Re: COLLISION 2

[John Denker <jsd@MONMOUTH.COM>]

At 09:20 AM 11/28/99 -0500, David Abineri wrote:

> 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.

Not really.

> 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.

That statement depends on an unreasonable approximation.  In the real world
you can never drop the pin at "just the right moment".  You should imagine
that the loop is bigger than the pin, so you have some nonzero time for
dropping the pin.

This will allow you to visualize the pin *rattling* in the loop, which is
an important part of the physics.

> Given that an elastic collision never happens,

It can happen to an exceedingly good approximation.

> 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.

You need to think in terms of some explicitly dissipative device such as a
shock absorber.

> Is the difference in the losses of kinetic energy manifested primarily
> as heat (deformation) or sound?

If you do it right, it is manifested has heat in the shock absorber, not as
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.

Actually there would be a significant difference if you tried to do it
without a shock absorber.  Remember, we must suppose there is some slop in
the loop.  The cars bang many many times as the pin rattles in the loop.

> I have tried thinking of the deformation as a giant spring between the
> two cars. In the elastic case the energy stored in the spring is
> recovered as kinetic energy. In the case of the inelastic collision, the
> spring finds itself permanently caught in a compressed condition as they
> couple and cannot come apart. Is this a reasonable model to present to
> beginning students?

No, because it is reversible in principle.  You need dissipation.