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[Phys-L] Re: Animated Math - comments



On 07/04/05 19:05, jbellina wrote:
I'll just jump in briefly. If you want to describe a temperature
change you would say it is so many Celsius degrees. If you want to
describe a temperature,you would say it is so many degrees Celsius.
Sadly that gets abbreviated into symbols that are not clearly defined.
The intent is correct, the symbolism is easily missinterpreted.

Further, students often have difficulty distinquishing temperature from
change in temperature. So having a symbolism that easily confuses
these is not pedagogically wise.

I'm not following this. We can agree that conceptually
there is a distinction between temperature and change
in temperature ... but I'm not convinced that the units
of measurement should bear this burden.

I've spent mannny years in physics labs where temperatures
are measured and talked about all day long, without ever
hearing "T" and "delta T" measured in different units. If
you tried to convey the distinction using the terminology
suggested above, nobody would understand.

By analogy,
-- Conceptually we distinguish between an angle
(as in orientation) and the difference between two
angles. Surely "turn left 30 degrees" is different
from "turn to a heading of 030 degrees". But people
measure both of them in the same _units_.
-- Conceptually we distinguish between my net worth
and my profit or loss on a given transaction. But
people measure both concepts in the same units.
-- Conceptually we might measure altitude relative
to the center of the earth, or relative to mean sea
level, or relative to the floor of the laboratory, and
it is important to distinguish the various cases. But
people measure all three concepts in the same units.
-- Conceptually there is a huge difference between
energy, enthalpy, free energy, and free enthalpy. But
people measure all of them in the same units.
++ Et cetera. Additional examples are a dime a dozen.

The only real counterexample I can think of is torque
(measured in newtonmeters) as opposed to energy
(measured in joules) ... although even in this case,
both can perfectly well be measured in kg m^2 / s^2.

=====================

This brings to mind one of my favorite sayings:
==> There is more to physics than dimensional analysis.

We can all agree that units are meaningful, and names
are often meaningful ... but there are limits! If I
introduce you to Mr. Smith, you shouldn't assume he
works in a smithy. Similarly, the variable-names in
computer programs ought to give some hints about the
meaning of the variable, but they cannot possibly
convey everything there is to know; otherwise names
would be as long as paragraphs, or perhaps much longer.

Don't try to make the units encode everything you know
about the situation.

If there's confusion between "T" and "delta T", don't
blame the units. The problem lies elsewhere, and the
solution will be found elsewhere.

============================================

Regarding the phys-l server: It has been doing some
weird things. Msgs I've posted have arrived many
hours late, and out of order. This is very unusual.
They seem to be going into the archive promptly, so
I know they were received OK ... they're just not
going out promptly. Maybe Dan has an explanation;
I don't.
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