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I, like John and others, try not to use the word deceleration and try toget
the students not to use it either. Of course, it occasionally slips outof
my mouth, but when it does, I try to immediately 'subtitle' the word withdifficult
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
time understanding that a ball thrown straight up is acceleratingdownwards
while on the way up. Eventually you have to deal with the actual throwingdownwards
of the ball so that there is first an upwards acceleration, then a
acceleration that persists until the ball is caught--during which there isa
short lived upwards acceleration again, and of course there is that zerotime,
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
discussing that a = delta(v)/delta(t) such that some finite delta(t) needsabout
to be considered and therefore there is a measurable delta(v), talking
the ball changing direction, moving to forces and realizing that the forcethink
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
we can describe 5 ways in which an object can accelerate.intro-course
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
level. Example, projectile motion with Corriolis effects.)and,
One can spend a lot of time just trying to deal with the first 3 above
IMO, using the word deceleration to describe the ball moving upwards willcan
increase the confusion over the direction of the acceleration. Yes one
define deceleration as a 1-D acceleration, opposite the direction ofmotion,
that causes an object to slow down, but I prefer to stick with 'when anthe
object in straight line motion slows down, the acceleration is opposite
direction of motion'. To deal with cases (4) and (5) in intro classes, mythe
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
direction of motion and slowing down is an acceleration opposite thewith
direction of motion.
A bit long-winded, but now in week-6 of the semester and still dealing
student conceptual problems in this area!the AAPT.
Rick
(No College mandated disclaimers--yet!)
*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
www.saintmarys.edu/~rtarara/software.html
NEW: Mac versions of Lab Simulations
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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