According to the Subject: line and the previous discussion,
we are mainly discussing the statement:
momentum does not dissipate as heat. [1]
I don't see what the fuss is about.
Sure, the long passage quoted in the original post is
open to misinterpretation, but I reckon _anything_ can
be misinterpreted if you work at it hard enough.
Statement [1] is 100% true and important. Specifically:
-- Both energy and momentum can flow across the boundary
from one system to another, but
-- Mechanical energy can be _dissipated_ in such a way
that it disappears from both systems ... whereas
momentum cannot be _dissipated_ in this way.
I speak of "mechanical energy" because in problems of
this kind, students are expected to quantify only the
mechanical energy, in contrast to thermal energy which
they are /not/ expected to quantify. You will notice
the statement of the problem dit not include any info
about heat capacity or other information that would
permit a quantitative discussion of thermal energy.
We agree that the quoted passage does not do a good
job of _explaining_ why mechanical energy can be
dissipated while momentum cannot ... but it was not
intended to be an explanatory passage; it is just
a lab exercise question.
I don't think a microscopic exegesis of this passage
is worth the trouble, but if you insist on asking,
I would say that the most dubious word in the whole
passage is the word "because", which is dubious in
the following way:
-- Truly, the loop-de-loop experiment is different
from the dart experiment.
-- Truly, momentum does not dissipate as heat, not
in this context or any other.
-- It's not clear that the two experiments are
mainly different "because" momentum cannot be
dissipated as heat. I would say that the dart
experiment contains built-in procedures that
(approximately!) account for dissipation of
mechanical energy by extrapolating back to t=0,
so that the question of momentum dissipation or
lack thereof does not arise ... in contrast to
the loop-de-loop experiment where extrapolation
is not done and would not tell us what we need
to know anyway.