Re: [Phys-l] Deceleration or Negative Acceleration

["John Clement" <clement@hal-pc.org>]

The evidence shows that just changing the definition or starting at a
different point does not avoid or prevent misconceptions.  The idea that
deceleration is negative and acceleration is positive is already embedded in
their brains because of common usage, and no amount of telling or redefining
changes this.

However, using things like McDermott tutorials, Real Time Physics Labs, Hake
Socratic Dialog Labs, Modeling... can change the way that they think about
it.  Few books really bring out the connection between + and - in one
dimensional cases and the vector nature in more dimensions.  Whether one
should start in 2 dimensions or 1 is not well known.  But we do know that
just presenting math definitions is not productive in building
understanding.  Students treat the math as a set of incantations to do
things without considering what it means.  This was quite clear from the
studies done by Joe Redish with his MPEX evaluation.

The solution is long term and not simple.  There are allied misconceptions
that also occur, and to treat all of them one must get students into a
different mode of thinking, or change their paradigms about science and
math.  Just making one definition change has very little effect by itself.
For example when one is on the negative side of the X axis, students will
tend to reverse the signs of lots of kinematic quantities even though the
motion is identical to a situation on the positive side.  They will say that
the velocity is zero at time zero if a position time graph goes through the
origin.  The flip side of this is that they want all graphs to go thorough
the origin.  And of course there is the way they want acceleration to be
zero whenever velocity is zero such as at the top of a trajectory.  They
have to start thinking and analyzing rather than just recalling what they
thought they saw.

I always tell them that deceleration is not a scientific word, so we will
not use it.  We will use acceleration, speeding up, and slowing down.
However their common usage of these words is deeply embedded and still come
out just as they want to use "straight line" as a synonym for horizontal
line.  This latter one is especially resistant to change.  I like the term
flat line because some students simply do not have the word horizontal in
their active vocabularies.

John M. Clement
Houston, TX


> All of this has been enlightening regarding common misconceptions that
> can confuse students, and possible ways out of the confusion.
>
> If we start with the definition of acceleration as the change in the
> VECTOR quantity velocity WRT time, I think we avoid the misconception.
>   A lot has been said using the term speed, so that is an addition to
> the confusion, which is unnecessary.
>
> If a=(delta v)/(delta t), the minus sign possiblilites become
> apparent, provided you insist that change in a vector quantity is
> always the 2nd value minus the 1st value.
>
> If a is negative, it is because v2-v1 is negative.
>
> Assuming (as in the discussion) that you have picked + velocity to be
> associated with increasing distance along some sign line, v2-v1 may be
> negative as a result of each permutation of the +/- choices which
> yield a *difference* which is (-), I think students can be drawn to
> pick all the possibilities and describe their physical realities.  Karl
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