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



Just to further pound home my point, this is a perfect teaching example
about the impossibility of an immovable object. F = ma says that any mass
can be accelerated with even the most minuscule of net forces. THAT is the
teaching moment, in my opinion.

2011/2/23 Mike Viotti <mike.viotti@gmail.com>

I totally agree that situations like these present teachable moments, but
they're not physics. By definition they can't be! We can't have an
unstoppable force. We can't have an immovable object. Use the student's
question to foster critical thinking, fine. But it's not physics. By
pretending that it is, we're really depriving the student the clarity he/she
deserves. We say that physics is used to model the real world, so how can
we possibly extend its uses to something that is not an element of the real
world?

Massless pulleys (etc.) are different, because we acknowledge that the real
world doesn't exactly work that way, but we have to start with simple cases
and work our way up. We are honest with students about this; we don't pull
the wool over their eyes. In my view, there is a distinct difference
between approximations that simplify a complex problem to a manageable one
and artificial constructs that attempt to physically explain something that
cannot possibly exist. The separation between those two groups is not
trivial.



On Wed, Feb 23, 2011 at 2:16 PM, John Denker <jsd@av8n.com> wrote:

On 02/23/2011 11:25 AM, Mike Viotti wrote:
Here's an equally valid question:

What happens if a blorp comes into thermal contact with a slurg and all
the
marfs are turned into storgs?

Who cares? There's no such thing as either an unstoppable object or an
unmovable object. Everything can be stopped and everything can be
moved.
As John pointed out, this is not a physics question, it's a philosophy
one.
Just so long as students understand the domain of the problem, you can
tell
them whatever you want. Hell, go ahead and agree that the world will
end.
But no matter what you tell them, it isn't physics.

I wouldn't have said that. The example is not equally valid.
There *are* physics problems that closely correspond to the
original question.

As I see it, this is a case of the student's reach /slightly/
exceeding his grasp.

IMHO it is a glorious achievement that humans can even imagine
the idea of infinity. We can sometimes make sense of things
like infinity over infinity or zero over zero ... and we can
systematically separate the cases that make sense from the ones
that don't.

On the other side of the same coin, this is a tricky business.
The fact that students have a hard time with it does not come
as a surprise, and does not bother me in the slightest.

Slurgs are not physics, but lim(Δy/Δx) is physics. When the
student starts asking about such things, it's a teachable
moment ... or at least a motivational moment. Tell 'em the
precalculus course and especially the calculus course will
allow them to make sense of such questions. Newton invented
calculus so he could do physics, specifically so he could
deal with physics problems of this ilk.

Another well-regarded resource suitable for high-school (and
younger) readers is:
George Gamow
_One, Two, Three ... Infinity_
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