My textbook (University Physics by Young and Freedman, 13E) showed that if a ball rolled down an inclined plane, friction would point uphill. Below are the relevant diagrams:
It then says that "If the ball were rolling uphill without slipping, the force of friction would still be directed uphill.” I’m presuming that the ball spins with clockwise rotation looking from the side. By this token, friction will point upward along the incline because friction resists motion and will exert a torque about the center of mass that counters the clockwise rotation.
However, the ball is also translating uphill, and shouldn’t friction want to counter that translation by pointing downhill? How do I know which type of motion friction wants to oppose?
And also, I had assumed that the ball was rolling with clockwise rotation. What if it’s counterclockwise? In that case, friction would point downhill, would it not? Yet my book seems to imply that friction will point up no matter which direction the ball rotates in.