Re: crash tests

[Lawrence C Leung <l.leung@juno.com>]

	I wasn't referring to the Energy of the crash. I was referring to the
Appearance to the driver. I was trying to make the point that many people
think that they are going into accidents at rates of speed much greater
than their actual impact speed so I was trying to STRESS what the
impending crash would LOOK like to the driver. Most drivers really
inflate their accident speed, so they think that their car was better at
survival than what was indicated in the NHTSA crash tests. Since MOST
drivers, upon seeing that they are about to crash will, at the least,
lock the brakes (even then there is some braking, actually more than you
think) they tend to crash at speeds much slower than the 55-65 MPH that
they thought they were doing as they crashed. They forget the braking
part. So, a 30 MPH header is actually more valid than it appears. 

	Now, as for the energy of the crash, the only sane (i.e. reasonable
cost, wrecking only one car) way to approximate the energy of two cars
colliding into each other at 30 is to run a car into a fixed barrier (or
fully loaded semi, if the trailer moves, it's motion absorbs energy,
invalidating the results). Here, Consevation of Momemtum suggests that
the immovable nature of the barrier means little energy absorbtion by the
barrier, (momemtum of barrier = 0 before and after the crash) all of the
energy is absorbed by the crashing car, which is why the enegy effect on
the crashed car is doubled, approximating the effect of a head on crash
of a car into a like size car travelling at the same speed as our test
car. 

BTW  (the total energy of one car doing 60 crashing into a barrier is 4
times the energy of one car doing 30 into the same barrier, from KE  =
1/2 mv^2). Thus, for two cars doing 30 head on, the energy is only double
that of the single car doing 30. So, the most severe crash in this list
is the 60 MPH one, not the double 30 one. THIS is why it hurts more to
strike a tree rather than another vehicle. Not that you're likely to be
happy with any of these results. 


.. Be aware that two cars travelling towards each 
>other,
>each at 30 MPH is an extremely severe collision, visually, it would 
>LOOK<<<<<<<<<<<<<<<<<<<<<<<<< (Note this emphasis, LOOK not SAME effect)
>like driving into a parked car while you were doing 60 MPH, brakes 
>OFF. 
>
>I believe physics shows us that 2 cars head-on each going 30mph is not 
>the
>same as 1 car going 60 into a parked car.  Each car absorbs 30mph, not 
>1
>absorbing none (unless it is a semi-truck) and the other car absorbing 
>all.
>Small point, but a common misconception.  
>
>
>Tom E. Thomas
>Data and Video Network Engineer
>BadgerNet | BTM | DTM | DOA | State of Wisconsin
>tom.thomas@doa.state.wi.us
>