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I think most of our current textbooks don't do
it justice. It's a lot harder for new students to swallow than either
N2 or N3. I start out in my non-honors HS class (after enough
introductory stuff that the students understand velocity, at least in
one dimension) with conservation of momentum as an empirical
observation (We don't know why it's true. I just seems to be the way
the world works.). We spend about 2 weeks investigating this idea
both in class and in the lab. From there the progression to N3, N2
and N1, in that order, is pretty straightforward (I didn't say easy,
but at least it isn't just holy writ). But N1 still has baggage with
it that the students have a great deal of trouble with, so I
sympathize with them, by pointing out that Aristotle didn't think
there was anything like N1, and it wasn't until Galileo that anyone
gave the idea of inertia any serious thought, and that was 2000 years
after Aristotle! So, I ask them, is it reasonable to ask you to buy
into the idea of N1 in a few class periods when all of civilization
took 2000 years to first get there? Once I have disarmed them with
the idea that it is OK if you have trouble with this concept, then we
do some classroom examples with relative motion that play with their
minds a bit and illustrate that, if, as they saw at the end of the
momentum section, force is something that *changes* momentum, then a
constant momentum must imply the absence of force. A few reminders
that constant momentum doesn't necessarily mean zero momentum and
most of them are comfortable with the idea. That, and lots of
reinforcement usually get almost all of them on board by the end of
the first semester. But I don't think it is reasonable to expect them
to buy into the idea, which is *so* non-intuitive, without some
serious work on their part.