just learned: release -> let fall, haha

let fall at the same time and hit the ground at the same time，haha

here is something related to "fell to the ground and fell on the ground", even though it might be a little different than what you mean here.

http://www.english-test.net/forum/ftopic63935.html

btw: above web site is pretty good for learning grammer and asking related questions.

here is another interesting question about drop on the floor~~

http://answers.yahoo.com/question/index?qid=20110614125443AAhMEKl