Re: [Phys-L] defining energy

[Bruce Sherwood <Bruce_Sherwood@ncsu.edu>]

Perhaps Antti's question wasn't directly addressed. As you say, "mass is
defined in the rest frame", that is, the frame in which the total momentum
of all the particles is zero. I didn't say it explicitly, but what I
intended was to compare a non-rotating wheel in its rest frame with a
rotating wheel in its rest frame -- if the axis isn't moving the total
momentum of the rotating wheel is zero. Necessarily, this rotating wheel
has greater mass than the non-rotating wheel, by an amount 0.5*I*omega^2.

Bruce


On Wed, Oct 30, 2013 at 2:12 AM, Savinainen Antti <
antti.savinainen@kuopio.fi> wrote:

> Hi,
>
> Bruce Sherwood wrote:
>
> "A rotating wheel has slightly more rest mass than a stationary wheel, but
> it is usually convenient to call this extra energy "rotational kinetic
> energy" and calculate it from 0.5I*omega^2."
>
> Could you elaborate a bit? I have thought that kinetic energy does *not*
> add to the rest energy in the relativistic perspective as the rest mass is
> defined in the rest frame. Rotational energy can be viewed as the sum of
> kinetic energies of "small" parts of the wheel. Hence, it would not add to
> the rest mass...or just mass as the qualifier "rest" is unnecessary as
> eplained by Taylor & Wheeler in their splendid book Spacetime physics
> (1992).
>
> Or perhaps I am missing something?
>
> Regards,
>
> Antti
> --
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