Re: Why work before energy in texts

["Carl E. Mungan" <mungan@USNA.EDU>]

> Now if I slide the book across the table slowly,  there are again two
> forces acting -- my hand and friction -- and the work done by each force
> cancels.  See the book does not continue to move --  the level of energy is
> not increased.  {A little lie to be corrected later -- necessary because I
> haven't introduced KE yet}

Clearly wrong. The book warms up. It does gain energy. Yes, the
pseudowork done by each force cancels. But what most people simply
call "work" does NOT cancel for the applied force of my hand and the
friction. Apply first law of thermo assuming things happen fast
enough that there's no heat transfer:

W_net,ext = change in E

But change in E is positive (book warms up at constant speed).
Therefore W_net,ext = W_app + W_f is nonzero, so they can't cancel.

This brings me to my question:
HOW MANY OF YOU TEACH ABOUT PSEUDOWORK (WHATEVER YOU CALL IT)?

To restate the same question another way:
HOW MANY OF YOU DO EXAMPLE PROBLEMS INVOLVING NONCONSERVATIVE FORCES
THAT ACCELERATE AN OBJECT (ie. any forces in Physics I except gravity
and springs)?
Typical examples are slide with friction, elevator going up, clay
hitting ground, figure skater pushing off, piston and gas, car
accelerating, box on conveyor belt or in back of pickup, etc.

> What is the difference between gravity and friction?   With gravity I can
> get a subsequent increase in the level of energy and with friction I can
> not.   Let me call such forces conservative and non-conservative.

Next question. At the macroscopic level, every force except gravity
and springs (and electric next semester) is nonconservative.

Yet at the microscopic level, all of these nonconservative forces
(kinetic and static friction, tension, normal force, drag force,
applied force, etc) are all conservative. (After all, they are just
electric forces.) I think this is what enables me to bridge the
work-energy theorem (which is about the pseudowork) with the first
law of thermodynamics (which is about the real work).
IS THIS CORRECT? JOHN M, BOB S -> help?

Last question. HOW (IF AT ALL) DO YOU INSERT ROTATIONAL KE AND
TORQUES INTO YOUR DISCUSSION OF WORK AND ENERGY? Is there such a
thing as pseudo-rotational-work? eg. I can imagine a rope wrapped
around a pulley and rotating it frictionally. But we know that the
pseudowork and real work are different for friction. And so...?
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/