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Re: Force or acceleration first (was The sign of g)

Now this is interesting...we've come back to the discussion that had the
subject line "Re: kinematics, traditional or not". As I noted in my post
<http://lists.nau.edu/cgi-bin/wa?A2=ind0208&L=phys-l&P=R25334>
in which I tried to addressed Tim's problem with force first, there are two
ways to approach the problem: (1) empirically or (2) theoretically. I see
we now agree.

That said, let me restate my position:

I think one can do it either way.

I needn't say any more but I'd like to expand upon my reasoning to follow
option (2).

In my case, I will have already exposed my students to the empirical
methodology when they empirically identify that the ratio of distance/time
is constant for my toy cars. As I point out to my students, it may fun to
go through this empirical process for each problem we encounter but it can
become quite tedious, especially when the problems are ones we will rarely
see again (like being asked how long it takes an object to fall).

Consequently, rather than identify the specific relationship between the
distance traveled by a falling object and the elapsed time, I prefer to use
the topic of "falling objects" to introduce them to a much more powerful
technique, i.e., the use of general laws and theories.

We all know why such an approach is so powerful but I don't think students
get an opportunity to really see it. I use Albert Einstein's analogy about
how from the top floor window of a building, one can see the tops of smaller
buildings nearby. However, one cannot see the top of one's own building.
To do so, one must climb to the top of a taller building. The taller
building not only allows us to see the top of our previous building but also
several more buildings, which were too tall to be seen from our previous
building.

I think the topic of falling bodies allows me to demonstrate this quite
well.
____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; 570-422-3428; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301

-----Original Message-----
From: Tim Folkerts [mailto:tfolkert@FHSU.EDU]
Sent: Wednesday, September 04, 2002 6:23 PM
To: PHYS-L@lists.nau.edu
Subject: Force or acceleration first (was The sign of g)


The discussion of "g" seems to me to be a bit like the chicken & egg
question. There are two intertwined ideas that can't really
be separated.
You either have to say "you will understand acceleration
better after you
understand force" or " you will understand force better after you
understand acceleration".

Several people have argued that you need forces before it
makes sense to
discuss g (or even a), but I don't agree. Perhaps its because I'm an
experimentalist, but I definitely see theories following from
experiments.
In this case, the experiment is that I can drop 100 different objects,
measure x vs t, calculate a, and find that (within limits)
all of them have
an acceleration downward of 9.8 m/s^2. I don't need to know
anything about
"force" or "Second Law" or "gravity" to notice this pattern in nature.

I'm starting to see this as a theoretician vs experimentalist
perspective
on the question. So instead of the biological question
"Which came first,
the chicken or the egg?" we might ask "Which came first, the
experiment or
the theory?"

My own answers are
* Two birds a lot like chickens mated and produced an egg. That egg
produced a bird that was enough different than either one
that it was a new
species, that has come to be called a chicken. Thus the egg
came first by
1/2 a generation. However, since that occurred millions of
generations
ago, the half generation difference is of little practical
concern. Eggs
lead to chickens and chickens lead to eggs. All modern
chickens and modern
eggs are so completely intertwined that it makes little sense
to talk about
one without the other.
* You need at least some observations of patterns before you
think to form
a theory. However, we have become so accustomed to expecting
patterns that
often we will make up theories because they are analogous to previous
theories, or just because they are mathematically elegant. Theories
suggest experiments, and experiments suggest theories. The two are so
completely intertwined that it makes little sense to talk
about one without
the other.


For the case of g, the two perspectives might be summarized
as follows:

Experimentalist: "Falling things are worth looking at, so we measured
several things falling, and all had a = 9.8 m/s^2. Taking
into account N2,
let's try to come up with a theory of gravity that would cause such an
acceleration. WOW! That means F = mg and we have a simple
explanation that
agrees with the data."

Theoretician: "Let's assume that gravity causes a force of a
very simple
form, say F/m = g. Then according to N2, things should accelerate
downward uniformly. Let's try to confirm that. WOW!
Everything we check
does fall with a = 9.8 m/s^2, so our original conjecture must
have been
correct, and g = 9.8 m/s^2"

We measure things because we hope to find patterns. Or we
propose patterns
and hope that measurements fit our predictions. Both are common and
acceptable ways for science to proceed. So it is perfectly
reasonable that
g can be an experimental result to which we try to fit a
theory of gravity,
or g can be a theoretical prediction that needs testing.

--------------------------------

There is a second related, but different, question - will
the students
understand better with one approach over the other? The more common
textbook approach seems to be the "Experimentalist" approach
- present an
experimental result that g = 9.8 m/s^2 and seek the theory to explain.
I've said about enough and I'm running out of free time, so
I'll stop and
see what you all think about these ramblings.


Tim Folkerts


Department of Physics
Fort Hays State University
Hays, KS 67601
785-628-4501