Why do cars get deformed in high-speed crashes?

[Alacritous]

I have a question. Let's say a car is moving at 100 miles an hour and hits a 15 inch thick steel beam, the car would most likely go flying back and get deformed (assuming the beam is being held into place somehow, like being bolted into the ground or something). Based on my understanding of Newton's 3rd law of motion (which is limited), I thought that this was because of the density and strength of the steel, and not because of the speed of the moving object. Someone in another forum said I am wrong and that if the car was moving fast enough, it would penetrate the steel (he used the world trade center as an example which seems to prove me wrong). Could someone explain this to me?

 

[KingNothing]

Yes, if it were moving fast enough, it would knock the pole over. It does depend on the car's momentum (product of speed and mass) and the steel beam's strength. The faster the collision, the more BOTH objects will be deformed.

EDIT: However, a 15-inch thick steel beam would probably just cut the car in half.

 

[russ_watters]

Also, since most of the energy is absorbed through plastic deformation or structural failure, cars in accidents don't tend to bounce back very far.