I appreciated the various ways of viewing mass that John D. itemized. I
was less thrilled with his comment that he is not comfortable defining
mass in terms of amount of material. I'm not saying to substitute
amount of material in place of all the other things in John's list. I'm
just saying it should be added to the list. Why?
(1) It's the way students learned it in chemistry. And at most places
they still teach chemistry before physics. So students are going to
come into physics class thinking of mass as how you measure the quantity
of something.
(2) If we are ever successful at getting US households to have balances
in their kitchens, that will really cement the idea that mass is a
measure of how much stuff you have. We already label packaged food
items with the mass.
(3) I believe it is the way Newton viewed it, and that is why he was
talking about volume and density. He knew mass had something to do with
size (volume) but he also knew the need to include density. Had he
known about protons and neutrons and what atoms are, I suspect he would
have been able to better describe what he probably had in mind.
(4) Although some dictionaries have a special physics definition for
mass that describes inertial mass, most dictionaries do describe mass as
quantity of material. I know we do not turn to dictionaries to find the
definitions of scientific things. Therefore I mention this not to
invoke an authority; rather to point out that this is what is believed
by the people we are trying to teach. We need to meet them where they
are, and fix their misconceptions and expand their correct conceptions.
In this case I don't think they have a misconception; they're conception
just needs expansion.
My old edition of Webster's Collegiate Dictionary says all three of
inertia, quantity of stuff, and gravity. It's a rather nice statement
so I will quote it here...
"the property of a body that is a measure of its inertia, that is
commonly taken as a measure of the amount of material it contains and
causes it to have weight in a gravitational field, and that along with
length and time constitutes one of the fundamental quantities on which
all physical measurements are based"
(5) Depending on how the NIST experiments turn out, we may end up
defining the standard kilogram as a certain number of atoms (probably of
silicon).
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu