Re: why pseudowork (NOT)

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

I have a problem with mechanism.  Assuming momentum conservation then the 1
kg block moves backwards with speed v.  This also conserves the CM motion so
we can deduce no external forces.  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.  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
(but by what?).  But I can't imagine how such a collision could happen.  The
motion implied for the 1 kg block is the same as though it collided with a
fixed wall.  How can the 2 kg block effect the 1 kg block this way?  In
other words, the only way for the 2 kg block to stop dead in this collision
is to conjecture external forces acting, and in that case, momentum need not
be conserved so we don't know what the velocity of the 1 kg block really is
without knowing the details of these forces.  We cannot answer the work
questions without these forces either.

That's my interpretation--but I may be wrong ;-(

Rick

----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, October 29, 1999 10:28 AM
Subject: Re: why pseudowork (NOT)


> Both Ludwik and Rick find my problem flawed.  I posed it precisely because
> I thought the physics to be almost transparent.  Am I missing something?
> Would someone please explain the perceived difficulty?
>
> > A 2 kg block and a 1 kg block head toward each other with equal and
> > opposite velocities of magnitude 1 m/s.  After colliding, the 2 kg
> > block is at rest.  How much work was done on the 1 kg block?
>
> BTW, it will help my diagnosis to know as well how much work you would say
> is done on the 2 kg block.
>
> John Mallinckrodt      mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm