Re: Kinematics First

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

> -----Original Message-----
> From: Rick Tarara [mailto:rtarara@SAINTMARYS.EDU]
> Sent: Tuesday, September 17, 2002 8:54 AM
>
> I just don't see how you can introduce Newton's Laws successfully if
> students don't understand ACCELERATION.

I agree it can be difficult to see how this can be done.

>  I spend considerable
> time working
> on just the concepts of speed, velocity, acceleration.

I suspect you'd need to spend less time if you presented it the other way.

>  If
> students can
> consistently recognize situations in which an object is
> accelerating I'm
> happy.

I'd argue that one needn't have a formal definition of acceleration in order
to do this.

>  If they can then tell me the direction of the
> acceleration, they
> 'have it'.  At that point I can introduce kinematics and then
> dynamics.

It isn't clear to me why they must master this point *before* dynamics
rather than hand-in-hand.  It also isn't clear to me why students need a
formal definition of acceleration before they can identify the direction of
acceleration.

>  I
> want them to recognize situations where a non-zero Net Force
> must exist or
> to recognize situations where the forces must vector sum to
> zero.

Note that this does not require an ability to identify the direction of
acceleration.  It only requires them to recognize when the velocity is
changing.

>  I want
> them to recognize when a set of forces will cause a change in
> the motion.

Again, this does not seem to require an ability to identify the direction of
acceleration.

> Since these dynamics goals require an understanding of
> acceleration, I still
> think acceleration is the right starting point.

Yes, some understanding of the difference between "force" and
"effect/acceleration" is necessary as well as an understanding that the
"effect" is not the same as "speed".  These ideas can be explored at the
same time, I think.  If you introduce it before, I don't believe students
really recognize *why* they need to be so picky about the definition of
acceleration.

> Unless you have tested (conceptual style questions) for student
> understanding of acceleration, you would probably be surprised at how
> difficult a concept it is.  I use my favorite motion--ball
> thrown straight
> up and later caught--to test this.  It is amazing how
> difficult it is for
> students to understand that (out of the hand) the ball is always
> accelerating downwards.  We go over and over 1-dimensional
> accelerations (if
> an object speeds up the acceleration is in the direction of
> motion and if it
> slows down the acceleration is in the opposite direction of
> the motion) but
> the concept that the ball going up is accelerating downward
> is _really_
> difficult.

I've used a question like this on my students.  Last year, after a somewhat
haphazard approach using dynamics first (my first try), 58 out of 59
students correctly answered this question on a test (the one who got it
wrong never came to class).  However, only 52 out of 59 students correctly
answered "downward" when asked about the ball at its highest point.
Furthermore, when asked if the acceleration was constant or changing on the
way up, only half of the students answered correctly.  I hope to improve
upon this this semester.

>  Ultimately, one shouldn't jump into the
> kinematics equations
> without knowing that most of the class can deal with
> acceleration on this
> conceptual level.

I agree with this 100%.  That is why I leave it until after dynamics.  I
believe dynamics allows students to explore acceleration on a conceptual
level.
____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; 570-422-3428; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301