Hey, I recently got Schaum's Outlines: Enginering Mechanics: Statics and Dynamics and started reading up on it... Here's what confuses me from the text: "A couple consists of two forces equal in magnitude and parallel, but oppositely directed." Would that be similar to saying "A couple is pretty much a third-law force pair"? That's what I "got" from that sentence, but hey, maybe I'm wrong. Now, it proceeds to talk about replacing a single force: "A single force F acting at point P may be replaced by (a) an equal and similarly directed force acting through any point O and (b) a couple C = r x F, where r is the vector from O to P." Can someone explain why it would be justified to replace a single force by a couple? (Maybe I need a clarification on what a couple actually is) Thanks a lot for any responses.