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



This gets into a common preconception that students have. When doing a lab
where they explore acceleration, force, mass they should always predict
(hypothesize?) what they think they might see. This should be an
acceleration vs mass, and acceleration vs force graph. Surprisingly many
just draw a straight line in a vs m so the acceleration goes to zero at a
specific mass. They are surprised to see that it never seems to go to zero.

When you tell them the equation, they miss this idea. But if they do the
lab and figure out the equation, they have a much better view of what goes
on. Similarly they can see that almost zero force does not produce zero
acceleration. So the idea of an immovable object is not actually possible,
although there may be objects which are immovable by you with your existing
tools.

Actually looking at the end points of a give graph is an important way to
figure out what the equation might be. So the limiting cases while not
necessarily possible, are important considerations.

John M. Clement
Houston, TX


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.