that will be
available to you when you graduate. Also in the case of physics, the jobs
often pay so badly that it is not easily possible to pay for housing.

The solution is for UK higher education to focus on offering more direct
vocational courses, allowing a select (priviledged) few to study academic
subjects into higher education, and take on those kind of jobs.


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  #5  
Old 01-03-2005, 10:49 PM
Martin Hogbin
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Default zero frame of rotation?


"Rocket Scientist" <[Only registered users see links. ]> wrote in message news:cr6v1a$6mg$[Only registered users see links. ].pol.co.uk...

I believe that it is possible for a rapidly rotating mass
to change the local 'absolute zero' frame of rotation.
Try searching using Lense Thirring.

Martin Hogbin


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