Well, my physics teacher taught us about rotation the other day and I came across a scenario where a sphere and a ring roll down a friction-less inclined plane from a point of absolute rest. I found it counter-intuitive as I started to think about why would they start rolling in the first place. There is no off-set force that can provide torque. So, I raised my hand and argued that the normal force and gravity pass through the center of mass of the body, so the sphere ought to just slide down the plane. Then he drew a diagram and a gravity vector going straight down the center of the sphere and said gravity provides torque if we consider the point of contact as the pivot. At that point I didn't know what else to say and pretended to be satisfied by that. But, I'm not and that's why I'm here. Please help!