Re: [Phys-l] A simple (?) question about speed

[Rick Tarara <rtarara@saintmarys.edu>]

----- Original Message ----- 
From: "Paul Lulai" <plulai@stanthony.k12.mn.us>
> These difficulties are one of the reasons the fci seems to be a good
> measure of instruction.

The FCI is much more complex and much more subtle than the difficulties 
being discussed in this thread.  The FCI probes DEEP understanding of 
Newton's Laws, but really little else.  Seeing it as some kind of assessment 
panacea is seriously over-reaching here.  Acceleration--understanding it--is 
the key to getting started in understanding motion.  Now if students don't 
understand 'constant' or 'initial' and 'final' then there are perhaps more 
problems than most physics teachers can really handle.  OTOH, understanding 
that in any sequence of events one can pick any number of 'initial' and 
'final' times to analyze, IS a physics question and something you may (will) 
need to address explicitly.

Two simple mantras about acceleration seem to help.  'An object is 
accelerating if it speeds up, slows down, or changes direction.  For one 
dimensional motion, if an object speeds up the acceleration is in the 
direction of motion and if it slows down the acceleration is opposite the 
direction of motion.  As simple as these two statements may seem to you and 
me, they are not at all obvious in application for students.  Given many 
different motions (the motion of a vertically thrown ball is a good example) 
student will have trouble identifying both whether or not the object is 
accelerating and if it is, the direction of the acceleration.  IMO, that is 
where you really need to spend time and effort at the start of a course in 
kinematics/dynamics.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
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