Re: Newton's first law

[Maurice Barnhill <mvb@UDEL.EDU>]

Leigh,

I am painfully aware that many students do not believe Newton's First
Law after we are through with them.  I just disagree with you about
where the problem comes from.  My belief is that students come to us as
Aristotelians.  By the time they get to my class in college they may
have changed their minds, but certainly when they come to their first
physics instruction they are convinced *by their own observations* that
the natural movement of an object is to be at rest, and something must
be done to things to get them to move in any other way.  Aristotle was
no idiot, and that description of motion as seen on Earth is by far the
most natural one *until you are guided into thinking more deeply.*  On
the other hand, worrying about finding all the forces or exactly how you
define an unaccelerated frame is something we have to teach them about.
I don't believe that we should attempt this instruction when they are
struggling with an Aristotlean world view.

I do mention the issues, however, especially given my particular
audience.  I just don't emphasize them or in particular test on them.  I
want them to concentrate on first things first.

Introductory teaching necessarily oversimplifies something.  Otherwise
we would not need advanced courses.  The art is in finding what can and
must be simplified, which depends on the audience of course.  It is also
important to warn them that things are being simplified so that they
don't get the idea that they know physics after only one course.

I have also been the advisor of all physics majors at UD for as long as
anyone, including me, can remember.  With only 30-40 majors total, I try
to be a "mentoring" advisor rather than a "rule-giving" advisor, so I
have a fair idea of what bothers them in higher-level courses.  I
haven't heard anything to persuade me to change the way I handle this
issue.


Leigh Palmer wrote:
> >     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.
>
> This isn't a philosophical issue; it's a physical one. Newton's laws
> only work in an inertial frame of reference. Failure to point out
> that the mundane laboratory is *not* an inertial frame will mislead
> students (quite properly) to distrust Newton's laws and, by extension,
> all the rest that you tell them. They may not believe, deep down,
> that physics is a description of the real world. I have found second
> year students who were very successful in physics courses who really
> did not believe Newton's second law was a good description of the
> real world!
>
> I think this is not a small point, and it takes very little time (say
> three minutes initially and occasional reminders ten times or so when
> a teachable moment arises) to include it. I introduce the topic by
> stating Newton's first law and then demonstrating it by holding my
> office keys at arm's length and releasing them. I point out that my
> demonstration has failed to instantiate Newton's first law, and that
> there is a good reason, a reason we will account for soon by
> introducing the gravitational force concept. The teachable moments
> arrive frequently enough after that introduction that I only need to
> pull my office keys out of my pocket and hold them at arm's length to
> wordlessly reiterate the point (sometimes I even drop them again).
>
> Leigh

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