Re: [Phys-l] definitions ... purely operational, or not

["Dr. Richard Tarara" <rtarara@saintmarys.edu>]

----- Original Message ----- 

> *Agreed*
>
> particulary if you have in the back of your mind that your scale is 
> properly calibrated, there are no buoyancy affects to worry about, and 
> many of the other items that John Denker mentioned in his criticism, etc.
>
> I think that defining as M times free-fall-acceleration is a cleaner way 
> to encapsulate all of that in a definition.  I'd have no problem with 
> defining it *my* way then telling a class that this pretty much amounts to 
> saying what the "spring scale reads", where I can mention the above kind 
> of caveats in passing.
>
> Joel Rauber

There have been too many posts on this and I've lost my place, but 'free 
fall acceleration' in which frame--earth frame or astronaut's frame.  After 
all, they ARE falling, accelerating at 8-9 m/s^2.  I tend to view that (from 
the earth--my choice) as them being in 'free fall' but clearly a bathroom 
scale (falling with them) will read zero.  For students--in the astronaut's 
frame there are no forces acting on them, but in the earth frame they are 
accelerating and there is a gravitational force acting.  Like the falling 
elevator we still have to explain the accelerations.  If we move into the 
rotating frame of a 'rotor-ride' and want to include the 'centrifugal 
force', what is the agent of that force?  There is no third law pair!  Yes 
of course we know that Newtonian Physics isn't the 'answer' and that 
accelerated frames introduce a lot of non-Newtonian stuff, but I would also 
agree with some other posters here that this may well be too confusing for 
HS students, intro level students in most courses.  To me the great 
advantage in a typical physics major curriculum is the spiral approach.  One 
can learn the Newtonian model and work mostly at the Algebra/simple integral 
level of sophistication in first year courses and then get much more 
sophisticated in second year mechanics, then really sophisticated in 
graduate level mechanics.  Same for E&M and Thermal.  KISS is a very 
appropriate approach for 1st year courses and almost a necessity in courses 
for non-science majors (general education science courses) and probably 
equally appropriate for pre-med and engineering physics courses.  This 
doesn't preclude doing some work from accelerating frames in such courses, 
but should allow for the choice to back off and always look at the 
accelerating frame from the fixed frame.  Look at the falling elevator from 
someone in the building, the space station from the point of view of someone 
on earth--then explain in those terms what is experienced by those in the 
accelerating frames.  I would say these two choices--stay in the 'fixed' 
frame for analysis or freely move between frames (again for analysis) is 
what drives this constant debate about the meaning of 'weight'.  We all--and 
the older ones amongst us ;-) are firmly entrenched--in our preferences!

Rick

Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
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