Re: deceleration

["John S. Denker" <jsd@MONMOUTH.COM>]

I wrote:
> > *) Terminology: Note that I said "slow down" rather than
> > "decelerate".  I'm becoming more and more convinced that
> > "deceleration" is a dirty word.

> > Any (any!) change in velocity is an acceleration.  An acceleration
> > in the direction opposite to the velocity is still an acceleration;
> > it causes a decrease in speed.

David Bowman wrote:
> Why is this word any dirtier than the phrase 'slowing down'?
>
> I realize that John has not advocated banning the word 'decelerate'
> from physics classes; he just claimed it was a "dirty word".  But I
> want to take the opportunity to spout off, anyway, on the practice
> of banning certain words from the physics classroom.  I'm sort of
> dubious of attempts to forbid the use of certain vernacular terms
> (heat, decelerate, centrifugal force, etc.)

1) I know exactly what centrifugal force is, and I use it
in the technical sense all the time.
  http://www.monmouth.com/~jsd/how/htm/motion.html#sec-coriolis-effect
  http://www.monmouth.com/~jsd/how/htm/motion.html#sec-centrifuge

This is the best-case situation, where there is no real
conflict between the technical meaning and the non-technical
meaning(s).

2) Heat has several vernacular meanings, one of which agrees
quite closely with my friends' technical usage.  (There
are other vernacular meanings, such as hot weather ==
"heat wave", that are problematical.)

This is a difficult situation.  It is a challenge to
overcome common misconceptions such as heat==temperature
but there is a hope of doing it, whereupon the non-technical
meanings can be seen as poetical metaphorical distortions
of the technical meanings.

3) "Elastic" is another problematic word.  The technical
meaning is not at all what people expect.  People think
you're nuts if you tell them that steel-on-steel is
more elastic than rubber-on-rubber.

4) "Deceleration" is definitely an odd duck.  The problem
is that students tend to assume that a deceleration must
be the opposite of an acceleration.

> My attitude is that it
> is best to defuse the potential of vernacular words for harboring
> or abetting misconceptions by confronting head-on any possible
> confusion that may be contained in the word or caused by a mismatch
> between a word's vernacular meaning and its technical physics
> meaning.

I agree with the sentiment, but ....

> I prefer to render the potentially offending word
> (misconceptionally) impotent by *carefully defining* its meaning
> with sufficient emphasis.

Agreed, *carefully defining* is better than banning.  But
when I try to carefully define "deceleration", I wind up
with something that is so non-useful that banning it would
be superfluous.

> In the case of the word 'decelerate' I
> would prefer to *define* it as meaning exactly slowing down or
> decreasing the speed.  The opportunity can then be used to explain
> how such a situation happens when the acceleration vector and the
> velocity vector have an obtuse angle between them (i.e. a negative
> dot product).

According to that definition, a deceleration is a special
case of an acceleration.  It is !!not!! the opposite of
an acceleration, just as dromedary is not the opposite of
camel.  Half of all accelerations are decelerations.

The puzzling thing is that AFAIK there isn't a Latin-based
word for the opposite of deceleration.  If somebody can
find/invent one, please let me know!

> Similarly, I would explain how speeding up occurs
> when these vectors have an acute angle/positive dot product.

But if speeding up results from a forward acceleration,
why not just use the obvious corresponding term "slowing
down" for the rearward acceleration?  According to my
friends Strunk and White, if you want to communicate
parallel ideas, it is good style to use parallel words.

> The vernacular meaning of speeding up *must* be distinguished (in the
> students' minds) from the physics technical meaning of the rate of
> change of the velocity.

Yes!

> I would prefer to expend instructional
> energy on emphasizing the definitional nuances that lead to confusion
> (if they go unnoted) to expending it on enforcing a ban on vocabulary
> terms that have been placed on some index of forbidden words.

I'm not into "banning" and "enforcement".
I'm just saying that when a word is understood to be
useless and unhelpful, it will die out on its own.

> Besides, once the students leave the protective confines of their
> physics classes, they will co back to the outside world where the
> banned words and vernacular meanings are still used all the time
> anyway.

Agreed.  But it's not worth anybody's time to spend a
lot of time defining "deceleration" or "phlogiston"
beyond the point where students know they should _avoid_
using such terms in technical applications.

The most important point I'd like to make concerns _my_
speech not students' speech.  I have a bad habit of using
"acceleration" and "deceleration" in ways that suggest
they are opposites.  Every time I do that I feel guilty,
because I feel I'm perpetuating a troublesome misconception.

> > This posting is the position of the writer, not that of Moe, Curly, or
> > Shemp.
>
> I was hoping this example would have come up soon.  But what about
> Larry?

Different surname.

This posting is the position of the writer, not that of Larry, Joe, or
Curly Joe.

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