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Re: [Phys-l] Weight?



According to the UMPERG group g should be defined as 9.81 N/kg on the
surface of the Earth if it is used in the equation F_g=mg. This is because
they have found that g=9.81 m/s^2 is extremely confusing to students. I can
remember thinking as a student it is stupid to calculate F_g = m x 9.81
m/s^2 for an object which is at rest because it is not accelerating. Also
unit conversions are often just incantations. So the proposed formulation
makes more sense to students. The idea that N=kg m/s^2 is a very foreign
incantation to them.

I would use a_g to notate the free fall acceleration. Again this is inline
with the idea that all accelerations should be notated a, all forces F, all
energies E, with appropriate subscripts. The separate symbols for the same
type of quantity make students think they are different things, and serve as
a barrier to integrated thinking.

I am not claiming it is "truth" or "correct" merely that research points to
it being pedagogically better. You are free to disagree, but if you want
evidence go to the UMPERG web page and their various papers.

The big problem is that the vast majority of intro physics students in an
algebra based course do not have proportional thinking. So many things
become incantations rather than understood concepts. Many of them exhibit
the characteristics of concrete operational thinking so multiple steps are
extremely hard. Three variable equations such as a=F_net/m are opaque and
have no meaning to them. Shayer & Adey's "Really Raising Standards" has
good information on this problem.

All too often the debate is really about what "is is".

John M. Clement
Houston, TX



Quick question: You say that mg is the pull of gravity, I'm gathering
from what you said below you mean the integrated contribution of all mass
elements constituting the earth. Does that mean that g is not the free-
fall acceleration of an object?