Re: [Phys-l] Teaching Special Relativity

["Richard Tarara" <rbtarara@sprynet.com>]

----- Original Message ----- 
From: "John Mallinckrodt" <ajm@csupomona.edu>
> The same cannot be said for "relativistic mass increase," which has
> no observational basis.  What we observe is that objects moving at
> high speed have more momentum than that predicted by Newtonian
> mechanics.  There are two possible responses to this fact: 1) let the
> mass of an object increase with velocity or 2) alter the dependence
> of momentum on mass and velocity.
>
> In either case the correction is exceedingly simple and noticeable
> only at high velocities.  The first response, however, leaves one
> with myriad difficult if not unresolvable questions:  How does one
> actually measure the mass of a rapidly moving object?  Where does the
> extra "stuff" come from?  What about gravitational effects?
>
> The second response sidesteps all of those problems.  Why on Earth
> would anyone not choose it?
>
> John Mallinckrodt
> Cal Poly Pomona


But who chooses it when momentum is introduced?  I can't think of any text
that I've seen that doesn't define momentum as mv.  This is what the intro
student (indeed myself) comes to special relativity with.  The magnetic
field example (again I must say that I've calculated and set the fields for
bending high speed protons using 'relativistic mass' and been quite
successful in getting the beam to the target) presents us with a measured
velocity but a momentum that has increased non-linearly with that velocity.
Again, I'm not saying that we should teach relativistic mass to physics
majors, but really, how are our gen-ed students, with the hard won idea that
momentum is mass x velocity (actually work to convince them that they
instinctually have a great feel for this) supposed to deal with the
observations.  I'm quite willing to use some 'weasel' words--the result is
as though the mass has increased--but that is, in my mind, the quickest and
easiest way to present the effect of very high velocities in light of the
student's knowledge, background, and the level of detail (or lack thereof)
that is possible within the framework of the course and the students.

Rick