Re: Fwd: Audi A8 Achieves Highest Possible Safety Test Rating

[Andrei Kogan <abk@phy.duke.edu>]

At 09:39 AM 9/16/98 -0700, you wrote:
>Robert Myers decided to speak these words:
>
>>None whatsoever, Richard, that is, if you are referring to the head-on
>>vehicle 35 mph crash being equivalent to the 35 mph barrier crash.  The
>>physics says that is a true statement.  Team Door Handle has nothing to do
>>with it.
>
>
>Im confused now.  Wouldnt the momentum of the two cars add together and 
>cause what would be equivalently a 70 mph collision?  What am i 
>missing...its been years since my last physics class...
>
>later...
>
>
>Michael Sheridan Williams
>ICQ# 11740998
>
>1985 4000 S Quattro 185,000+ miles, Koni Yellows/Coilover (2B), strut 
>brace, Sport 8000 Tires, K&N, MOMO 4pt harnesses, Hella XL Driving lights 
>(excuse me, auxilliary low beams) (2x135w), magnecor wires....<out of 
>breath>
>1986 Oceanic Blue, 4000CS Quattro--Parting Out(ask if you want anything)
>My father's: 1986 4000 CS Quattro--Graphite 161,000 miles
>
>http://members.aol.com/daserde2


If we restrict the discussion to a non-relativistic case (The relativistic
limit   was considered by Igor Kessel --see ref 1.)  then the average force
experienced by an object involved in a collision with another object is
given by

(delta P)/(delta t)

(ref 2.) where delta P is the change in the momentum of the object, or just
its initial momentum if the object is at rest after the collision. That is,
the collision time matters as much as the initial momentum, if you move
very slowly but stop abruptly, it will still hurt.

When comparing the two cases when the  "opponent" is either a brick wall or
an identical vehicle, one may argue that the collision with a vehicle lasts
longer, because most vehicles are softer then most brick walls, give or
take a few vehicles or walls. ( ref 3.)

The initial relative velocity of the two colliding objects is therefore not
the relevant parameter. Instead, for a given collision time, and if the
object ends up at rest, the only relevant quantity is the initial velocity
of the object. It seems counterintuitive because when viewed from inside
the other car (car B) , the first one (car A) appears to go twice as fast.
There is no problem, however, because in the frame of reference of the car
B , both cars after the collision are moving backwards with a speed equal
to half of their relative speed prior to the collision.


References

1. Igor Kessel, quattro list, sept 15
2. L.D. Landau and E.M. Lifshits, Course of Theoretical Physics -- volume
1: Mechanics
3. X. Y. Zet, "Comparative study of Young moduli of  modern automotive
frames and typical construction materials", Ph.D. thesis, unpublished