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Re: [Phys-l] irresistible force v. immovable object

Here's one last reply from me before I step back into the shadows and let
you all go on with the discussion.

If a student came to me and said, "Yo teach, what happens when an
unstoppable force meets an immovable object?" I'd say "well, what's an
unstoppable force? Can any force truly be unstoppable? How do you know?"
The discussion would progress from there. Same thing with an immovable
object. "How is the object immovable? Why can't I move it?" Newtonian
mechanics (and even GR, if I'm not mistaken) does not allow either of these
things.

Really examining these questions on a deep level is not trivial, and it is
certainly not just "doing what is in the book" - it is authentic scientific
thinking. If the student clarifies her question and arrives at what John D
believes the underlying "real" question might be (infinity times zero, or
something along those lines), we can talk about calculus and limits. I love
having that discussion with students! If the student wants to talk about
quantized space-time, then that's fine, we can talk about that, though I
don't think I would introduce it to the student myself because I don't think
it's an exploration worth going into regarding this particular question.
There's nothing corrupt about any of it, it is just a matter of answering
the question that is asked.

But, if a student wants to TRULY know about an immovable object, then I (and
physics in general) cannot help her. Physics is a set of rules that models
the way the universe works, and I don't have a rule that describes an
immovable object. It's not excessive rigor, and it's certainly not a cop
out! Grasping that there is no such thing as an immovable object or
unstoppable force (whatever that's even supposed to be) is not easy. Please
don't brush it off as something that students will automatically understand
on the deepest possible level. I had to think about it for a while before I
really understood it, and I wager many students are the same way. If you
think that the Newtonian answer to the question isn't enough to entice a
student into further exploration, then I suppose you have a reason to bring
up quantized space-time; but would you really jump straight there? I think
there's more value in getting the student to ask the right question rather
than telling him an answer (albeit an interesting one) to a question he
didn't ask. I don't think you're giving the Newtonian answer enough credit
in its depth.

Chuck, you claim (and I agree with you) that if physics can't be made
interesting and relevant, it shouldn't be taught. How exactly is
introducing quantized space-time relevant to this specific problem? How
would you use that concept to answer this specific question? Even giving
you the benefit of the doubt, I think this is very student specific. One
might take that answer and say, "Wow, my physics teacher just made physics
so interesting! It turns out space-time is quantized! That's CRAZY!"
Another might say, "Man, my physics teacher is an ass. I asked him about
unstoppable forces and immovable objects, and he just told me something
about the Planck length scale. Can't these guys give me a straight answer
about anything?"

Given what I know about physics, which I'm sure is not as much as many
members on this list, I feel comfortable claiming that truly immovable
objects and unstoppable forces are not physics, they are philosophy; the
question itself is useful for physics thinking in as much as it leads to
understanding this. Beyond that, physics can provide no true answer.
Physics has boundaries. There are things we cannot prove, and there are
limitations on the things we can prove and the models we use. Educating
students as to the role of physics is one of our chief goals as teachers, as
is generating motivation for future involvement in the discipline. We can
do both without sacrificing either.

On Wed, Feb 23, 2011 at 5:54 PM, chuck britton <cvbritton@mac.com> wrote:

At 4:58 PM -0500 2/23/11, Mike Viotti wrote:
It was specifically aimed at Chuck who mentioned
that infinitesimals might not be allowed because space-time is discrete.
Ask yourself this: what is more important to an introductory physics
student, the concept that a small net force will accelerate even a massive
object, or the reconciliation between QM and GR in terms of quantized
space-time?

Yeah, this might indeed have been one of my problems with teaching
intro physics for those decades.
I always wanted to do more than the 'book'.
I'll say that it worked for a number of kids who went on to Harvey
Mudd, CalTech, MIT and such
as WELL as some who were right in off the tobacco farms and
considered NCSU to be a great school before they moved on to do
'Great Things' in Silicon Valley or such.

I was fascinated early on by infinities and such, and didn't consider
them to be the 'Corrupters of Youth' that many of Cantors foes did.

Too much rigor teaches Rigor Mortis.
If Physics can't be made interesting and relevant - it shouldn't be taught.

(Just My Opinions)
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