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

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue May 29 00:31:40 EDT 2007

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 
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).

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.
If the satellite is large enough and orbits close enough to the Earth, 
this gravitational field gradient would tear the satellite apart.

For an observer located on the Earth however the motion of the satellite 
can be accurately described by Newtonian mechanics where the centripetal 
pull of gravity acts on the satellite causing it to have a centripetal 
(radial) acceleration as it orbits the Earth.


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