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Re: [Phys-l] Invariant mass and relativist mass...




----- Original Message ----- From: "John Denker" <jsd@av8n.com>

Last but not least, there is an Occam's Razor argument.
-- Introducing relativistic mass, plus time-dilated clocks, plus
FitzGerald-Lorentz contracted rulers is not a complete description
of relativity. You also need to say something about the breakdown
of simultaneity at a distance. And then you need to worry about
transverse mass versus longitudinal mass versus rest mass. And
so on.
-- In contrast, a modern approach to special relativity is much
simpler. Basically you need to say there are 4 dimensions instead
of 3 (which is trivial), and you need to say that rotations in
the xt plane have a "+" sign in one place where spacelike rotations
would have a "-" sign. That's it. One flipped sign. This gives
you the geometry and trigonometry of spacetime, and essentially
everything else follows from that. I'm not saying you have to derive
everything from simple axioms (although you could). My point is
that Minkowski spacetime is just not very different from Euclidean
space, and this greatly reduces the amount of stuff that needs to
be remembered. By the same token, it reduces the number of ways
that mistakes and so-called "paradoxes" can creep in.

First--I'd like to hear from _anybody_ teaching HS or general education physics who approaches special relativity from JD's point of view (4D geometry). How does it work?

Second--the paradoxes are the 'fun' part of this material--at least for the students named above.

Third--the primary point one is usually making with these students is that the phenomenon of high speed motion deviates from the Newtonian model that they've been working on and thus requires a different model. What they will immediately 'demand' is proof that indeed the 'phenomenon' exist. Experiments with atomic clocks and the lifetime of muons are offered as direct verification of problems with clocks while the twin paradox is the seminal example of the ultimate consequences of 'time dilation'. One can then turn to particle accelerator work (was a PSSC film on this as well as the muons) that measures velocity and momentum of particles moving very fast. The experiments clearly show that momentum rises must faster than velocity at high velocity. For the student group in question--comparing this to their ideas of momentum (p = mv) the conclusion _has_ to be that mass has increases. So I am totally unconvinced that the approach which is best for a Modern Physics Course for physics majors is the approach one should take (or even can take) with HS and non-science students. 3-D geometry is a mystery to most and a 1/R^2 dependency is as difficult to fathom as Jackson's E&M was to most of us. [And to anyone who found Jackson's E&M a snap--a big, loud, raspberry to you! ;-) ]

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
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Free Physics Software
PC & Mac
www.saintmarys.edu/~rtarara/software.html
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