Re: SI and nothing else

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Ludwik K. wrote:
> In other words, these Planck's units, or the GeV-based units, can not
> possibly, even in principle, be used in a starting chapter of an introductory
> physics course.

Duh.

>                 Too much previous knowledge is required to make them
> meaningful. Let us agree that at least three basic units are necessary to
> make a useful transition from common knowledge and experience to physics.

Agreed.

> I am talking about mechanics. And at least four are needed to include E&M,
> when we want to each physical quantity to be dimenionally unique. CGSE had
> three units and cm was shared by distance and by capacity.

If you want each distinct physical quantity to be dimensionally unique you
will have to have significantly more than 4 base units.  In the current SI
system (with 7 base units) we still have that the distinct quantities of
torque, work, energy, and heat all sharing the same dimension.  We have the
distinct quantities of angular momentum and action sharing the same
dimension.  We have the distinct quantities of planar angle and solid angle
sharing the same dimension.  We have the distinct quantities of
frequency and (radio)activity sharing the same dimension. We have the
distinct quantities of electrical resistance and reactance sharing the same
dimension.  We have the distinct quantities of energy density and pressure
and other forms of stress all sharing the same dimension.  We have the
distinct quantities of acceleration and gravitational field strength having
the same dimension.  I could keep on going but I hope you see the point.  In
order to make each quantity each have its own unique dimension you would have
to greatly increase the number of base units used.  With each extra base
unit used there would be a corresponding extra dimensioned universal constant
of nature that would have to be memorized.  This is a prospect that I, for
one, do not relish.

David Bowman
dbowman@gtc.georgetown.ky.us