Re: Newton's first law

["John M. Clement" <clement@HAL-PC.ORG>]

This is a very broad difficult issue, as many texts differ in their
treatment of the first law.  From a pedagogical point of view, one answer is
quite clear.  The statement that N1 defines an inertial reference frame is
far to difficult and abstract for the beginning student.  Most introductory
lower level texts start by defining it as "A body in motion tends to stay in
motion, and a body at rest tends to stay at rest when the net force is on it
is zero".  Often the last clause is ignored.  Even this is far
to abstract for most students.

A good method of introducing the first law is via a laboratory exercise
where they see, using low friction carts that indeed things tend to stay in
motion.  Then they need to see cases with obvious forces which are in
balance.  Examples of these would be inclined ramps with friction where the
object travels at a constant speed, or balanced opposing fans on carts.
They are then confronted with the reality that balanced forces have no
effect on the motion.  These sort of exercises are the basis for Workshop
Physics, Real Time Physics, Modeling, and some other curricula.  Essentially
you can introduce N1 without talking about it, then later you give the
definition.  This is in line with the method in Arnold Arons book.
Introduce concept first, definition later.  I would also like to point out that
conventional demonstrations have low effectiveness in improving conceptual understanding,
while interactive lecture demonstrations are much better.

Now at this point N1 is essentially the zero force case of N2.  For a
beginning course in physics, this is a good point to stop.  I would defer
the discussion of inertial reference frames to later courses, for the
physics students.  Since the beginning students come into the usual intro.
course with essentially no understanding of physics concepts, the main job
confronting the instructor is to get the student to confront their
misconceptions, and gain a connected framework of ideas.

I currently teach both an advanced and regular HS physics course, but in the
past I have taught engineers, and non science majors at a University.  I
have come to realize that there is little difference between the
misconceptions held by HS students, and first year college students, and the
average HS physics course does almost nothing to improve student
understanding of the concepts.

John M. Clement
St. Pius HS, Houston, TX

> I'm interested in how folks introduce Newton's first law (N1) to
> their classes. (Presumably the answer depends on the level of the
> class, so include that info in your answer.)
>
> A typical textbook says something to the effect that "N1 gives us a
> criterion for determining if a reference frame is inertial"
> (paraphrasing from Tipler). I have several problems with that:
>
> (1) I like to distinguish "laws" (things about the world that do not
> follow from mathematical deduction using definitions) and
> "definitions" (useful but ultimately arbitrary decisions physicists
> make). The above phrasing seems to reduce, maybe not to a logical
> definition of "inertial frame" itself, but certainly at least to an
> operational one.
>
> (2) Even as an operational definition, it puzzles me. Suppose I plunk
> you down in some cubicle attached to some reference frame. You cannot
> look outside (at the "fixed stars" presumably--BTW why can we say
> they are "fixed"?), but I'll otherwise give you whatever instruments
> you want. Your assignment, should you choose to accept it, is to
> *experimentally* determine if your reference frame is inertial using
> NI alone. Presumably you may find there is a net force on some test
> object, so I will also allow you a test object, as well as a spring
> to cancel that net force out. My challenge thus boil downs (I think)
> to two related questions: (i) how do you choose a suitable test
> object; and (ii) how do you know you've measured every last force on
> it?
> --
> Dr. Carl E. Mungan, Asst. Prof. of Physics   410-293-6680 (O) -3729 (F)
> U.S. Naval Academy, Annapolis, MD 21402-5026     mailto:mungan@usna.edu
>              http://physics.usna.edu/physics/faculty/mungan/