Date: Thu Feb 21 2002 - 19:40:44 MST
As a first cut at it we can assume all of the collisions are elastic
and frictionless. This is not a very good estimate since cars are
designed to crumple and absorb energy, and you're not made of rubber
either, but can give us a ballpark figure.
When the 4000 pound car travelling at 35 mph hits the 3000 pound minivan,
if the collision were elastic and no energy was lost in the crushing
metal, the result would be the minivan going at 40 mph.
When this 40 mph minivan then hits your 220 pound body, the result is
you getting bounced forward at a speed of 75 mph!
If you figure the minivan is going slower, it will still be approximately
true that you are given an impact which would send you forward at almost
twice the minivan speed, if you were made of rubber.
Most of this impact is absorbed by your body and represents energy
imparted to you. So you take the minivan speed after the collision,
multiply by about 1.8 to get your velocity, then do 1/2 * m * v^2 to
get the energy you received.
If you figure the minivan was really going maybe only 20 mph after the
collision then you'd be given an impact which would send you off at
about 37 mph, essentially all of which energy gets absorbed by your
body both when the minivan hits it and when you fall to the ground.
Converting to metric units, this is a velocity of about 13.5 m/s and
a mass of 100 kg, for a total of about 13700 Joules of energy.
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