Re: Student Questions

[John Barrer <forcejb@YAHOO.COM>]

Point well taken; I was unclear. I'm not advocating
the def'n of force via ma, simply that it's helpful to
start by identifying which objects are interacting
with a mass at rest (at first). Hold a mass in your
hand, ask the question. Hold a spring with the same
mass attached to the other end of the spring, ask the
same question. The idea is to build the concept that a
force results from the interaction (with or without
contact) between two objects. These interactions can
often (always??) be thought of and experienced as a
push or a pull. Before moving to acceleration, the
conceptual goal is to associate balanced forces with
unchanging motion. That is a BIG leap for many
students. John Barrere--- "John S. Denker"
<jsd@MONMOUTH.COM> wrote:
> Tina wrote:
>
> > > I am currently teaching forces to my general
> physics
> > > class.
>
> John Barrer wrote:
> > Start by getting them used to the question "What
> > objects are interacting with the mass?"
>
> If that means what I think it means, then I'm not
> convinced that's the right starting point.
>
> I don't subscribe to the school of thought that
> starts
> with position -> velocity -> acceleration and then
> _defines_ force in terms of F=ma.  (If force is
> defined
> that way then F=ma becomes a tautology.)
>
> Instead, I take the point of view (both for physics
> reasons and pedagogical reasons) that force exists
> unto itself.  (Therefore F=ma is a nontrivial
> expression, subject to experimental verification
> or falsification.)
>
> Specifically, I would hand the student one end of a
> rope.  I pull on the other end and say
>         "Feel that?  That's a force."
> I pull harder and say
>         "Feel that?  That's a larger magnitude of
> force."
> I pull off to the side and say
>         "Feel that?  That's a force in a different
> direction."
>
> You will note that these exemplary forces are
> defined
> absolutely without reference to any mass.


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