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



By contrast in 30 years of work in nuclear and particle physics, I've never
once heard or known of a working physicist using the concept of
relativistic mass. I'm not sure what the use of the concept would be even
in the situation you're talking about. If you want to ensure a
mono-energetic beam of electrons you might possibly bend them through 90
degrees in a dipole magnet, but then what you're doing if you select the
radius R is you are selecting a particular momentum. (and hence energy,
assuming a pure electron beam). I really don't see where the use of
relativistic mass is helpful or adds anything at all to that particular
practical problem. B/v isn't a constant but that's because p is not equal
to mv, but gamma * m * v.

Anyway, obviously some won't be convinced that mass should simply refer to
the amount of matter, or in other words the mass of an object in its own
reference frame, and are going to wish to retain the concept of
relativistic mass despite its shortcomings and the concern that conceptual
errors associated with relativistic mass do negatively impact student
learning.

TKP

On Wed, Jul 27, 2016 at 7:55 AM, Richard Tarara <rtarara@saintmarys.edu>
wrote:

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

free Physics educational software
http://sites.saintmarys.edu/~rtarara/software.html


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Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




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Todd K. Pedlar
Associate Professor of Physics
Luther College, Decorah, IA
pedlto01@luther.edu