"elastic off-center central collisions" (?) Hi, I'm new to the forums and, to a large extent, to physics in general. Anyway, I've been interested in the dynamics of simple collisions of spherical particles in 2D and 3D space. I've been searching for information on the collisions of this type and to such ends I recently bought a book entitled "Handbook of Physics". So you know, it's a recent edition (copyright 2006). So, to the specific question at hand: The book entitles a particular section on collisions as "elastic off-center central collisions", but simultaneously defines the terms "off-center" and "central" collisions as follows (direct quote from book): See the contradiction in the above? The section presumes to be about "off-center central collisions" but the given definitions of "off-central" and "central" contradict each other as far as torque is concerned. Has the book juxtaposed some of it's terms? Or, perhaps there is something I am missing. The terms in the general local section are "straight-line collision", "non-central collision", "collision normal", "central collision", and "off-center collision". Can someone who's physics savvy help me discern the nature of this dilemma? Do I just not know the terminology? It seems like a pretty blatant logical self-contradiction to me... The confusion is detrimental in two respects: Firstly, I cannot know for sure what particle conditions they are actually referring to thus cannot select a situation to apply it to. And secondly, even if I did apply it the contradiction suggests the possibility of other errors. Hopefully the information I have given is sufficient to resolve the matter, but if not feel free to ask me for more info, I'd be happy to provide it. Your general perspective on the subject of collisions is of course also welcome. Thank you for your time and for reading this thread. I await any replies you may have.