[time-nuts] FW: Pendulums & Atomic Clocks & Gravity

Bill Beam wbeam at gci.net
Tue May 29 10:08:52 EDT 2007


On Tue, 29 May 2007 22:27:42 +1200, Dr Bruce Griffiths wrote:

>Bill
>Bill Beam wrote:
>> On Tue, 29 May 2007 16:31:40 +1200, Dr Bruce Griffiths wrote:
>>
>>   
>>> Ulrich, Didier
>>>
>>> Talking about forces, gravitational fields etc makes no physical sense 
>>> if the observer's reference frame isn't specified.
>>> For an observer in/on a satellite orbiting about the Earth with their 
>>> reference frame fixed with respect to the satellite.
>>> There is no gravitational field, whatever methods chosen to measure a 
>>> gravitational field (within the satellite) will always produce a null 
>>> result.
>>>     
>>
>> Not true.
>> Very simple experiments will show occupants of the satellite that they
>> are in a non-inertial reference frame.  (Release a few test masses
>> about the cabin and you will observe that they move/accelerate for no
>> apparent reason, unless the satellite is in free fall which you'll know soon
>> enough,)  The experimenter must conclude that the satellite is undergoing
>> acceleration due to the influence of an attractive (gravitational) field.
>>
>> Just because NASA calls it 'microgravity' doesn't make it true.  It means
>> NASA is wrong.  Weightlessness is not the same as zero-g.
>>
>>   
>Only, if you insist on sticking to Newtonian physics with all its 
>attendant problems.

This discussion began as a classical problem.  The relativistic effects
are many orders of magnitude smaller than Newtonian (v/c=2.6e-5).
For example:  A test mass released on the Earth side of the satellite
cabin will advance in its own orbit a few mm/sec faster than one released
on the far side due to purely classical differences in orbits.  Easily observable
without even using a timepiece.

Once your feet leave the ground, not even Newtonian mechanics is
intuitive.  Who would have thought that 'putting on the brakes' to
leave orbit would cause a satellite to speed up....

>
>>> Pendulum clocks fail to work, given an initial push they will just 
>>> rotate around the pivot, provided the pivot suitably constrains the 
>>> motion of the pendulum (ie a shaft running in a set of ball or roller 
>>> bearings or similar and not a knife edge pivot).

Run the numbers - depends on how hard the push.
Consider sheeparding of material in Saturn rings by small moons.

>>>
>>> If, however the satellite acts as a rigid body and has a large enough 
>>> diameter then it would be possible for an observer on the satellite to 
>>> detect a gravitational field gradient.
>>>     
>>
>> Therefore, you must conclude that somewhere inside the satellite g is not zero.
>>
>>   
>A finite gradient doesn't imply that the field itself is nonzero, except 
>of course towards the extremeities of the satellite.

Of course it does.

If g=0 everywhere in the neighborhood of a  point then the gradient is zero.
Else, what is the meaning of gradient?

Grad not zero implies field not uniform implies not(field zero everywhere).

>>
>> Regards,
>> Bill Beam (PhD, physics 1966, past tenured Associate Professor of Physics)
>>
>>
>> Bill Beam
>> NL7F
>>
>>
>>   
>Bruce
>
>
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Bill Beam
NL7F





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