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Re: [Phys-l] E=mc^2 because E=mc^2?



The majority is still using "relativistic mass"!

Wow. That really puts things into perspective. Comparatively
speaking, that's the best argument for a speed-dependent mass
I've ever seen ... but in absolute terms, it is a travesty of
science. It reminds of of beer commercials that rely on
celebrity endorsements and peer pressure.


Actually this is a reasonable reason for using it in class so that the
students can be made aware of it. Of course you do not have to endorse this
usage. Physics terminology is often codified by consensus, rather than by
logic.

It is my sense that relativistic mass may be a good concept when introducing
HS students to relativity. It gives a concrete interpretation that might
make sense. I do not know of any research as to whether this would improve
student ability to understand or use the concepts in relativity.

One could hedge by proposing that it "looks like" mass increases with
velocity. In either case students need a firm model to be able to grasp the
concepts. So in an introductory course, particularly one with non physics
majors, the concept may be just what they need. When this type of thing is
done, the students have to be reminded that particular interpretations are
subject to revision, and as one gains more knowledge the model you are using
may change. Actually the research shows that physics majors also need the
same types of things that non-physics majors need, so it may also play well
with them.

For example students have to acquire the concept that waves travel at the
same speed independently of how they are produced, and that parameters like
pulse width, wavelength... do not influence the speed. Once this model is
firmly grasped, then one may introduce dispersion. They first have to
understand the first order effects before the smaller effects are
introduced. The first concept is absolutely vital to helping them with a
variety of misconceptions.

I was once asked by a colleague whether relativistic mass was incorrect and
should be avoided. I replied "yes, but..." Now I would reply that the
concept may be very useful, but some physics books do not endorse it. Until
we have more knowledge as to how students acquire relativity concepts, it's
usage is an open question.

John M. Clement
Houston, TX