1. The problem statement, all variables and given/known data In rotational dynamics, a typical problem would be along the lines of a ring with mass m and radius r rolling down a hill with angle θ to the horizontal. Find the acceleration of the ring. 2. Relevant equations Ʃτ=I[itex]\alpha[/itex] = r x (friction) ƩF = ma = mgsinθ - (friction) 3. The attempt at a solution These equations usually give the correct answer for the problem (plug in the numbers, isolate, solve for whatever the question asks). My question is that these equations don't seem to make sense if θ=0. That would imply that F = -(friction) and the ring is slowing down (if it was moving originally). However, that is not the case if there is no rolling friction. Why does this equation break down? A similar dilemma I have is given a yoyo on the floor, if you pull vertically up on the string, the yoyo rolls in a certain direction. However, there is no net force horizontally, so how does it roll? If the answer is that the torque causes it, when why would newton's second law even apply in the case of yoyos rolling down a string in midair?