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Re: definition of weight (again)



This definition seems to be close to the one in Halliday and Resnick (old
edition). They point out that they will generalize the definition in
chapter 16, but I can not find where they actually did that. Minds on
Physics (MOP) uses a generalized definition of weight as the total
gravitational attraction on a mass. This would include forces due to the
Moon, Sun,... including the Earth. In practical terms their generalized
definition is just as good as the simple one below for the first year HS
physics student. It would be nice if there could be complete agreement on
how to use the term as this would facilitate communication.

I think that more of the popular books use a definition that it is the
gravitational force rather than defining it as what a spring scale measures.
It is my impression that the former definition may be easier to use in
teaching first year students, but I do not have any research articles to
cite to support my belief.

One issue that I am fairly sure about is that many books could benefit by
using consistent terminology. The symbol F with a subscript should be used
for all forces F_g (gravitation force), F_n (normal force...). Students
become extremely confused when they see the equation F=mu N. They confuse N
(normal force) with the unit N (Newtons). This confusion may not be as big
a problem in advanced university classes, but it is extremely prevalent in
HS. The fact that N (symbol for normal force) should be italicized and N
(Newtons) is not is totally lost on students. They often do not notice the
context.

Traditionally most physics texts have been written by authors who were good
writers, physicists, and thought they knew how to get across the concepts.
Unfortunately, thinking you know how is not equivalent to actually knowing
how. Detailed observation, testing, and trying of various things is
necessary to find out what works better. Priscilla Laws commented that over
the years her intuition has gotten better, but she still tries things that
do not work. This type of research and development often requires more than
just a single talented individual. As a result I would tend to go with the
recommendations of the research groups. At the moment I find that the
recommendations of the U.Mass Amherst (UMPERG) group are very good, but I
certainly use material from other groups when I feel that it is appropriate.
MOP uses very consistent symbols, and the authors warn against some common
practices that they have found to be confusing to students. Unfortunately
most of their recommendations have been scattered throughout the teacher
guides for MOP, so it is not possible to just read an article or two on the
ideas.

At the moment I would like to see some of the research based material using
consistent terminology as I would like to combine some of these materials.
I can do some of this myself as I purchased Real Time Physics, TST labs, and
ILDs with the right to duplicate license and sources before some of them
were published. However it is still a pain to try to find all of the
inconsistent symbols and usage make it work with the books that I use.


John M. Clement
Houston, TX


Then I'm in the same minority because that's the way I have used
in Calc Physics classes for years. I do point out that it is the
same definition on other bodies, just replace Earth with ___ .
James Mackey


Joe Heafner wrote:

From: Larry Smith <larry.smith@SNOW.EDU>

This debate seems to resurface every year (or is it every
semester?), but
it would be nice if we could all agree on the definition of weight.

Weight is *the force on a body due to Earth's gravitational
interaction with that body*. Period. Yes, I'm aware of other
definitions that include *apparent weight* but this is more
trouble than it's worth IMHO. I completely ban the use of the
term *weightless* in my classes because it's not logically
consistent with the above definition. What most authors call
*weightless* is really more accurately *contact-forceless* (i.e.
the absence of a contact force whose magnitude may or may not be
equal to GMm/d^2).

Sadly, I don't think there is any closure to be had here. I'm
probably in the minority though. Chabay and Sherwood use the same
definition that I use.

Cheers,
Joe