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

• From: Bill Nettles <bnettles@uu.edu>
• Date: Mon, 29 Jan 2018 20:40:25 +0000

The original language was NOT "unstoppable force" but "irresistible force." I don't think those two adjectives are isomorphic. Rather than being a question/exercise, in the 3rd Edition of Copi it is in the text of the chapter, page 268, along with the answer "Everything!"

By the way, the book price in 1973 was \$8.50. Book inflation (size and price) is the irresistible force.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@mail.phys-l.org] On Behalf Of Anthony Lapinski
Sent: Monday, January 22, 2018 11:28 AM
To: Phys <Phys-L@phys-l.org>
Subject: Re: [Phys-L] Figuring Physics solution Jan 2018

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

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

Hmmmm...

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

On Mon, Jan 22, 2018 at 12:11 PM, Philip Keller <pkeller@holmdelschools.org>
wrote:

I have run into this issue in other contexts. You try to make a point
by raising a counterfactual -- what would happen if the universe were
different than it is? Sometimes this works, but as a teaching method
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
up your original questions: "No, no, no -- I just want you to
consider what would happen if the universe were different in the one way and no other!"
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
the threads of the tapestry are woven together.

In case you are curious, here are two counterfactual-type questions
that I 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
of the one indicated by Lenz's law? (Discussions here on this list
helped me with this one.)

On Mon, Jan 22, 2018 at 11:26 AM, Paul Nord <paul.nord@valpo.edu> wrote:

John,

It seems that the physics concept Hewitt is trying to show is simply
that '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
such
a
strange liquid existed, it wouldn't cool by evaporation?

Paul

(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
the entire car. That was an odd mistake for Hewitt.)

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

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

(I think the explanation given in the Physics Teacher is
conceptually
correct.)

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 energy??????

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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