How are the moon's craters related to the collision of a fly and a train?

A Diesel engine train speeding along a stretch of rail runs headlong into a fly traveling directly opposite the train - the fly striking the engine's windshield. As unlikely as it is to believe that the forces of the train and the fly are equal, Newton's Third Law says that is exactly the case. Asteroids and comets that strike the moon (note our picture) follow the same principle.

What is the key to understanding this seemingly inexplicable phenomenon? Well, an understanding of the second law of motion (Newton's Second Law) helps. It states that the acceleration change experienced by both objects will be equal to the ratio of each object's force and its mass; that is: a = F/m. How does this explain the conundrum? It's obvious that the two masses are drastically different - the preponderance of mass belonging to the train. And when you think about it, it's equally obvious to see that the acceleration of the train will change, only imperceptibly, while the change in the acceleration of the fly upon impact is large and deadly.

Therefore, the train has a tremendously greater mass while the fly has tremendously greater acceleration change. In the meantime, the forces of each one upon the other is the same. Do the math!