[Phys-L] Re: collision question (revisited)

[John Barrer <forcejb@YAHOO.COM>]

> On Fri, 18 Feb 2005 12:39:21 -0500 Bob Sciamanda
> <trebor@WINBEAM.COM>
> writes:
> > Certainly, the momentum of the entire closed
> system is conserved.  So
> > is the
> > energy.  This does not preclude dissipation.
> Dissipation of energy
> > does not
> > imply energy destruction - neither does
> dissipation of momentum.
> > (The original system momentum is not necessarily
> zero, but that is a
> > red
> > herring)
Consider a 0.25 kg low-friction lab cart crashing into
a stationary 1 kg cart. Before and after velocity
measurements demonstrate conservation of CoM p while
showing that about 80% of the CoM KE disappears
("dissipates"). It is no mystery that this dissipated
KE shows up initially as sound (vibrating air
particles) and internal vibrations of the plastic and
metal out of which the carts are constructed. Is there
not momentum associated with these vibrating
particles? While the time-average value of these
momenta is zero (due to the vector nature of p), the
instantaneous value(s) are not. It would thus appear
that these non-zero instantaneous vibrating-particle
momenta could not have resulted from a transfer of CoM
momenta, but rather from the transfer of some of the
CoM KE (but NO CoM p) to internal and external
vibrations, the results of which are time-varying
values of both instantaneous non-CoM KE and p. Is this
mental model correct/reasonable?  Of course, this
would lead to another question: Why can CoM KE be
transferred to internal vibrations but CoM p cannot?

Last year (with help from John Denker), I finally
realized that it is quite easy to transfer momentum
with no transfer of energy. It now appears to me that
the opposite is also true.

John Barrere University HS  Fresno,CA