> What I'd reasoned was that at the point of impact you as a vehicle have
> deccelerated from 75 to 0 and stopped at exacty the same point you
> would have if you'd hit a stationary wall. Hmmm, still not convinced
> about that "combined speed" thing

:-)

As for deceleration, you'd be right. But imagine being a wall, in a
stationary position, and having a car hit you at 75mph. You wouldn't move, or
change speeds, you're at 0 mph at all times. It would still hurt right? :-)

Basically in the "combined speed" case there is twice as much energy (in the
form of kinetic energy) that has to go somewhere, and now both cars absorb
that amount. In the case of one car vs. a wall there is only half of that
amount of kinetic energy. However the wall will feel it just as much as the
car :) You could say that half of your kinetic energy is transferred to the
object you hit, so if you hit another car you receive half it's energy plus
your own half, in the case of a wall you only get your own half and none of
the wall because it has none (it is not moving).

Of course it is not exactly half, it all depends on the type of object, its
mass compared to yours, etc. Obviously if you're a VW and hit a bicycle at a
combined speed of 100mph it'll be a different experience than hitting a big
truck at that speed :) Also a wall is a bad object to hit because it usually
does not move or buckle (so you absorb most of the impact), whereas another
car would at least "bounce" back a bit and forces are more equally
distributed.

Best thing to do though is not crash at all!

Regards,
Dennis

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