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Originally Posted by GTP owner
Maggot got it right. Two cars of exactly the same type and mass hitting each other at 100km is NOT equal to a 200km/h collision. If your car weighed substantially less than the opposing vehicle (say you are a barina and they are a truck) and you go backwards as a result of the collision at 100km/h, then, and only then is the force equivalent to a 200km/h collision.
So two equal cars colliding will have exactly the same crumple zones, so will be exactly the same as hitting a brick wall. There will be no reduction in force as a result of their crumple zone giving way unless you weigh more than they do.
This is why it is seen as safer for you to have a heavier car (as in a 4x4) because more of the force is transferred to the smaller car - ie. they go backwards, while you do not completely stop. As an example - you have a landcruiser and they have a barina. They will go backwards at 20km/h, meaning their impact is the equivalent of a 120km/h collision, which is unsurvivable. The cruiser decelerates to 20km/h, meaning the force is of a 80km/h collision, which will probably be fatal.
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When you are talking two passenger cars involved in a 100 km/h head on collision, individual vehicle weight have very little to do with it, neither will have a "survivability" advantage. This of course relevant to two passenger vehicles, if you make one a car and the other a B double, that is different.
I assume in your example that both vehicles are traveling at 100, the landcruiser will not decelerate from 100-20, it will stop (quickly). Considering the combined force of the impact is 200 km/h, both vehicles, regardless of mass still sustain a 200 km/h impact. Your example gives the false impression that you are more likely to survive in a landcruiser than a barina. This is a over simplified assumption and does not take into account effectiveness of crumple zones, safety restraints and force transference to occupants. Interestingly, statistically you are more likely to be involved in an accident in a 4wd and when you are you are more likely to be killed. Does not say much for the "safety of 4wd's theory" does it?
Looking at forces involved, the other car at same speed in opposite direction is still worse than stationary object. Although not twice as bad as the speeds involved would suggest due to other factors, but still worse. At least that is what all the research and study I have done on vehicle accident kinematics suggests. The end result is neither would be particularly survivable, in any vehicle.