> the car comes to a complete stop at the wall It's this assumption that causes the entire discrepancy. For a wall to be truly immovable, the ground must exert

> the car comes to a complete stop at the wall It's this assumption that causes the entire discrepancy. For a wall to be truly immovable, the ground must exert a great force into the wall to keep it in place, which goes into the car. But if the wall isn't anchored very securely (or backed by the ground), then the car will bust through it and continue moving forward. In the limiting case where the wall isn't attached to the ground at all, both the car and wall move at 50 mph following the collision. The dissipated kinetic energy comes out the same as in the car/car case.