Re: [Phys-L] The Physics Teacher and relativistic mass

[Richard Tarara <rtarara@saintmarys.edu>]

The 'real world' situation that has always seemed to me to call for 
velocity dependent mass is the following.  You have an electron (or any 
charged particle) accelerator and the ability to measure the velocity 
(in the lab frame) of the accelerated electrons.  In order to assure a 
mono-energetic beam you bend the electrons through 90 degrees with a 
radius R (again measured in the lab frame) magnet of field B which you 
measure.  Knowing the charge of the particle then  m = qRB/v.  Do the 
experiment and B/v is not a constant but rather increases as v gets into 
the realm of relativistic velocities.  I know one approach here is to 
only talk about the momentum but with all the other parameters measured 
or known, the velocity dependent mass seems to have some use.  I have 
also heard some well known CERN types talk about relativistic mass in 
this way, so even today some working physicists seem to still use the 
concept.

rwt

On 7/27/2016 7:25 AM, John Denker wrote:
> I will grudgingly concede that in an ivory-tower classroom situation,
> it is possible to /select/ a set of problems for which the notion of
> velocity-dependent mass gets the right answer.  In this artificial
> situation the question of velocity-dependent mass versus invariant
> mass becomes a matter of opinion to some extent.  However, the larger
> point remains: If you want to integrate the notion of mass with the
> rest of physics, and with general relativity in particular, then the
> spacetime approach (including invariant mass) is the only reasonable
> option.
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--
Richard Tarara
Professor Emeritus
Saint Mary's College

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