Hello, I have the third edition of Goldstein which I have been using to learn mechanics. I believe I have found an error in the book, however normally when I feel such things I tend to either be misreading the situation or misunderstanding the concept. I checked Professor Safko's site on Goldstein corrections but it made no mention of the error I believe I caught. Also, I am not sure what printing of the edition I have. The method for ascertaining the printing edition on the professor's site does not seem to apply to the book I have, but again it is possible that I am missing something obvious. However, since Professor Safko's site was last updated in 2010, and I very recently bought the book, I thought perhaps I have a more recent printing than those presented on Dr. Safko's site. That said, here's the issue. Beginning on pg. 47 in my book, the first two paragraphs state: My objection to this is the book's definition of cos theta. It appears to me that based on what has been written cos theta should be z/a. Am I wrong in this? I don't think so, because the calculation directly following this seems to coincide with the way I'm thinking about it. Please let me know if I'm off base here. Thanks.