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