The skater has another advantage - she can pull her arms in and see her rotation rate change relative to her surroundings.
You can also test your absolute rotation even if you cannot see the surroundings. I figure skate and can easily feel the difference when I pull my arms in when I am not spinning and when I pull them in after initiating a rotation. It requires deliberate force to bring them in if there is an initial rotation. It is obvious that I am supplying the energy for the faster rotation.
Bob at PC
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of John Denker [jsd@av8n.com]
Sent: Friday, December 02, 2011 6:18 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Coriolis effect puzzlement
snip------------------------
Consider the subsystem consisting of the skater (including
the weights). This subsystem is closed with respect to
momentum. No rocket thrust crossing the boundary. No
applied magnetic fields or anything like that.
Therefore the dL/dt of this system is independent of what
we choose as the axis for defining L.
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Here's another way of looking at it:
Consider the skater's view of the world, including her
view of the spectators, and her view of the fixed stars,
and her view of the gyroscope that hangs from her
necklace. In the initial situation, where she partakes
of the platform motion, it is totally obvious to her
that she is spinning.
If the platform is bowl-shaped in just the right way
("hydrostatic equilibrium") she won't be aware of the
centrifugal force, in which case she won't know or care
where the pivot-point is ... but she will *always* know
that she is spinning. She is spinning relative to any
inertial frame.