Re: ENERGY WITH Q

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

> Example: a block slides across a rough stationary table. How much of
> the mechanical energy of the block was lost via work and how much via
> heat?
>
> I think it is possible to answer this question. But it is certainly
> not "obvious" or "quite distinct" without considerable modeling,
> analysis, and thought.

I forgot to add a final comment, so I'll do so now. Now ask this
question. How much of the mechanical energy of the block was lost via
pseudowork?

This is a much easier question. If the table is level, the answer is all of it.

Which answer do you think will be easier for intro students to understand?

Unfortunately many texts take one of two unhelpful approaches here:

1. They throw up their hands and claim sliding friction problems are
very hard. Footnotes to AJP articles appear. Mysterious formulas are
pulled out of thin air with strange comments like "-f delta s" is NOT
the work done by friction but it does equal the mechanical energy
lost and the thermal energy gained.

2. They merrily switch back and forth from work to pseudowork, as Jim
G does below, using whichever formula seems to suit the problem and
never warning the student that there's any subtlety to sliding
friction problem.

Approach 1 is at one end of the scale: friction is way too hard to do
in the work chapter. Approach 2 is at the other end: it's trivially
easy to do.

Jim Green wrote:

> Integrate N#2 --> Int F*ds = Int ma ds
>
> We _define_ Int F*ds as work and Int ma ds = 0.5mv^2 as KE == energy -- by
> definition.

No, you just defined pseudowork.

> Some of this work we call PE and we lump the PE and the KE together -- This
> is not always helpful in an intro class.

Give me an example where it's not helpful.

> Thus the work done on the system = - the work done on the applicator.

There is indeed such a law for thermodynamic work.
There is however no such law for pseudowork, as many have already pointed out.

> Now some on this list will want to use "enhanced" language but this is
> quite adequate for an intro class -- and beyond.

And you have just given an excellent demonstration of why such
enhancements are very much needed.

John Denker wrote:

> I've been doing a survey, talking to physicists.  Not physics
> teachers, but people who do physics research for a living.
>  -- I ask them for the technical definition of "work".
>     100% of them answer "Force dot distance".
>  -- I ask them for the corresponding definition of "pseudowork".
>     100% of them give me funny looks.
>     0% have ever heard of the term before.
>     (Nearly 100% of them fall back to a non-technical
>     definition:  pseudowork is what management does :-)

I'm shocked that you of all people would stoop to this. Haven't you
said before that physics is not decided by polls?

Go back to your physics researchers. Ask them if if they know what
Newton's second law is. Ask them if they can integrate it over the
displacement of the center of mass of an object (whether rigid or
not) and can relate that to its bulk translational kinetic energy.
Tell them, "Oh so you do you know what it is."

The fact that the name "pseudowork" is a relatively new term is moot.
I can probably find other new terms in the physics education
community that *our* physics research friends also have not heard of.
Does that mean they're all useless? Is introductory physics now so
well explained that no innovations in our lexicon and texts are
needed?

ps: I don't fully appreciate the fact that you appeal to those who
"do physics research for a living" and call them "physicists." The
rest of us are not physicists? Teaching has disconnected us from real
physics? We no longer do physics research for a living even as
teacher scholars?
--
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/