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/