Consider a pulley fixed to the ceiling. A mass-less string is wrapped around it, with each side of the string hanging down either side of the pulley. Since the pulley has friction with the string, tension along the string will vary. Let's say the string is attempting to move clockwise, so the friction between the pulley and string is counter-clockwise. Which direction is the net normal force exerted on the string by the pulley? Is my assertion correct that the net normal force would be tilted towards the right (the side with greater tension in the string)? Also, today I suddenly found my self hung up over this seemingly simple notion: Why do we treat tension in the string as a scalar? For instance, when integrating over an element of the string, we treat the elements of normal force as vectors and the tension as scalar, why? I would appreciate if someone could give a really deep insight into this as I have an appetite for details!