A fundamental problem-solving skill in physics is the ability to choose
simplifying models that are both useful and appropriate. If you're going to
toss out immoveable objects, then you also need to throw out massless
pulleys and string, frictionless surfaces, zero resistance wires, etc.
Unfortunately, the choice of which simplifying model to use is usually
already decided for the student, and even then, seldom explicitly recognized
as being either a model or a choice. And students are hardly ever placed in
a position to evaluate a model's usefulness or to recognize when a model
fails.
-----Original Message-----
From: Mike Viotti
Who cares? There's no such thing as either an unstoppable object or an
unmovable object. (...)
But no matter what you tell them, it isn't physics.