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

[Mike Viotti <mike.viotti@gmail.com>]

Richard, I couldn't agree more.  Let's teach about the applicability of
models.  Let's say that we can treat the earth as immovable and the car as
unstoppable in their cases, and let's discuss the limitation of those things
(for example, we wouldn't treat the car as unstoppable if the mosquito was,
instead, a barrel full of water).

But let's not pretend to stretch the bounds of what is governed by physics
to areas where they cannot go.  We can TREAT the car as if it is
unstoppable.  Is it actually unstoppable?  No, of course not.  Okay, so what
if there was something that was ACTUALLY unstoppable?  Well, it turns out,
that nothing can ever be ACTUALLY unstoppable, so physics won't apply to
it.  My explanation and methods aren't incompatible with yours; the emphasis
is just different.

On Wed, Feb 23, 2011 at 3:15 PM, Zajac, Richard <rzajac@sal.ksu.edu> wrote:

> 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.
>
>
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