Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Centrifugal redux



On 03/20/2009 08:09 AM, Rauber, Joel wrote:

I'd add the proviso that you may choose whatever frame you like to do
calculations; however, you may not choose whatever frame you like as
your rest-frame. Or perhaps more precisely stated, you may not
choose whatever frame you like for a rigid objects Center-of-Mass
rest frame. That is uniquely determined.

No, it is not uniquely determined, as we can see from the following:
Today a certain object is at rest in the frame of the space station.
Tomorrow the same object is at rest in some terrestrial frame.

These are not equivalent. Assuming we arrange to have them coincide
as to position, they will have wildly different velocity and wildly
different acceleration.

The concept of "rest frame" is not particularly profound and the
term is not particularly clear. Usually it is better to say "lab
frame" if you mean lab frame, or "comoving frame" if you mean
comoving frame.

A particle is always at rest relative to its own comoving frame.
Things like _proper time_ are defined in the comoving frame.

Being at rest in its own frame does *not* mean that the particle is
unaccelerated! Really, really not. See
http://www.av8n.com/physics/spacetime-acceleration.htm
especially the proper acceleration
http://www.av8n.com/physics/spacetime-acceleration.htm#eq-moe-a