Is anyone familiar with vectors? Here's my problem: I have a position vector r, and the 2nd derivative = [-GM/(abs.value of r)cubed] x r. According to my textbook, this means the second derivative is a scalar multiple of r, and therefore: the 2nd derivative of r x r = 0 Can anyone explain this to me? Is it some rule dealing with vectors? This is really urgent... I would appreciate any help. I could email you a copy of my prject, if you don't understand what I am trying to say.