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RE:crash tests..uh oh.

At 09:21 AM 12/13/99 -0600, you wrote:
>Lawrence C Leung said:
>		... Be aware that two cars travelling towards each other,
>each at 30 MPH is an extremely severe collision, visually, it would LOOK
>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.  

Hey Guys,

Believe it or not, it gets even a little more complicated than this.  With
the risk of this starting to sound like the T*rsen list...

Yes, there is more energy stored in the one car going 60, than in two cars
going 30.... that's because the energy stored in a moving mass increases by
the square of the speed, (but mass only to the first power).  So, in
actuality, the single car going 60 has twice as much energy stored up as
the pair of cars going 30...

And, both cars absorb the same amount, no matter how fast either one is going.

BUT, it's not just the amount of stored energy which is dissipated which
determines the results of a crash-- it's also the amount of time in which
this energy is dissipated (dissipated power).  For example, if you are
going 60 mph and you put the car in neutral, you are hitting the wind, and
letting the impact with the wind *slowly* decelerate you to (almost) zero.
You would experience all of that energy being dissipated, but over a longer
time, so there would be no impact on your health...

So, the point is, that if you send two 30 mph cars into eachother head on,
the two cars will stop in a VERY short period of time (ie high dissipated
power).  This is because the two cars will end up stopping right about
where they hit-- they won't go sliding together in either direction, since
they are the same mass.

However, in the case of the car travelling at 60 into a stopped car (twice
the energy of the two cars travelling at 30 needing to be dissipated), the
collision will start to accelerate the stopped car, and the car travelling
at 60 will end up coming to rest somewhere far down the street from the
initial point of impact...

But, it's the initial impact that counts, and computing it requires
specific knowledge of the structure, materials etc., so I don't have the
means or reason to try and compute it right now.

The point is that the two scenarios aren't the same (two cars at 30, one
car at 60).  But to determine which one is worse is not that simple (but
it's probably the single car going 60).

In any event, if you are inside any of these cars, brake and steer, and you
won't have to find out.