[Phys-L] Re: Momentum

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 01:52 PM 12/6/2005,  Rick Tarara, you wrote:

> While I am not a Jim Green Disciple, I do think this topic speaks to his
> (and others) warnings about reifying energy.  It is by thinking of energy as
> something 'real' that is contained by the initial moving object, that then
> causes us problems in thinking about how this energy 'stuff' ends up being
> redistributed in the final system.
>
> I think there are many cases where modeling energy as 'substance like' works
> well, but this is not one of those.
>
> Rick


Rick is illustrating a trait I think I have identified here before, a few
times.    An explanatory model is applied to a case that is in some way
  pathological, and then, the writer, discomfited,  decides in good faith
  that one should avoid using that explanatory mode evermore.

Is THAT  the Jim Green reaction?

Perhaps one could just remember that an unnatural momentum transfer
has its explanatory difficulties?

I am now going to poke at the pathology, for what it's worth.
    If I roll one steel bearing at another similar one, I am not surprised
if the first stops dead, and the second continues at the same speed
as the first.

If I roll it fast enough, the balls yield at the contact spot, losing some
energy in the process. The first ball trickles on, and the second bounds off,
  almost as fast as the first was rolling.

One can visualize a graduated series of collisions, with the target
bounding off increasingly slowly, the incoming ball trickling on at increased
speeds with dents of increasing size.

This process evidently has a limit of some kind: that is when the two balls
  proceed from the contact at the same speed and velocity.
We realize in practical terms that this has  been the occasion of some
  violence.
  But that's easy to forget: for engineers as for physicists.

  Visualize John Denker's rail-road wagon, with some means of grasping
an impacting wagon and holding it rigidly.   This is not the way they are
designed - not if they are intended to couple at any appreciable speed
  at all.

Visualize how this motional limit creates an immense transient cyclic stress
  on the structures so that they may crack or deform at the coupling or
  adjacent fixtures.    It is not difficult to see that the rolling
stock makers
place dissipative elements in their couplings - it is self-interest after all.

But what about the engineer brought in to make the drive train of the Bede
BD-5 light plane behave?    The transmission shaft from the rear engine to
the rear pusher prop was breaking. Breaking in hours.
    He considered a sturdy tubular prop shaft - like the one on your older car.
But he settled on a slim aluminum shaft, a quill-shaft so to say,
when he found
that disabling one of the firing cylinders of the engine also saved the shaft.

That was not the end of his travail, no doubt, but it illustrates what
  happens when you try for a rigid connection from an impulsive prime motion.
Trouble. A transient resonance.
You can tune for resonance below the frequency of interest, or above.
But you want to dissipate the energy with suitable damping.

Low frequency is lower stress.  Ah well, there are numerous (conventional
tractor) BD-4s  which look - conventional - and very very few functional
  BD5's which look much more exciting. These fellows only work at all
  sensibly with small jets.



Brian Whatcott    Altus OK    Eureka!
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l