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Re: Mass




Can someone pleases tell me what "mass" is?

The symbol "m" is spread all over physics topics, but I don't know what it
is.

This is a problem that students commonly have. To introduce it to students,
you can get them to come up with an operational definition. Usually they
can come up with the amount of stuff, which is a fairly adequate
description. In reality mass is probably a theoretical concept as defined
by Lawson's taxonomy. This makes it difficult to understand. In practical
terms for matter it is roughly proportional to the number of protons and
neutrons. As to what it is, that depends on what is is. Mass is not a
thing (an object), but a quantity that can be measured. The difference
between inertial and gravitational mass is not worth pushing in general
introductory courses, because it is definitely a theoretical concept which
can only be easily understood by students above the formal operational level
(generally only 20% or perhaps fewer HS graduates are at the formal
operational level).

As to measurement the suggestion that it "is" what you measure in a
gravitational field using a balance is very sensible. This might even be
historically consistent.


Is it the same number in every equation wherein it appears?

It is the same type of quantity, but not the same number. One must be
careful here because students can have difficulty if it is referred to as
the same number. For example students will think of acceleration as the
same number and use 9.8 for all accelerations, when they should be thinking
of it as the same type of quantity.


Mass seems to be an invention by someone at some point for some purpose
but
I don't understand the process of the development of it's usage.

So far nobody on the list has actually come up with a history. From looking
at the OED the following seems to be the history:
It may have originally referred to barley-corn
Later it meant the amount of amorphous substance such as a lump of silver.
It is mentioned in 1700 as being used by philosophers as the amount of
substance.
I could find no reference to Newton in the OED, but it certainly was
referenced by Maxwell.
Essentially it probably was an early measurement of material based on barley
corns, using a balance.
The list of quotes and references is far too long to give here, especially
when one has to read the OED using a magnifying glass.

The religious use of the word goes back to early Christianity, and has a
different root in Latin.

There is an important difference between mass and charge. Students tend to
think of charge as a fluid and not consider the microscopic dimension.
Charge is definitely a multiple of the number of "excess" charged particles.
As a result charge becomes disconnected in their minds from the matter.
They think you can transfer charge without transferring mass. They will
also tell you that when you charge an object that 50% of the electrons have
been removed!!!!

Another note is that students will sometimes confuse the symbol m (should be
in italics) with the unit m which is not in italics.


Comments???

Jim


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen