As I am no physicist by any stretch of the imagination, the math involved most likely eludes me; however, I think I have a very gentle grasp of the topic.
To determine the lethality of a falling object (dropped or shot from an airplane), one would first have to figure the odds of that object actually striking someone. Then you would have to factor in (assuming a hit to someone) the object's shape and mass relative to wind resistance. Every object has a "terminal velocity." Regardless of how high you start an object will eventually reach the maximum speed it can travel before its wind resistance halts the acceleration. Once its terminal velocity is reached, the object will continue to fall at the same speed regardless of how much farther it falls.
Then you have to figure the stability of the object as it falls. Bullets fly straight when shot because they spiral (like a football). However, at a certain point, their path also deteriorates and the bullet or object begins to tumble. This usually causes greater wind resistance further slowing it's free fall. And, you would also have to factor the penetrating effect of the shape of the object. A blunt object will be less likely to penetrate (especially a hard head like mine) than a sharp object. Sharper objects can be more lethal because they can penetrate into deeper tissue where a blunt object MAY only give a severe headache.
But in short, it can and does happen. A buddy of mine was actually hit from a "falling bullet" New Years Eve in Jacksonville Florida (1992). It was a .38 slug (among many that fell) entered the top of his left shoulder and penetrated down his arm and lodged against the Humerus (bone in upper arm). While it did penetrate, it was no where near the impact or damage that would have occurred had he been "shot" outright.
