Re: force

[Dave Hamilton <djhamil@teleport.com>]

Refering to students difficulties with kinematics, Dewey writes:

> it is very clear that their
> "street" notions concerning acceleration (acceleration is speeding up,
> slowing down is *either* deceleration or negative acceleration, and there
> can be no acceleration if the speed or velocity is zero) is NOT a mere
> sub-set of the "scientists'" notion of acceleration.  Acceleration is *just
> not the same entity* for them as it is for "us."

This is fully consistent with the conclusions I have drawn from my 
research into students' preinstructional knowledge of motion.  Students' 
in my study seemed to think of motion events holistically.  For example, 
a penny tossed into the air is considered a single motion event and it 
does not necessarily occur to the student to think about the motion of 
the penny as changing through time (accelerating).  My best guess as to 
what the phrase "a change in motion" means to most students is that one 
retosses the penny in a different way.  This is surely a whole different 
paradigm from ours.  Consequently I have concentrated on presenting 
situations where students are forced to describe instantaneous changes in 
motion within a single motion event.  That's where the sonic rangers are 
exceptionally helpful.
> As is implied in the Chapter in the Fosnot edited book, a consideration of
> statics appears not to even yield a net force idea for many students, at
> least not one which shows up when they move to consider forces on moving
> objects.  Maybe the problem is that they do not see "no motion" as merely a
> case of "constant velocity."  The fact that they do not generally see "no
> motion" or "stopped" as such is also documented in the research literature,
> I'm pretty sure, but I can't give a ref right off hand.  Hence, they do not
> seem generally to be inclined or see it natural to extrapolate things
> learned about the static situation to the moving one.  This seems to come,
> looking back on thing, rather than when moving on to what they consider a
> different situation.
>
> Dewey


David J. Hamilton, Ed.D.                    "And gladly wolde he lerne,
Franklin HS, Portland, OR                    and gladly teche."
djhamil@teleport.com                          Geoffrey Chaucer