Re: pseudowork

[Tom Wayburn <twayburn@WT.NET>]

One last word:  The electric current entering your building because the
TV is on is my perfect example of work if the bldg is the Control
Volume.

Friction is an example of lost work and must result in an amount of junk
heat generated equivalent to the lost work --- depending on which
formulation you use.

Def.  Work is the ability to raise a weight crossing the control volume
surface unaccompanied by entropy (loosely speaking, but good enough for
science :-)

Regards / T

Please define pseudowork, someone.  (Carl?)   I have never heard the
term used.




----- Original Message -----
From: "Carl E. Mungan" <mungan@USNA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, November 30, 2001 1:22 PM
Subject: Re: pseudowork


> > A motionless box of mass m is dropped from an infinitesimal
> > (ignorable) height onto a rapidly moving conveyor belt.
> >
> > Let the 'system' be the box.... describe the gain in KE
> > that the box undergoes.
>
> This example is discussed in Sherwood's paper and/or Mallinckrodt's
> paper. Just to review:
>
> Assume belt is horizontal. Apply work-kinetic-energy theorem:
>
> pseudowork = uk*m*g*s where uk is the coefficient of kinetic friction
> and s is the horizontal distance it has moved in the lab frame from
> the drop point (assuming it has not yet reached the belt's speed)
>
> change in KE = m*v^2/2 relative to lab
>
> Equating enables us to relate v to s. You can then relate s and v in
> the lab frame to s' and v' in the belt's frame if that's what you
> wanted. If you want to know what happens after it stops slipping
> relative to the belt, it will depend on whether the belt is
> accelerating or not. (After all, there is an equal and opposite force
> uk*m*g backward on the belt and the belt motor may or may not be able
> to compensate for this.)
>
> Notice that I have *not* done any thermo above. The other half of
> this problem, which you did not ask for, concerns the fact that the
> block and belt warm up. Calculating this for say the block alone will
> require that you give me a model for the frictional interaction and
> for the thermal conduction. In most realistic models (ie. where the
> block is not taken to be a single particle), you will find that the
> work on the block is not equal to the pseudowork calculated above.
> (John D gave a few examples of models, and there are many others.)
> The difference comes about because the first law has a change in
> internal energy on the RHS, while the work-energy theorem used above
> does not. (Some people would say that the work-energy theorem cannot
> be applied if you use a non-particle model for the block but I
> disagree. However, this seems to have degenerated into a childish
> argument.)
> --
> 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/