Re: why pseudowork (NOT)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Fri, 29 Oct 1999, Richard Tarara wrote:

> I have a problem with mechanism.  Assuming momentum conservation then the 1
> kg block moves backwards with speed v.

There you go.

> This also conserves the CM motion so we can deduce no external forces.

That's what momentum conservation is predicated on.

> However, the brief collision of the objects should be viewed as
> adiabatic so no thermal energy (or internal energy) could be transferred
> {or as Jim would prefer, one object could not have heated the other} but
> somehow all of the KE of the 2 kg block must now be internal energy
> while there is no change in energy of the 1 kg block.

I'm confused by your analysis here.  There's no reason to believe that
there was no energy transfered from one block to the other.  It might be
that way; it might not.  The problem statement doesn't give you enough
information to know one way or the other.

> So to answer your questions using the (pseudo) work energy theorem, no
> work was done on the 1 kg block and a negative mv^2 worth was done on
> the 2 kg block

Exactly so.  Other work-energy relationships will give different results.

> (but by what?).  But I can't imagine how such a collision could happen.

I still don't understand the difficulty.  This is nothing more than a very
ordinary inelastic collision.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm