Re: ENERGY WITH Q

[Bob Sciamanda <trebor@VELOCITY.NET>]

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.
Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, October 30, 2001 9:28 AM
Subject: Re: ENERGY WITH Q


> At 08:30 AM 10/30/01 -0500, Carl E. Mungan wrote:
> > I'm confident you agree that for an object with or without internal
> > degrees of freedom and having total mass M, the net force (which is
> > of course external) on the object equals M times the acceleration of
> > its center of mass (CM). Now dot both sides with a differential
> > displacement of the CM and integrate. That gives the W-K thm.
>
> That's fine as stated.  Note the absence of internal degrees of
> freedom.  This is an important premise of the Work-KE theorem.
>
> > Applied to your cart example. If F is the *only* external horizontal
> > force on the cart, I hereby publicly guarantee it will gain KE,
> > regardless of what clock mechanism is inside it.
>
> No good.  The reasoning is invalid.  The premise of the Work-KE theorem
is
> not upheld, therefore we cannot conclude the conclusion.  This is a real
> problem.  It is not just a theoretical problem, because real-world
> counterexamples exist;  see below.
>
> > If you meant that there are other external horizontal forces, please
spell
> > out a free-body diagram for your clock-cart a bit more clearly.
>
> The cart was not a perfect example, so let me offer a better one.  This
is
> in some sense the reverse of the notorious skater who pushes off the
wall.
>          ***********************************
>          ***********************************
>          ***********************************
>          ****                        /R/
>          ****VVVVVVVVVVVVVVVVVVVVV====================|
>          ****                        \R\
>          ***********************************
>          ***********************************
>          ***********************************
>
> This object is freely-floating in a space station, so there are no
forces
> other than those explicitly mentioned here.  There is a massive frame
> denoted by "*" symbols.  There is a low-mass spring denoted by "V"
> symbols.  There is a low-mass plunger denoted by "=" symbols.  There is
a
> ratchet denoted by "/R/" that ensures that the plunger moves only
inward,
> not outward.
>
> I whack the end of the plunger with a hammer.  There is a large force
> applied over a nonzero distance.  I calculate F dot ds in the only way
that
> makes sense, namely the force (at the place where the force is applied)
and
> the distance (at the place where the force is applied).
>
> It is simply not true that calculating F dot ds tells me the increase in
> the kinetic energy of this object.  Some of the energy goes into
> compressing the spring.
>
> Nitpickers note:  The foregoing violates the spirit of the "public
> guarantee" because the gain in KE is much smaller than would be
predicted
> by the Work-KE theorem.  If you want to play lawyer and observe that the
> guarantee didn't state _how much_ KE would be gained, I will apply a
brief
> rightward tug to the plunger, reducing the KE to zero.  The total F dot
ds
> will be nonzero.  This totally and completely violates the guarantee,
> letter and spirit.
>
> ==============================
>
> > I assume this is simply a miscommunication and that you are too busy
> > to look over my long document more carefully.
>
> I read
http://physics.usna.edu/physics/faculty/mungan/Scholarship/WorkEnergyTheo
>
http://physics.usna.edu/physics/faculty/mungan/Scholarship/WorkEnergyTheor
y.pdf
> carefully enough to ascertain that the word "entropy" occurred only once
in
> this long document, in a footnote.
>
> Trying to explain thermodynamics without entropy is like trying to sell
> dehydrated water.
>
> The whole notion of using the Work-KE theorem to "derive" thermodynamics
is
> misguided.  Any resemblance to real thermodynamics is purely
> coincidental.  Introducing additional concepts like "pseudowork" isn't
> going to help.
>
> > I strongly object to texts which consider examples (in the mechanics
> > chapter) of say clay hitting the floor and then say (in passing, no
> > less!) that the floor does no work because it doesn't move. Is there
> > an introductory student alive who is supposed to make sense of this
> > before having been introduced to thermo?
>
> Huh?  I agree with the text.  If the floor doesn't move, then it doesn't
do
> any work.  Every introductory student I've ever seen can make sense of
> "doesn't move".  Work = F dot ds where ds=0.  What's objectionable about
that?