I said: "When we say that weight is what "the scale says" we can be
quite wrong, depending upon how the scale works."
Jim Green said, "Oh my! How can an arbitrary definition possibly be
wrong? If one says
that, by the word "weight", one means "what a bathroom scale reads",
that
is what is meant!!! How can that be "wrong"? Another may not like the
usage, but it is not "wrong"."
Well... I have an electronic bathroom scale in my bathroom that says my
weight is 90 kg. No matter what side of the weight argument you take,
I hope we at least agree that weight is a force, and it ought to be
measured in newtons or pounds, etc. Are you telling me that my weight
is 90 kg because that is what my bathroom scale says?
Okay, you may not like that example because it is a rehash of the
kg/pound dilemma. Here's another example, perhaps more to your liking.
Most of our electronic balances can be switched to read grams or
ounces. But making this switch does not convert them from mass meas
urers to force measurers. In both cases the measurement is made by
mass comparison.
I routinely switch one of them to ounces when I need to "weigh" a
letter or package that I intend to mail. (Why convert from grams to
ounces in my head, if the scale will do it for me.) Now suppose it
says that my envelope weighs 1.20 ounces. In my lab at Bluffton
College, that is probably pretty darn close. But if I take that same
envelope and same balance to the moon, it will still say that my
envelope weighs 1.20 ounces... and Jim, that is wrong.
Had I chosen to take an inexpensive spring balance to the moon to weigh
my envelope, it would have given a reading more close to 0.20 ounces,
and that would be more correct.
I'm sorry that this does involve a rehash of the kg/pound (i.e.
mass/force) dilemma, but that simply is the way things are. Since it
is true (as I said earlier) that determining mass is one of the most
accurate things we can do, it is customary for us to use a balance that
measures mass to achieve calibrated forces in our lab. Right now my
students are doing the centripetal force lab in which they use a spring
to provide the centripetal force on a revolving mass. They must
calibrate the spring by stretching it with a string run over a pulley
with known masses hanging on the string.
They typically determine the mass of the calibration masses by putting
them on the balance and reading grams. They convert these readings to
kg, then multiply by 9.80 m/s^2 to get newtons. (g = 9.80 here in
Bluffton, OH) However, they could switch the electronic balance to
read the weight of these calibration masses directly in newtons. But,
if they did so, the balance's microprocessor would use g = 9.80665
m/s^2 (the standard value) whereas in Bluffton, OH the acceleration of
gravity is really 9.80139 m/s^2. This error (which would indeed be an
error) is minor. But if the students took that balance to the moon and
did the experiment there, directly reading the weight in newtons, the
error would be major... about a factor of 6.
You indeed have to know how your scale works.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817