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Fwd: Re: little gee and its sign



At 11:12 PM -0600 9/10/01, Jim Green wrote:

I'm not sure I follow your desire to calling g a "gravitational variable".
The way I see it, I perform a number of experiments of objects in free fall
near the earth, and I measure their accelerations. If I can eliminate
and/or account for air, I find that all the objects fall with an
acceleration of 9.8 m/s^2. For convenience I give that acceleration a
symbol, g, and a name, "acceleration due to gravity". What else it that
but an acceleration?

I am not sure that I can understand this post -- Well I understand the
words, what I don't understand is why one would say this in public.

My students, too, go into lab and take data about where a freely falling
object is at various (equally spaced) times. They analyze this and take
differences and second differences to find the acceleration of the falling
object. How is this not an acceleration? I agree with Tim (except I don't
call it g).

Do you agree with the following statement of Galileo's law of falling
bodies? "All bodies in free fall (at a given location) have the same
acceleration; near the surface of the earth that acceleration is about 9.8
m/s^2, down." If so, do you object to teaching it to an intro class?

Does one really have to suggest that one consider an Atwood Machine -- in a
laboratory system: |g| is still ~9.81m/s/s but the acceleration is
(usually) not equal to that!

..which is precisely why I call the acceleration of freely falling bodies
a_g instead of g.

I just can't believe that a physics instructor would tell his/her class
that g is acceleration!!!!!

I agree they shouldn't, but so many books (and, I might add, videos such
as Mechanical Universe) have equations like y = (1/2) g t^2. No doubt it
is pedagogically better to avoid g in those equations and discussions but
it is understandable why some teachers would want to match the printed
materials.

I also don't understand why an instructor would introduce the concept of
acceleration with free fall -- pedagogically this is nuts.

I don't see where Tim said he _introduced_ the concept of acceleration with
free fall. I don't, and I doubt he does either. But it _is_ an important
special case that merits discussion, IMHO.

Larry