https://www.physicsforums.com/attachment.php?attachmentid=7470&stc=1&d=1155157461 spool.JPG Q: if the thread(see attach) is pulled in the direction shown, which way will the spool roll? i know that the spool will roll to the right according to newton's law (for every action there's an equal and opposite direction). but there's also a moment apply to the spool (the moment arm is from center to the bottom of the thread). could you say that there's is an equal and opposite moment(with moment arm from center to the top of the thread)? What law states this? -what else is this question trying to get you to understand, if any?