Re: deceleration

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

I, like John and others, try not to use the word deceleration and try to get
the students not to use it either.  Of course, it occasionally slips out of
my mouth, but when it does, I try to immediately 'subtitle' the word with
our in class usage--'1-dimensional acceleration opposite the direction of
motion' where the 1-dimensional part is assumed because of the examples we
use.

This is really a tough conceptual area for students.  They have a difficult
time understanding that a ball thrown straight up is accelerating downwards
while on the way up.  Eventually you have to deal with the actual throwing
of the ball so that there is first an upwards acceleration, then a downwards
acceleration that persists until the ball is caught--during which there is a
short lived upwards acceleration again, and of course there is that zero
velocity point at the peak of the motion that disturbs them to no end
despite numerous ways used to describe why the ball is still accelerating
downward at that point.  [On the last point--graphing velocity versus time,
discussing that a = delta(v)/delta(t) such that some finite delta(t) needs
to be considered and therefore there is a measurable delta(v), talking about
the ball changing direction, moving to forces and realizing that the force
of gravity does not 'shut off' at the top of the motion, etc., etc. still
does not remove this confusion from some heads!]

Students come to College, out of science programs and HS physics programs
NOT understanding acceleration.  Maybe even this is not surprising.  I think
we can describe 5 ways in which an object can accelerate.
1)  Speeds up (1-D)
2)  Slows down (1-D)
3)  Changes direction with no speed change (2-D usually)
4)  Speeds up AND changes direction. (2-D example: projectile on downward
part of trajectory.)
5)  Slows down AND changes direction. (2-D: projectile on upward part of
trajectory.)

(Extensions of 3,4,5 into 3-D are, I think, usually beyond the intro-course
level.  Example, projectile motion with Corriolis effects.)

One can spend a lot of time just trying to deal with the first 3 above and,
IMO, using the word deceleration to describe the ball moving upwards will
increase the confusion over the direction of the acceleration.  Yes one can
define deceleration as a 1-D acceleration, opposite the direction of motion,
that causes an object to slow down, but I prefer to stick with 'when an
object in straight line motion slows down, the acceleration is opposite the
direction of motion'.  To deal with cases (4) and (5) in intro classes, my
approach (and that of many authors) is to always separate the motion into
horizontal and vertical components so that one can talk about the vertical
acceleration and the horizontal acceleration and therefore stick with the
more accessible 1-dimensional ideas of speeding up is an acceleration in the
direction of motion and slowing down is an acceleration opposite the
direction of motion.

A bit long-winded, but now in week-6 of the semester and still dealing with
student conceptual problems in this area!

Rick

(No College mandated disclaimers--yet!)

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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www.saintmarys.edu/~rtarara/software.html
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----- Original Message -----
From: "Joseph Bellina" <jbellina@saintmarys.edu>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, October 09, 2002 8:59 AM
Subject: Re: deceleration


> What disturbs me most about this exchange is the rampant mix of scalar
> and vector terms.  It seems to me that deceleration is a term derived
> from the rate of change of a scalar quantity, speed.  And so there is no
> real connection to the rate of change of vector quantity, velocity. But
> perhaps I am trodding on old ground.
>
> cheers,
>
> joe

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