Re: Newton's first law

[Leigh Palmer <palmer@SFU.CA>]

>     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