Re: A weighty subject

[Robert Carlson <Raacc@AOL.COM>]

In a message dated 10/16/99 4:30:44 PM Central Daylight Time,
ajmallinckro@CSUPOMONA.EDU writes:

> 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),

Scales will read whatever you calibrate them to read.

>  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.

My copy of Serway, Physics for Scientists and Engineers, Third Edition,
states, " the weight of an object is defined as the resultant gravitational
force on the object due to all other bodies in the universe!", page 103.  The
exclamation point is Serway's not mine.  So, which definition is he using?

Serway then goes on to more completely define g in terms of Newton's Law of
Gravitation on pages 361 and 362.  However some texts go further, including
corrections due to Earth's rotation.

>  > ...
>  > 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."

I prefer weight, it is much shorter than "the force of gravity."

>  > 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.

The advantage is that it is shorter and means the same thing.  Why do
mathematicians define m = (delta y)/(delta x)?  I conclude it is shorter, and
easier to say slope, than delta y divided by delta x.

  >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.

Everyone would also agree that weight is the force of gravity if they would
read their physics texts.

>  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.

It is not my definition.  I do not presume to be on the level to make
fundamental physics definitions.  I will leave that to those with much more
knowledge than myself.  Perhaps this is you John, convince me.

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