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Huw and Robert are arguing basic physics:
> Sorry, Huw. Your reasoning is flawed.
> Imagine a car striking an immovable barricade.
No barricades involved, so why "imagine" any?
> In effect, each car becomes the other car's immovable barricade.
Nope. In the 60/0 collision, the CAR being hit ends up moving at 30 mph
(less friction losses to the road) in the direction the 60 mph car was
going. Try it any time. maybe with something cheaper than cars, and
slower, and smaller...
My semi educated $.02:
Hey, you guys got most of this stuff right, it's the details that make the
difference. Crumple zones deforming absorb energy from the 0.5MV^2
equation so that resultant energy of system that a passenger would
experience is less than apparent with non compressible vehicles (no
crumpling of car(s)). Earlier someone also mentioned that the rate of
deformation and "big hit" energy transfer also strongly correlates to the
level of damage to occupants. Energy can be related back to force applied
over time, and the end result ... F=M*A, so if the overall energy that the
passenger sees is still big, his or her mass doesn't change and the only
thing left to do is accelerate ... passengers head, body, etc (the thing
that really matters here). There's lots of other variables also - the
resultant force vector is determined by angles between the hittee's - if a
car traveling down the road strikes a car at less than directly head on,
the force vector has a component that goes perpendicular to the road, which
also lessens the head on impact energy component (car(s) tries to spin as a
result). All of the variables get real ugly to try and predict theory on -
better to instrument and do empirical parametric studies with models and
scale up uncertainties and etc. unless you have lots of bucks to crash
Great way to demonstrate is the first year engineering school experiment of
protecting an egg from 30 foot fall where winner has the lightest
protection system with an undamaged raw egg. Lengthy deformation period of
"cage" resulting in lower accelerations for the eggs crash protection
system, ie shell. I think I used some perforated styrofoam cups filled
with shaving cream and guide vanes for flight control - very light, very
effective. BTW, the floor in this case was uncompressible so the system
was easier to design for!
-Randy - long time removed from school, so go light on flaming possible
errors in phsyics memories!
'90 200Q w/ lots of mass