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Re: Inertia





On Fri, 28 Nov 1997, Leigh Palmer wrote:

Durig Lewis makes an important point:

I have a couuple of questions:

How do we define force?
How do we define mass?

I have often heard physicists define each using Newton's second law, not
realizing their circular argument.

I think one should spend some considerable time at the outset in
an introductory physics course helping the student to recognize
this important fact. The argument my teacher made when I first
encountered this dilemma was that one of these quantities must
remain undefinable. In fact he introduced us to the old standards
of mass, length, and time as standards of "three fundamental
undefinable quantities".

I think we become so used to not needing definitions when using
these quantities that we lose sight of their ambiguous status.

Leigh


More can be said, Leigh. The "three fundamental undefinable quantities" is
a misleading way of putting it. Usually they are refered to as the three
fundamental quantities which are given "operational" definitions, in terms
of a procedure or process for comparing to a reproducible standard.
Indeed, this distinction is built into our official standards for units,
as summarized by the SUN (Symbols, Units and Nomenclature) document of the
International Union of Pure and Applied Physics, found in the back of the
Handbook of Chemistry and Physics. And of course, there are more than
three of these fundamental quantities.

In the philosophy of science courses I took when I was an undergraduate,
the meaning of F=ma was discussed at length, because there's a vast
literature on the question of whether it is a definition of mass, of
force, or a law of nature. The bottom line is that it both a law of nature
and a definition. If you choose to take force as fundamental, then this
defines mass. If you take mass as fundamental, then it defines force.
Acceleration is previously defined, of course. But F=ma also says more,
for it tells us how the three quantities relate in nature.

There's many scholarly papers on this, and the reference someone gave to
Arnold Arons' book is a good start toward going beyond the inadequate
treatments found in physics textbooks.

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
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