[Phys-L] Re: Momentum Agina

[Bernard Cleyet <anngeorg@PACBELL.NET>]

I think Dan has the answer -- I'll add one more obvious item.

Very simply KE is dissipated until both are moving at the same speed
whether it's thru gross inelasticity (one ball is sticky putty) or it's
plastic (or not) deformation of a steel railroad car coupling, or
whether it's completed slowly or quickly.

The "maths" is so transparent I don't see anything more to it.


for your curiosity:

http://inventors.about.com/library/inventors/bljannycoupler.htm

Very familiar to those who travel in so called primitive countries where
the intercars are "open air".

Here's one w/ movies!


http://www.sdrm.org/faqs/couplers/

Because it's done so slowly and even then w/ a resulting jerk, I suspect
there isn't a "soft" dissipater.

Here's a page that may answer the soft dissipation question.


"In contrast[,] the Janney couplers (see below) encourage violent
encounters in order to engage the coupling fully."


http://en.wikipedia.org/wiki/Coupling_(railway)

In the 60's the English couplers (chain and buffer) were very visible. I
don't remember what they're like now.


bc, who's seen (experienced) [re. movies] several times when bogies are
exchanged to fit soviet gauges and visa versa.


p.s. You all know why there's lotsa slack in the couplers?


Dan Crowe wrote:

> The short answer is that completely inelastic collisions are possible
> only for a subset of all the possible conditions listed by David.  If
> mechanisms are not available for the transformation of the necessary
> amount of kinetic energy of the railroad cars, then the cars cannot
> couple.
>
> Daniel Crowe
> Oklahoma School of Science and Mathematics
> Ardmore Regional Center
> dcrowe@sotc.org
>
> -----Original Message-----
> From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
> Behalf Of David Abineri
> Sent: Tuesday, December 06, 2005 4:32 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: [PHYS-L] Momentum Agina
>
> Thank to those who responded to my question.  May I rephrase it now
> thanks to clarifications by several of you?
>
> If a stationary railroad car is struck by a moving railroad car of equal
> mass and coupled to it, why is that regardless of the material of which
> the cars are made and regardless of their aerodynamics and regardless of
> the density of the atmosphere in which they are moving and regardless of
> whether there is an atmosphere or not that half the kinetic energy is
> lost (converted to other forms)?
>
> To both my students and to me, this does not seem reasonable and yet, if
> momentum is conserved and the masses are equal this must happen!
>
> How does one explain to a high school class of bright students why,
> regardless of all these factors and some I haven't mentioned, exactly
> half the kinetic energy is always lost?
>
> Thanks again, I appreciate your thoughts and I hope the question is more
> clear.
>
>     David Abineri
>
>
> --
> dabineri@fuse.net
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