I used to teach that mass is what you measure on a beam balance. I
don't do that anymore. Here are some reasons why...
(1) Many students will never use a beam balance. Electronic balances
have taken over. And the truth is, a single-pan electronic balance is
just as good of a way to measure mass as a beam balance. They both
truly compare masses.
As I mentioned in another thread a long time ago... a beam balance
compares two masses that are displaced in space whereas a single-pan
electronic balance compares two masses that are displaced in time. For
the beam balance to work properly the gravity has to be the same in the
two locations during the comparison. For the electronic balance to work
properly the gravity has to remain constant over the duration of the
comparison.
Today you simply cannot state that a beam balance measures mass whereas
an electronic balance measures weight. Both balances make a mass
comparison. They are both equally good as measuring mass.
Therefore, if you want to define mass in terms of balances, don't say
mass is what you measure on a beam balance. Just say mass is what you
measure on a balance.
(2) Point (1) is more relevant than you might think. In the quest to
eliminate the platinum-iridium cylinder as the world's International
Prototype Kilogram (IPK), one of the projects currently being carried
out by NIST is a project to define the standard mass electromagnetically
in the manner in which an electronic balance works.
Here are two of several NIST pages that discuss this...
(3) Chemistry books and dictionaries define mass as "quantity of
matter." This is a good definition. I believe it is the answer to Jim
Green's original question of what mass is.
(4) Another method being examined at NIST for replacing the
International Prototype Kilogram (IPK) is "counting atoms." This method
would very clearly be in unison with the idea that mass is quantity of
matter. This is mentioned in the first NIST reference above.
(5) This is just a common sense argument. Suppose we have the IPK and
we also have two cylinders made of the exact same material, but these
two other cylinders have the same mass as each other and together have
the same mass as the IPK. In other words, the two additional cylinders
are each one-half kilogram. In reality, how does the IPK differ from
one of the half-kilogram masses? Are we going to say, well it differs
because it takes two of the smaller cylinders to balance it on a beam
balance? I don't think so. It's more simple and intuitive than that...
the IPK has twice as many atoms.
Of course this is the whole basis behind the mole concept and the atomic
masses listed on the periodic table. We use mass to determine how many
atoms are present in the sample. This is why chemists are defining mass
as quantity of matter.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu