David Ramsey wrote:
>
> And all this time I thought we were talking about a new plastic car and an
> old steel car hitting each other. OK so the plastic car will crumple, the
> air bags will inflate, and the tank will still be moving after the plastic
> is a flat piece of road kill. By using F=MA, lets say F1=F2 is car one and
> car two arriving at the same point at the same time and coming to rest , it
> follows that M1A1=M2A2. now if M1 is 10 times more than M2, then A1 is 10
> times less than A2.
Lots of inaccuracies here though. Or should we say
"inapplicabilities" because the math is right but doesn't
accurately model anything that is important to
vehicle accident safety.
First and most obvious, the mass difference is
not nearly as much as some people think. I know that
10 times in your example was an exaggeration, but
realistically the mass difference is not that high.
Old cars had more steel in them, but new cars have more
"stuff" in them. If I recall most muscle cars (an oft-quoted
heavy car) start life around 4000 pounds, most rice rockets
(oft-referred to as crushboxes) start life around 3000 pounds.
The mass difference is not insignificant, but
generally very overhyped.
The other point, which is the important point, the
"A" that matters is the the acceleration of the
person inside the car. Your calculation talks about
what happens to the car, which negates the obvious
benefits of crumple zones.
In your "plastic car and steel car crash", even if
the steel car is still going 20 mph after the impact,
it's very likely that the impact was more severe for
it's occupants than those in the plastic car. (hose
out the inside of the car and sell it again)
If you need a real-world example, consider the case
of jumping off your house roof onto a trampoline,
versus onto a six inch mattress.
You bouncing off the trampoline is much like the plastic
car entering the collision going forwards and finishing the
collision going backwards. Overall the trampoline exerts
more energy into you than the six inch mattress, but
the peak instantaneous acceleration is very low.
Hitting the mattress is the same as the "solid
steel Packard". Almost no crumple zone, all the
deceleration happens in a couple of inches, then
your ankles become the crumple zones.
Jumping off my roof onto a mattress is only
something I'd do if my house was on fire, but the
kids will jump on the trampoline all day just
for fun.
There is a lesson in there, just waiting to be
absorbed. ;> ;> ;>
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
The above matches my computer.
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