[Will]

Renegade, almost right. First, to get Joules out of E=(0.5)*m*(v^2), you need mass in kg (you got that), and v in m/s (not km/hr). 100km/hr == about 27.8 m/s.

So, in the hypothetical full head-on collision (which, by the way, would almost never happen, nearly all are offset collisions), the two cars would have:

E=(0.5)*2000*(27.8^2) = 772840 J
E=(0.5)*1000*(27.8^2) = 386420 J

And right at the point of impact, things get fuzzy. In an ideal collision, the larger vehicle would have 386420 J of energy remaining in its direction, and so the COMBINED speed would be v=sqrt(386420/1500)=16 m/s = 57.6 km/hr

However, this would not be an ideal collision. The cars bounce and deform, there's friction from the road, most likely there will be a conversion of some forward momentum to torque, making the vehicles spin, etc. This is where the more intangible stuff begins to take effect, like how well materials will crumple up around the frame.

What would likely occur in the full head on collision would be the lighter car would begin to turn away from the larger, beginning from the rear. So, the rear of the lighter car will arc around, possibly until the direction the lighter car is facing is the same as that of the larger, but probably more at a perpendicular angle. Basically, the lighter car is designed to get the most of that impact energy dispersed on something other than the passengers. The bigger vehicle will not turn as much, probably won't crumple as much, and a lot of the decelleration gets applied to the squishy passengers.

Regardless, in this particular situation, at that high speed, none of the passengers is going to feel "fine" afterwards. So, what it comes down to is, do you want to have a lower chance of injury, or do you want to point at the other guy's car and say "I win!" (likely while wearing a neck brace)?

Oh, and the semi... have you ever seen just how much open space is in and around those cabs? This is the one case where having a lot of size (as in volume) and mass would pay off. Because semis are higher off the ground, some of the impact energy goes into trying to LIFT the semi, and so gets dispersed. Then, there's a good sized space between the front of the truck and the cab, spreading out the impact energy over time. Plus, the mass is so much bigger than any conventional vehicle because of the cargo, the reduce in speed is not going to be very high. But does this mean that you're going to start driving a semitruck around? What happens when everybody does this?

*/really now, end thread hijack*