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Re: [Phys-l] order of topics (was Newton's first law)



On 11/03/2011 08:56 AM, Bill Nettles wrote:
If we want to build the idea of masses or charges interacting with
gravitational or electrical fields, resulting in forces, shouldn’t we
introduce fields first?

The short answer is that there are eleventeen factorial ways of
re-ordering the material ... and they all involve tradeoffs.
They all have advantages and disadvantages.

The only thing that has ever worked AFAICT is the spiral approach.

There are plenty of good reasons for starting with the fields,
including the fact that you can visualize the electrostatic
field using various hands-on demos, especially during the
winter when the indoor air is dry.

There are also plenty of good reasons for not doing that,
including the fact that for 99% of the practical applications
of gravitation, you don't need to worry about the fact that
it is a "field". That is, you don't need to worry about the
fact that the g-vector is a function of position. It's just
a constant vector. There are plenty of people who have no
clue about vector fields, or even about scalar fields, but
that does not prevent them from solving practical problems
involving gravitation.

It's true as a matter of history that the guys who figured
out the laws of motion, Galileo and Newton, used cosmological
evidence to show them the way ... and on cosmological length
scales you do have to treat the g-vector as a vector field
... but again I say that retracing history is rarely the best
way to teach a subject or to do practical problems.

Returning to the main point: There are eleventeen factorial
ways of re-ordering the material.

Indeed one perfectly reasonable starting point doesn't mention
forces at all, but rather concentrates on /energy/. There is
quite a lot you can do with this, including pendulums and
even interrupted pendulums:
http://www.av8n.com/physics/loop-de-loop.htm
This has the advantage of minimizing the amount of concepts
that need to be introduced before getting to the first
examples ... but still, it is just one of many perfectly
reasonable approaches.