Re: kinematics language

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Let me suggest that one not get carried away with all this
'correct-speak' ESPECIALLY if teaching a conceptual course--either at
the HS level or to a Gen-Ed population at the college level.  To get
across to these students, careful vector language and notation is not
going to do the trick and avoiding the vernacular won't be very
effective.  For acceleration, it seems enough to consider whether an
object speeds up, slows down, or changes direction.  The first two have
an implied one-dimensionality about them that can be made more explicit
by considering two dimensional (projectile-type) motion by separating
the horizontal and vertical motions--which should be OK with simple
motions [we had the discussion long ago about how a real bullet's
aerodynamic lift does couple the two.]  With these limitations, then if
something speeds up, the acceleration is along the direction of motion,
and if something slows down, the acceleration is opposite the direction
of motion.  When the direction is changing, then we have to look more
closely, but in the simple trajectory problem we model it with no change
in the horizontal speed while on the way up the object slows down and
while coming down the object speed up--implying that the object is
always accelerating downward.  As I've posted before, throwing a ball
vertically up and later catching it, is a motion that can take half a
semester to fully analyze.

The point here is that if you are not teaching at the vector math level,
trying to be  _too_ careful with language may be more confusing to
students since you can't fall back to the precise mathematical models
that might make it all clear.

Rick

***************************************************
Richard W. Tarara
Professor of Physics
Dept. of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu
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www.saintmarys.edu/~rtarara/software.html
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> -----Original Message-----
> From: phys-l@lists.nau.edu: Forum for Physics Educators [mailto:PHYS-
> L@lists.nau.edu] On Behalf Of Daniel S. Price
> Sent: Wednesday, August 28, 2002 10:59 AM
> To: PHYS-L@lists.nau.edu
> Subject: Re: kinematics language
>
> Thank you, Mr. Titus, for addressing the terminology issue.  Each
year, and
> this year in particular, I have taken great pains to eliminate terms
which
> carry multiple meanings in both the vernacular and in more "technical
> settings."
>
> > I suggest never using the word "deceleration".  If one means,
slowing
> > down, then I suggest saying "slowing down".  If one means, an
> > acceleration in the -x direction (or along whatever axis), then say
> > that.  If one means, an acceleration that is opposite to the
> > velocity, then say that.  Deceleration is often used with different
> > meanings. It's no wonder students are confused.
>
> I have found this to be entirely true, and have not introduced the
term
> 'deceleration'.
> When students use it (familiar as they are with automobiles), I
encourage them
> to restate their meaning.  [Do they mean "slowing down" or
"accelerating in
> the negative direction"?]
>
> > The words "increasing" and "decreasing" can be confusing if one
isn't
> > careful to distinguish between the magnitude of a quantity and the
> > quantity.  I use this example in my class: If your checking account
> > is in overdraft protection with a balance of -$400 and you deposit
> > $300, has your balance increased or decreased? <increased>  Has "how
> > much you owe the bank" increased or decreased? <decreased>
>
> This year, I have made every attempt to expunge "increase" and
"decrease" from
> my discussions for the exact reason you cite:  magnitude versus
direction.
> Referring to values as becoming "more positive" or "more negative" is
a bit
> awkward (right now, anyway) but provides apt descriptions, and, I
hope, will
> lead to better understanding.