Re: [Phys-l] Teaching Special Relativity

[Moses Fayngold <moshfarlan@yahoo.com>]

--- On Thu, 7/2/09, John Mallinckrodt <ajm@csupomona.edu> wrote:

"How does one actually measure the mass of a rapidly moving object?"

A good question. Then  this one should be equally good: And how does one actually measure the invariant (rest) mass of a photon?
    On the other hand, one does not have a problem in actually measuring its relativistic mass m(rel) = h*w/c^2. 
 Based on this, should we discard the notion of the invariant mass for photons and use only their relativistic mass?  


  "Where does the extra "stuff" come from?"

> From the extra energy if we use m = E/c^2 as universal relation.
And the extra energy comes from the work done to launch the initially resting object. A simple extension from introductory general physics, not requiring any space-time geometry etc. 


"The second response sidesteps all of those problems.  Why on Earth  
would anyone not choose it?"

Probably because the second response does not really sidestep all of those problems, as seen from the above comments.

Moses Fayngold,
NJIT  

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