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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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