Re: [Phys-L] blue road physics

["John Clement" <clement@hal-pc.org>]

Indeed why would he use an "outdated" concept.  It would be used if it were
useful in helping students understand the topic.  Actually if to teach
relativity one should use a research based method which has been shown to be
more effective.  I understand there is a McDermott tutorial on relativity
which would be ideal.  If it uses relativistic mass then that is the tactic
that they found works best.  The concept of relativistic mass probably helps
students visualize what is going on.  While a purely mathematical
formulation does not.  The big lesson from PER is that students need to
conceptualize rather than just dealing with equations.  So bundling the
relativistic effect into the mass is not necessarily wrong.

In physics we do these sorts of things all the time.  We have massless
strings, frictionless surfaces, point particle molecules...  In other words
we use a conceptual model to build the math and the concepts.  Then with
subsequent courses we change the models to account for smaller effects.  So
at first we establish that the speed of a wave is independent from
frequency.  But when we get to the rainbow this has to be relaxed slightly.

One way of building a concept is by anchor and bridging analogies.  The
student starts with something they believe and is then challenged by similar
but slightly different cases until bridged to the final case with the
general law.  So relativistic mass just might be a good stepping stone on
the way to better understanding.  Do I have the research that shows this?
No, but someone else might have it, or they might have research which shows
the opposite.  How you should teach is really determined by experiment and
NOT by logical thought.  So until we have evidence for one method over the
other it is anyone's opinion.  So does anyone know of the relevant research
in this area?

John M. Clement
Houston, TX

> Is anyone composing a response to Elisha Huggins's article on 
> relativistic mass and mass of a photon in November 2012 ,The 
> Physics Teacher?  First he ignores C. G. Adler's paper (Am. 
> J. Phys., 55(8), August 1987) which addresses the problems 
> associated with "relativistic mass." Next, he asserts that 
> using relativistic mass is just another correct way to do the 
> physics. Finally, he uses special relativity in an 
> accelerating reference frame to talk about trajectories of 
> photons.  He never approaches relativistic momentum as 
> _gamma_mass_velocity.
>
> He wants to introduce SR early, an admirable thing.  Why 
> would he use, at best, an out-dated concept (see Adler cf. 
> Feynmann Lectures of 1963), and at worst, a mistaken concept 
> ( versus GR)? Adler points out that even Einstein discouraged 
> the velocity-dependent mass.
>
> It seems to me that his approach is similar to saying the 
> Bohr model is right simply because it gives the right energy answers.