Darren New <dne### [at] sanrrcom> wrote:
> Invisible wrote:
> > Hmm. But is your planet rotating? Or is your planet actually stationary 
> > and the universe is rotating around it? ;-)

> The planet is rotating.
> http://en.wikipedia.org/wiki/Foucault_pendulum

  Some people actually have misconceptions related to this, or more precisely,
related to inertial coordinate systems. Some people think that basically any
coordinate system, even a rotating one, is inertial, and can be used as the
basis of any inertial calculations.

  Of course this is not so. A coordinate system which is not intertial is
an accelerating coordinate system, and a rotating coordinate system (such
as one fixed on Earth) is accelerating. The difference to a truely inertial
coordinate system is that this acceleration can be measured from within
the coordinate system itself. The focault pendulum is one example of how
this can be measured.

  These people often also have the misconception that it's not possible
to say if you are in a truely inertial coordinate system or not. But of
course this is not so, as it can be trivially measured: If there is no
any kind of acceleration you are in a truely inertial system.

  (If I'm not mistaken, the idea in general relativity that gravitation
is inertial movement and not an accelerating force, is related to this.)

-- 
                                                          - Warp

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