Hello, I find this very fascinating. I read somewhere that car manufactures uses the principle of center of mass (Or maybe center of gravity, I do not know if they are the same) . I researched about this on the net and I simply can't understand it. From Wikipedia, In physics, the center of mass, or barycenter, of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero. Questions? What is relative position? Another one, In the case of a single rigid body, the center of mass is fixed in relation to the body, and if the body has uniform density, it will be located at the centroid. So what is uniform density? Sorry for these questions. Ok? I just want to play like this man. http://upload.wikimedia.org/wikipedia/commons/b/b3/Bird_toy_showing_center_of_gravity.jpg http://forum.allaboutcircuits.com/attachment.php?attachmentid=47922&stc=1&d=1351338995 I can't resize the photo. sorry.