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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 anstudent's
unstoppable force. We can't have an immovable object. Use the
question to foster critical thinking, fine. But it's not physics. Byhe/she
pretending that it is, we're really depriving the student the clarity
deserves. We say that physics is used to model the real world, so howcan
we possibly extend its uses to something that is not an element of thereal
world?real
Massless pulleys (etc.) are different, because we acknowledge that the
world doesn't exactly work that way, but we have to start with simplecases
and work our way up. We are honest with students about this; we don'tpull
the wool over their eyes. In my view, there is a distinct differenceone
between approximations that simplify a complex problem to a manageable
and artificial constructs that attempt to physically explain somethingthat
cannot possibly exist. The separation between those two groups is not
trivial.