fine i've pulled the mechanics textbooks out of my bookshelf the two textbooks by feynman and goldstein refer to a center of gravity i've added them to the article with quotations and page numbers on the other hand the following textbooks define the center of mass but not a center of gravity arnold mathematical methods of classical mechanics marion and thornton classical dynamics of particles and systems kleppner and kolenkow an introduction to mechanics jose and saletan classical dynamics a contemporary approach i haven't decided what to do with them