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When we were talking about a beaker of water on a pan balance, with
a steel ball having been lowered intoit, I considered the beaker of
water as the system, hence my system was what the engineers call a
control mass. Because the steel ball was in contact with the
system, momentum was flowing from the steel ball into the system.
You insisted on considering a control volume to be the system. To
my mind, by doing so, you complicated the problem in that you then
had to concern yourself with momentum flow in the string that was
supporting the steel ball. Why the insistence on a control volume
in this case? To me, your introduction of the equation dE=PdV+TdS
indicates that you are comfortable with using a control mass as your
system.
[.....] rotating bullet [.....]
I can see considering the bullet to consist of two parts, a nose part
and a tail part for instance, and each of those parts can be a
subsystem, but I'm not familiar with the way you used the term
subsystem as a subset of the degrees of freedom. Is that a common
use of the expression?