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Re: [Phys-L] Figuring Physics solution Jan 2018

The immovable object: Immovable with respect to what?
The unstoppable force: there cannot be a force unless it would be
something acting on a mass and changing its velocity or direction.
If we cannot determine whether an object is moving, then we cannot
determine what an unstoppable force would be.

So we ignore all of that and try it anyway and get comingling of molecules,
none of which will be at rest.

So this might be something approached "in the limit," and practically any
astronomer can tell some stories about some limits which are pretty far
out, such as colliding black holes, colliding galaxies; And closer to home
we know of devastating collisions between meteors and this ball we ride on.

On Mon, Jan 22, 2018 at 1:28 PM, Paul Nord <> wrote:

What would happen if an unstoppable force hit an immovable object?


It was an odd number, so the answer was in the back!

Are you going to leave us in suspense, Anthony? What was the answer?


On Mon, Jan 22, 2018 at 11:27 AM, Anthony Lapinski <>

The universe is complicated. Nobody fully understands it (or ever will).
Most intro courses try to simply things.

In my college logic book there was this question:

What would happen if an unstoppable force hit an immovable object?


It was an odd number, so the answer was in the back!

On Mon, Jan 22, 2018 at 12:11 PM, Philip Keller <>

I have run into this issue in other contexts. You try to make a point
raising a counterfactual -- what would happen if the universe were
different than it is? Sometimes this works, but as a teaching method
frequently runs into trouble. Your students (or your colleagues on the
list serve) start thinking about the implications of what you have
suggested and they start finding all kinds of ways that your little
counterfactual leads to weird results. It is tempting to try to prop
your original questions: "No, no, no -- I just want you to consider
would happen if the universe were different in the one way and no
But it is no use...your audience persists in their reasoning and your
counterfactual leads to not just the limited conclusion you hoped for
but a
lot of other conflicts with the laws of physics.

But maybe that is not the worst thing that can happen. It shows that
threads of the tapestry are woven together.

In case you are curious, here are two counterfactual-type questions
tried (and failed with) in the past:

What would happen if an object exerted forces on other objects but
experienced no reactions?

What would happen if induced current flowed in the opposite direction
the one indicated by Lenz's law? (Discussions here on this list helped
with this one.)

On Mon, Jan 22, 2018 at 11:26 AM, Paul Nord <>


It seems that the physics concept Hewitt is trying to show is simply
'liquids cool by evaporation because the faster molecules leave the
surface.' That concept seems fundamentally sound.

We might like to add a few caveats about surface tension, energy
distribution, and equilibrium. But isn't it basically true that if
strange liquid existed, it wouldn't cool by evaporation?


(P.S. I had a bigger gripe with December's Figuring Physics when it
referred to the momentum of the windshield and not the momentum of
entire car. That was an odd mistake for Hewitt.)

On Sat, Jan 20, 2018 at 10:18 PM, John Denker via Phys-l <> wrote:

On 01/20/2018 08:59 PM, Jeffrey Schnick wrote:

(I think the explanation given in the Physics Teacher is

It's not correct, conceptually or otherwise.

As Robert Cohen pointed out at the beginning of this thread,
when the molecule leaves the liquid, it loses its binding
energy (van der Waals or whatever). If it loses energy
but there is no cooling, what happens to conservation of

Also, if the explanation were correct it would apply
equally to evaporation (sublimation) from a solid.
It's even more obviously wrong in that case.
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