Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: terminology: period vs. wavelength

These discussions of terminology seem to keep coming up - "period", "simple
harmonic motion" "weightless" "heat".... I don't usually get involved, but
I DO think that terminology is important. Not that I expect a student to
be able to spout back the exact definition of every word or law in the
book, but that such definitions are available should clarification be
sought. A lawyer can find the exact definition of "grand theft auto", a
pharmacist doesn't have to debate what "codeine 100 mg t.i.d" means. Why
should physicists keep arguing the same point over and over? When we
discussed "weightless" some time ago, it wasn't that the various sides
didn't understand each other, but rather that each side thought it had a
better definition. Why not slug it out, pick a winner, and then accept the
results? Unless new evidence or insights have been gained to necessitate a
new definition, accept the consensus and move on!

The dictionaries are on my side, they specifically say the root of period
periodus and this specifically refers to a time interval.

Not all dictionaries say so.

Before there was Latin _periodus_ there was Greek
_periodos_ "circuit"
_peri_ "around"
_odos_ "way" (same root as "odometer")
giving no reason to restrict the concept to temporal periods.

Who cares what the dictionaries say? I find etymologies fascinating, but
just because "period" once meant "way around" doesn't mean that has
anything to do with the current popular or technical definition. We all
know that physics uses words in ways that are different/more precise than
what the dictionaries will say.

At a minimum, we should consult physicists, rather than dictionaries. Or a
technical dictionary. In one of his books, Feynman made a comment to the
effect that polling a larger and larger group of non-experts doesn't get
you any closer to the correct answer.

I think most physicists would be perfectly comfortable saying that in a
crystal lattice, the atomic positions are given by a periodic function of
and Y.

But that is a significantly different use of the word. Heck, it actually
IS a different word. Yes, physicist are comfortable using the adjective
"periodic" for both space and time. But the noun "period" is invariably
reserved for a time interval. Ask 100 physicist to define "period" and
I'll bet you that every one will say a time interval.

Nonetheless, I still expect students to work problems in context. If I
ask students for the wavelength of a sound wave I expect them to report a


and if I ask for the period of a sound wave I expect them to report a

Not so clear.

But the professor DEFINED the word as a time. The text DEFINED the word as
a time. The examples I'm sure they did in class and homework used this
definition. What is unclear?

... and if I don't ask a more-specific question I shouldn't expect a
more-specific answer. The same applies to open-ended questions about
physics terminology. The terminology is highly irregular, always has
and probably always will be.

What is "open-ended" about a question like "Mark the period"? Can't we
expect some degree of uniformity? Suppose I ask a student to mark the
period on a diagram, and he marks the amplitude but says "The terminology
is highly irregular, always has been,and probably always will be. My fried
told me last night that this was the period, so I'm just using his
definition instead of yours."

Granted this is an absurd extreme, but we need to agree on our terminology.
Or at a very minimum to be self-consistent within a course.