>[David]
> >>I have tried thinking of the deformation as a giant spring between the
> >>two cars.///
>
>What can have heated the spring 'very hot'?
> Did we not work an example case of a spring as heavily loaded as it
>could be - yet the temperature rise in degrees could be counted on
>the fingers of one hand? David was postulating only one compression
>stroke or one compression and one relaxation stroke, respectively.
Then at 02:19 AM 11/29/99 -0700, Jim Green wrote:
>Brian, David is postulating an appropriately constructed "giant spring"
>which is strong enough to stop moving freight cars!!! This "giant spring"
>could not be ideal as its internal energy must increase by ~half the KE of
>the freight cars, which it could do by being flexed, which it clearly
>reasonable because it is constructed of flubonium.
I agree that a steel spring which adiabatically and elastically stopped a
rail car would not get very hot. That is, if it is big enough to remain in
the elastic limit it has a big enough heat capacity to not get hot.
This misses an important point. The point is that to perform the function
David required, something grossly inelastic must occur. This whole thread
revolves around explaining and modeling inelasticity. Therefore the
earlier calculation of the elastic properties of steel is
irrelevant. Either you need a shock absorber (which would get hot, while
the steel did not get very hot) or you could perhaps try to rely on the
very slight internal friction of the steel, in which case the assumption of
"one compression and one relaxation" is where the problem lies, because the
two rail cars would oscillate about their CM for many cycles, gradually
heating the steel.