Full Speed Ahead!

Perfect thinking! That was correct!

In this situation we also have an elastic collision. The lighter (green) car and the heavier (blue) car collide and bounce off of each other. After the collision, they move in opposite directions. Here however, the heavier (blue) car is clearly much more slowly after the collision and the lighter (green) car moves a lot faster after the collision. You can see that in the video:

The two cars' kinetic energy and momentum have both changed due to the collision. However, the sum of energy before and after the collision is, of course, the same. The same goes for the momentum. The sum of momentum before the collision is the same as the sum of momentum afterwards. This means: Altogether, both energy and momentum are conserved in the system (law of the conservation).