okay bringing up an old topic but a car (lets just say 2000 kg) drives into a concrete wall (essentially immovable) at 40 km/h. the same vehicle (make and model not the product of the colliision!) drives head on into another identical vehicle traveling in the opposite direction; both at 40 km/h. Which does more damage? They are equal no? Argue me if im wrong but also couldnt you argue that the vehicle crashing into the wall has momentum lost to recoil, rolling away from the wall after it has hit? All of course depending on the construction of the car right? those cars in demolition derbies are built (impracticably for the transfer of momentum and safety's sake) to withstand heavy blows, so couldnt that vehicle lose some momentum to recoil on colliding with the wall?