Re: A weighty subject

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sat, 16 Oct 1999, Robert Carlson wrote:

> This 1894 definition is clear to me.  Weight is the force of gravity and not
> limited to Earth and objects on the Earth.  It is consistent, as Leigh says,
> with most physics text books that I have read.  I do not see any reason to
> alter this definition.

This is just an anecdote, but I think it is pretty representative of what
intro texts do:  The book we are currently using (Serway and Beichner, 5th
Ed.) defines (on page 119) "weight" as the magnitude of the "force of
gravity" where the "force of gravity" is the mass of the object times the
acceleration of a freely falling object which, they say, has an
acceleration "acting" (sic) toward the center of the Earth.

I see lots of potential here for confusion down the line.  I happen to
like the identification of weight with the magnitude of m*a_freefall
because that *is* what a scale reads (i.e., they are using my definition),
but the author's imply that a_freefall is directed toward the center of
the earth implying that weight is defined BOTH as "what a scale reads" AND
as "the force of gravity" as will be later identified with GMm/R^2.  So
which definition are they using, yours or mine?  I *really* don't know.

> ...
> Some have advocated not using weight at all, but instead, advocate saying the
> force of gravity.  I say weight is the force of gravity.

We know and that is certainly your right.  We would have it be otherwise
since there is already a perfectly good name for the force of gravity
(i.e., "the force of gravity") and a very meaningful alternate use for the
name "weight."

> Would those advocating not using weight also advocate not using
> acceleration, but instead always say the time rate of change in
> velocity?

Why would we do that?  I see no advantage to be gained.  Everyone that I
know agrees that acceleration IS the time rate of change of velocity.
I've never heard anyone suggest a good alternate use for the word
acceleration.

Look, I'm willing to let you define weight as something *other* than what
a scale reads, but I don't think it is unreasonable for me to ask you to
understand why lots of people think there are good reasons *not* to.  You
don't have to agree, but you should at least understand our point; it does
not appear to me from the above that you do.

> Others have advocated redefining weight as the force a scale reads.  I see
> nothing in the definition, weight is the force of gravity, regarding scales.

Of course not, it is a different definition.

> While scales may be properly used to measure weight, they are not necessary
> (and not included) in its definition.

No.  They are not included in *your* definition.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm