Re: [Phys-l] Deceleration or Negative Acceleration

[John Mallinckrodt <ajm@csupomona.edu>]

I'm pretty much with Rick on this one.  IMO it is highly  
counterproductive to talk about "negative acceleration" because it  
wrongly suggests that a vector can somehow BE "negative" and confuses  
direction (an inherent property of a vector) with algebraic sign (a  
property of its components in an arbitrary reference system.)

I find it useful to emphasize the points Rick made about what happens  
when the angle between the acceleration and velocity vectors is 0  
("speeding up"), 90 degrees ("turning"), or 180 degrees ("slowing  
down") and then to go on to point out, more generally, that when that  
angle is acute we have "turning and speeding up" and when that angle  
is obtuse we have "turning and slowing down."

I don't, however, think that we can stamp out the use of the word  
"deceleration," which, in common parlance, means "slowing down."  So  
I think we need to make sure that students understand how to  
translate that common interpretation into the language of physics.   
Quantitatively, we have

   deceleration ==  - d/dt | v_vec |

so that a "deceleration of 3 mph/s" means that the magnitude of the  
object's velocity is decreasing by 3 mph each second and "a  
deceleration of - 3 mph/s" means that the object is "not  
decelerating"--i.e., "speeding up" at a rate of 3 mph each second.   
Although I certainly wouldn't want to belabor the point, it should be  
perfectly clear what we mean when we say "the acceleration is 10 m/ 
s^2, west and the deceleration is 6 m/s^2."

John Mallinckrodt
Cal Poly Pomona

Rick Tarara wrote:

> I don't like either term.  In our usage, accelerations in the  
> direction of
> motion cause object to increase speed while accelerations opposite the
> direction of motion cause objects to decrease speed.  Accelerations  
> at right
> angles to the direction of motion cause a change in direction.
> Accelerations at any angles other than 0, 90,180,270 are dealt with by
> breaking the motion into orthogonal components and analyzing the
> accelerations along each of those directions with the above rules.
>
> All of that is a mouthful and requires some considerable effort to  
> ingrain,
> but it eliminates a lot of other problems, not the least of which  
> is the
> freedom to choose positive/negative directions in any particular
> situation/problem.

in response to Peter, who wrote:

> > I instituted the use of "Reading Questions" in my classes this  
> > year, and I
> > am getting some of the most interesting questions this semester  
> > from my
> > students.
> >
> > This is one that I've not run into before...  A student has asked  
> > which is
> > proper terminology:  "Negative Acceleration" or "Deceleration".
> >
> > Of the textbooks on my shelf, about half are for the first and the  
> > other
> > half seem to use both interchangeably.  Interestingly, those that say
> > "Negative Acceleration" is the proper term are all of a more recent
> > vintage.
> >
> > Is one or the other really more used/acceptable?

A. JOHN "Slo" MALLINCKRODT
Lead Guitarist, Out-Laws of Physics
     http://outlawsofphysics.com
Professor of Physics (Ret'd), Cal Poly Pomona
     http://www.csupomona.edu/~ajm
Consulting Editor, AMERICAN JOURNAL of PHYSICS
     http://www.kzoo.edu/ajp