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

• From: John Denker <jsd@av8n.com>
• Date: Mon, 22 Jan 2018 17:16:27 -0700

On 01/22/2018 03:03 PM, Jeffrey Schnick wrote:

Why are you writing about a uniform-velocity gas?

Velocity is a typo. I should have said speed. My bad.

As for the rest, I was trying to be as charitable as
possible to Hewitt. In computer science there is a
class of problems called NP-complete that are (probably!)
rather hard. Hewitt's question belongs to a class I
call ESP-complete, because the only way to begin to
it.

Hewitt didn't say anything about a uniform-velocity gas or liquid in
the question under discussion.

Not in the "question" strictly speaking ... but his
proffered /answer/ makes it clear that he imagined
the escape probability to depend on speed, perhaps
normally, perhaps always.

It wouldn't be fair to students to expect them to start
from Hewitt's answer and work backwards, but it's more
than fair to Hewitt.

Even /with/ the answer I find it impossible to make
sense of the question, even under the most charitable
assumptions.

In particular, he seems to think his liquid can evaporate
with no latent heat, so calling it a liquid instead of a
dense gas seems to be a distinction without a difference.

==

I did not previously publicly discuss the interpretation
that says the hypothetical liquid has a wide range of
speeds, but all speeds evaporate equally. So here goes.
This doesn't make any sense either. You could go in
there with a siphon and remove a parcel of liquid, with
the original liquid speed-distribution, and this would
leave the temperature unchanged ... but we call that
siphoning, not evaporation. At some point you have to
convert that parcel from liquid to vapor, which involves
the latent heat somehow. So either something cools, or
you provide heat from outside, or ... I give up. It
was meant to be a simple question, and nobody has yet
suggested anything resembling a simple-yet-correct
interpretation.

=====================

If anybody can come up with a set of assumptions that
allow the question to be mapped onto real physics,