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Re: (dv/dt) terminology: opportunity for improvement



The word "abruptness" came to my mind.

"John S. Denker" wrote:

Hi Folks --

A) In physics, we need a name for the quantity (dv/dt).
It's the same quantity that shows up as (F/m) in Newton's
second law. We like to use reference frames that lack
this quantity.

B) Alas, calling this quantity "acceleration" sometimes
causes problems for students. They think that acceleration
denotes an increase in speed, and that it is the opposite
of deceleration. This leads to wrong physics conceptions
in two ways:
-- Speed is a scalar, whereas velocity is a vector.
The quantity under discussion is a change in velocity,
and must not be confused with a change in speed.
-- We don't want two words (acceleration and deceleration)
when one word will do. A force in the forward direction
causes an increase in speed, while a force in the rearward
direction causes a decrease in speed, but we want to use
the same word to describe both of these cases (plus a
third case, namely sideways force).

C) The origins of the students' misconceptions are clear:
-- The words acceleration and deceleration clearly look like
they mean "more speed" and "less speed". That's exactly
what they ought to mean, based on the Latin root "celer".
-- That's exactly what they do mean, in everyday conversation
outside the physics lab ... and even sometimes inside the
physics lab! If we use words inconsistently, we shouldn't
be surprised if the students are confused.

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

The obvious way of dealing with a situation like this is to
coin a new term that more clearly expresses what we mean.
I'm not 1000% sure what the new word should be, but here are
some proposals for you to think about.

1a) The first candidate is "mutovelocity" meaning, literally,
change in velocity, where "muto-" is the combining-form of
the Latin word for change, and we all know what velocity means.

1b) The foregoing probably benefits from elision to "mutocity".
We don't want a word that sounds _too_ much like velocity; we
want it to be related but easily distinguishable.

Remaining drawbacks include:
-- no good adjectival form. We need something to use in the
replacement for expressions like "unaccelerated frame".
Saying "unmutovelocitous" isn't exactly poetic.

2) Another candidate is "afflection". That comes from the
Latin root "flex" as in bending, referring to the fact that
a change in velocity causes a bend in the particle's world
line (except on a set of measure zero, namely directly-forward
and directly-backward afflections).

Additional minor advantages include:
++ It resembles ordinary words, perhaps a blend of "effect of
attraction" and this resemblance is non-misleading.
++ It even starts with "a", so that the usual symbol (formerly
an abbreviation for acceleration) can be re-used as the
abbreviation for afflection, with no change in meaning.

For practice, try saying:
Force is mass times afflection.
Unafflected reference frame.
The afflection due to gravity.

*) Additional suggestions are welcome.

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

It is not required that the new word take over entirely from
the old word. The new word will be of value even if it is
only used temporarily, in emergencies, when it is necessary
to emphasize the distinction between a change in speed and a
change in velocity.

To say the same thing another way: Often it helps to be
ultra-precise when introducing new concepts. This may require
using special words or even long adjective-laden phrases.
Later, after the concepts have solidified in the students'
minds, we can revert to ordinary ambiguous words; students
can figure out from context what is meant -- !!after!! they
understand the concepts.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.