I don't really have a specific problem, but for example, I was doing a problem where a constant force unwounds a spool of wire (a disk). The force pulls at the top of the disk to the right and the force of static friction is also at the bottom to the right, the same direction as the sphere is moving. But in an example we did in class where a sphere rolls down an incline, the force of static friction was to the left, up the incline and the opposite direction of the way the sphere was rolling. I'm really lost as to how to find out which was static friction is supposed to go, can anyone enlighten me? I wasn't sure where else to post this question so I'm sorry if it is in the wrong place.