When calculating how long it takes a dropped ball to reach the ground, we
implicitly choose to treat the earth as an immoveable object.
When we ask students to calculate the impulse suffered by a hovering
mosquito that gets smushed onto the windshield of a moving car, it is useful
to treat the car as unstoppable.
The immoveable earth and the unstoppable car can both (in separate
instances) belong to the category of useful models, and constitute valid
physics.
How much more difficult must it then be for a student to grasp the
distinction you propose, when either "artificial construct" can constitute a
valid approximation? A lesson in recognizing when either approximation is
valid is truly a lesson in doing good physics.
-----Original Message-----
From: Mike Viotti
Sent: Wednesday, February 23, 2011 1:29 PM
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.