Re: ENERGY WITH Q

["John S. Denker" <jsd@MONMOUTH.COM>]

At 09:01 AM 10/30/01 -0600, RAUBER, JOEL wrote:
> But is it true that F dot ds_cm is Delta KE?  Which I think is the claim
> Carl is making.

Good question.

At 09:35 AM 10/30/01 -0500, Bob Sciamanda wrote:
> The integral of the net external force over the trajectory of the system
> CM will ALWAYS equal the system KE change.
> See previous (1999?) list discussions.

1) Wrong answer.

Consider a flywheel, starting from rest.  I spin the thing up by applying
equal and opposite forces to opposite points on the rim.  The net force is
zero.  The net force on the CM is certainly zero.  The motion of the CM is
zero.  The net external force dotted on the trajectory of the CM is zero
squared.  Yet the kinetic energy increases.

2) Even if some statement along these lines were true, it would be useless
except in certain unrealistically-simple situations.
 -- It might be useful if we knew _a priori_ that nonthermal energy
transfer were the only thing going on.
 -- It might be useful if we knew _a priori_ that thermal conduction or
similar purely-thermal energy transfer were the only thing going on.
 -- But in general we don't know any such thing in typical thermodynamic
situations.  I apply a force to some point on the system.  I have no idea
what the CM is doing.  I don't know the instantaneous force on the CM, and
I don't know the instantaneous motion of the CM.

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

Look, you can't do thermodynamics without entropy.  You just can't.  Give
it up now, and save yourself a lot of bother.  Without discussing entropy,
you can't explain the distinction between a spinning flywheel and a warm
flywheel no matter how carefully you keep track of the macroscopic force
and macroscopic motion.
  http://www.monmouth.com/~jsd/physics/thermo-laws.htm#sec-flywheel