I apologize in advance for not exactly adhering to the template, but the question I have here arose from my attempts to solve the following exercises, so please bear with me. (Edit: I also apologize if discussion of a concept belongs in a different forum, as this is not exactly a homework problem, just a confusing aspect of physics that I need to understand in order to solve a homework problem.) In Sears and Zemansky's University Physics, 12th ed., I have come across two problems involving rotational motion that kind of confuse me: 5.96) Consider a banked, wet roadway, where there is a coefficient of static friction of 0.30 and a coefficient of kinetic friction of 0.25 between the tires and the roadway. The radius of the curve is R = 50 m. (a) If the banking angle is β = 25°, what is the maximum speed the automobile can have before sliding up the banking? (b) What is the minimum speed the automobile can have before sliding down the banking? 5.119) A small block with mass m is placed inside an inverted cone that is rotating about a vertical axis such that the time for one revolution of the cone is T [figure omitted]. The walls of the cone make an angle β with the vertical. The coefficient of static friction between the block and the cone is μs. If the block is to remain at a constant height h above the apex of the cone, what are the maximum and minimum values of T? The only roadblock preventing me from solving these problems is understanding how exactly static friction plays a role in these problems. I figure that, above a certain rotational speed, fs points down the incline in both problems, working in conjunction with the normal force to keep the object in rotational motion, as seen in this free body diagram. But, on the other hand, if the car stops moving in the first problem, or the cone stops rotating in the second, the object starts to slide down the incline, opposed by a frictional force pointing up the incline, as seen in this other free body diagram. The only problem I have is, what happens as rotational motion begins? The frictional force before rotational motion starts points in the opposite direction as the frictional force while rotational motion occurs, so what exactly am I missing in the middle that causes the switch to happen in the first place? And, to tie it back to the original questions, how does that shift determine the minimum or maximum rotational velocities necessary for the objects to keep vertical equilibrium? This particular concept has been bothering me for about a week and a half, so any help would be greatly appreciated! If it helps, I already understand how rotational motion is maintained at constant speed, it's just when I have to find a range of speed that things fall apart in my head.