Re: COLLISION 2

[brian whatcott <inet@INTELLISYS.NET>]

At 09:20 11/28/99 -0500, David Abineri  wrote:
> Thanks to all who responded to my collision question.
>
> Let me try to take it a little further if I may.
>
> The difference between two cars that couple together upon colliding and
> two that don't can be quite a simple mechanism that simply latches the
> two together.  For example, a loop on the one car is caught by dropping
> a "pin" through it on the other car at just the right moment.
>
> Given that an elastic collision never happens, I am surprised by the
> apparently large difference in surviving kinetic energy between the two
> cars hitting each other and the two cars hitting each other with this
> "pin" dropped to catch the loop making it inelastic.
>
> Is the difference in the losses of kinetic energy manifested primarily
> as heat (deformation) or sound?  It does not appear to me that these two
> collision would sound very different from each other nor that
> significantly more deformation would take place.
///
>        David Abineri     dabineri@choice.net

I think David's students have gained a significant insight into the
impacts of heavy vehicles.
  For those who have lived within a mile or two of a marshalling
railhead or even a rail station, it will be no surprise that the
initial push of a loco on a long laden string of rail wagons is
a noisy and prolongued ricochet. And these vehicles feature heavy
springs behind their couplings.
There are industrial trucks with simple ring and pin connectors
and these rattle badly if coupled impatiently. It is certain that
most of the kinetic excess energy is lost in heating the couplings
if they are rationally designed with lossy compliance intended to
 dissipate reasonable impulses.

A design that attempts a solid coupling soon
is subjected to extreme stress and consequent breakage.
In this respect, it is interesting to attempt to tow a vehicle with
a steel cable. Bad jerks are hard to avoid. In contrast, Coast Guard
vessels use a nylon warp to reduce the stresses experienced in heavy
 seas - their concern is that a long tow can store too much energy which
rebounds if the attachment is lost.

So compliance is not just a matter of softening the ride for road cars
but a question of providing a reasonable life for their suspensions.

It is not a question of thermodynamic compression heating either - as
the recent thread demonstrated, this is usually a rather modest quantity
 for liquids and solids. Rather an explicit frictional or viscous damping
method is employed.





brian whatcott <inet@intellisys.net>
Altus OK