Carl,
There is a historical reason for Newton's first law to be both first and
worded the way it is. Aristotle talks about the absurdity of those
philosophers that say that a body put in motion will continue in straight
line motion. He says that if that were true when you throw something it
would leave the earth and continue on out of the celestial spheres. Thus
for unnatural motion (not up or down) an object needs a mover to cause it
to move.
Newton used this wording to show his contemporaries that he was replacing
Aristotle's physics with one of his own, and that the way out of
Aristotle's conundrum was an outside force. His second law defines that
force as being proportional to the rate of change of motion (momentum to
us). Etc.
I usually save a full blown discussion of the inertial frame until I am
talking about rotating frames of reference because it give me a counter
example. It is hard to see why something is important until you can see
that it is not always true!
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU