The Movie: Janek decides to take down the Engineer's spacecraft with Prometheus, because he's apparently a really good guy. He crashes into the spaceship, and the flaming mess of both ships fall directly down. The semi-circle alien ship/temple rolls toward Shaw and Vickers and they run away from it. In a straight line.
Real Life: If Isaac Newton were alive today, he would ask us what is so hard to understand about his first law. Objects in motion stay in motion, and that includes kamikaze spaceships. When the Prometheus collides with the Engineer ship, it is moving on a parabolic arc, and the wreckage would have continued on that trajectory, landing many miles away. Instead, the ships fall sharply to the ground, blatantly flipping off the laws of physics. And the laws of common sense are next on the list, because Shaw and Vickers forget they inhabit 3-dimensional space when running from the spinning Engineer-ship. Oy.