Let me try. When you "couple" the cars, you are constraining the collision so that the final velocities will be half the original velocity of the moving car (assuming conservation of momentum). In an important sense, you have constrained the "material of which the cars are made," at least at the point of contact.
skip
-----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: 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.