Re: [Phys-l] force or mass

[John Denker <jsd@av8n.com>]

On 11/06/2008 06:04 AM, Rick Tarara wrote:
> I'll 'stick to my guns' too.  Clearly some groups use the pound as mass, 
> other groups (engineers and intro physics textbook writers) take it as 
> force.  I will continue to go with the engineers and intro books.  Students 
> come to us with the clear knowledge that WEIGHT is measured in pounds. 
....

If you want to persuade anybody, you'll need a better 
argument (see below).  Students "come to us" with innumerable 
misconceptions, and it would be very bad policy to 
redefine basic physics to match students' naive beliefs.

....
>  They 'know' pounds to be weight!

Yeah, but:

*) They 'know' that _elastic_ is soft and squishy.

*) They 'know' that red means hot and blue means cold.

*) They 'know' that heating something raises its temperature.

*) They 'know' that the bubbles in boiling water are air bubbles.

*) Etc. etc. etc. etc. etc.

===========

We agree that students "come to us" with misconceptions, and
we must take that into account ... but taking it into account
does not mean letting the inmates run the asylum.

We take each misconception as a starting point, and then move
away from it as quickly as possible.

> weight of 1 kilogram is 2.2 lbs,

That is going from bad to worse.  Even if/when we use lbf as
a unit of force, it is a unit of _force_ not a unit of "weight".
The weight of a kilogram will be dramatically unequal to 2.2 lbf
on the surface of the moon, or in the weightless environment of
a space station.

===========================================

On 11/05/2008 09:04 AM, Polvani, Donald G. wrote:

> >   Currently (2008), here at work engineers still compute forces
> > in pounds and mass in slugs as in the B.E. system, 

That's a reasonable argument for /mentioning/ the use of "pound"
as sloppy shorthand for "pound-force".

OTOH mentioning does not mean emphasizing.  There are lots of things
that are common in aviation that we would not want to inflict on
introductory physics students.
 -- speed in knots
 -- pressure in inHg
 -- pressure altitude in "feet" that may be dangerously different
  from real feet.
 -- et cetera

===================================================================

Bottom line:  Technically and legally, the pound is a unit of mass,
and always has been.  It is defined in terms of the kilogram.

On 11/06/2008 06:50 AM, Vern Lindberg wrote:

> > > try googling kilogram-force.

An excellent suggestion.

Using kilogram as shorthand for kilogram-force makes just as
much sense as using pound as shorthand for pound-force, i.e.
not much sense.