Re: Newton's first law

[Maurice Barnhill <mvb@UDEL.EDU>]

Many students have trouble believing Newton's First Law is true even
after being told about it.  After all, in the real world friction of
various kinds makes it *untrue* if you don't know to or don't agree to
ignore these forces.  So I emphasize the idealization required to find
the law.  I talked about sliding a block on a surface using successively
larger amounts of grease to make the surface more slippery.  I also talk
about movement in "outer space." I show movement on an air track, and
then detach the air supply slowly so the students can see the build-up
of friction.  None of this is perfect.  Space movement is not really
unaccelerated and I may be building trouble for myself when I talk about
circular motion. In the air track demo the size of the friction is going
the opposite way to what I wish them to visualize.  But in both cases I
am working with things they know about and which are visual concepts
instead of abstract ones.  All of mechanics can be discussed with
situations the students are familiar with, even though the results of
considering the situations carefully may be very unintuitive to them. I
like to take advantage of such situations as much as I can.  There is no
such familiarity with electricity and magnetism -- what little they know
is very far from the basic concepts.

     Granted I am working with honors students.  I have even once or
twice had classes with no student who would defend the existance of
outward forces in circular motion!  Not even by accident on exams.

     In any event I de-emphasize the philosophical issues, e. g. the
rigor of defining inertial frames with Newton's First Law, that we
ourselves find so interesting.  It's a useful "throw-away" line to set
up for special relativity when we get to it, but I wouldn't ask them to
remember it.  I will restate it when we actually use it and go from
there.  There are much simpler, much more important issues to handle in
their first physics classes.

"Carl E. Mungan" wrote:
> 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/

--
Maurice Barnhill (mvb@udel.edu)
Department of Physics and Astronomy
University of Delaware
Newark, DE 19716