Sorry Frosty, but you asked for it!

There's some good physics in your comment about the Fords and Commys;

Newton's Three Laws of Motion.

3rd Law - Big car hits little car = little car hits big car = same force on each!

( Newton's First Law; If object A puts a force on object B, then B puts an equal and opposite force on A OR For every action there is an equal and opposite reaction )

BUT, by the 2nd law - the "same force" on the little car will produce a large acceleration (of that car), and the "same force" on the big car will cause a smaller acceleration (of the bigger car).

( Newton's Second Law; If a (net) force acts on a mass, then the mass will accelerate. The size of the acceleration is in inverse proportion to the mass - the bigger the mass the smaller the acceleration, and vice-versa )

This means that if it was a head on collision say, the Hummer will slow down a little, and the smaller car will slow down a lot, depending on the actual difference between the two masses of the cars.

(The second law can be rewritten mathematically to come up with an equation called the Law of Conservation of Momentum. This equation, or formula, can easily work out the exact changes in the speeds of the two cars, given their masses and their speeds at the time of impact.)

And finally the 1st law - the passengers in the Hummer will keep moving at their initial speed ( the Hummers speed of impact) until they hit their seat belt.

( Newton's First Law; A moving object will continue to move at a constant speed, and in a straight line, if there is no net force acting on it. And, an object at rest will remain at rest if there is no net force acting.)

When they hit the seat belt, then the belt will be moving a bit slower than them, because the Hummer has slowed down in the collision. So now the seatbelt will stretch as it slows the passengers down to the new speed of the slowed down Hummer. Because the Hummer only slowed down a bit, they should not hit the windscreen ( hopefully not ).

But in the small car, since it slowed down a lot, then the seatbelts have to slow down its passengers by quite a larger amount than for the Hummer. If this speed change is too large, then the seat belts might have stretched so far that the poor passengers end up hitting the dash or the windscreen before they come to the same speed as the small car after impact. This speed might even be in the reverse direction. This is as what happens when a car meets a loaded semi in a head on collision!

And as far as side impacts go, you can imagine the car sliding under the Hummer ( if its midships ). This is not going to do the small car's passenger's heads any good. But it could also mean the Hummer could get knocked over onto its side. Again, this is going to depend a bit on the mass difference between the two cars, and the actual speed of the smaller car as it hits the Hummer.

In this case I am pretty sure there is a fair amount of research to say that the passengers, or driver, in a 4WD, on the side that hits the ground, could get badly injured. Mainly because they are 'falling' from a higher place compared to a standard car. Although these days with those side curtain airbags they will stand a better chance of surviving.

Hope this makes sense! Its a bit long winded - as usual! Millsy.