Ok, I missed last class period, but I have the general idea of this I think. I just don't know how to put it all together for this one... A rock that weighs 40 N is dropped from a cliff 20 m above the ground. What is the kinetic energy of the rock when it gets to the ground? (disregard air friction). Ok, I know that F=MA, I know that KE=1/2mv^2, and I know that PE=mgh. Am I missing something here, or am I just retarded? lol. I got the an answer of 10290J and I don't even remember how!.. and I really don't think this is correct. Can anyone help? Thanks!