Question: Refer to https://www.physicsforums.com/threads/rolling-motion-of-plank-and-cylinders.93329/ I am able to understand all the points until the point v_roller+rω=v_plank. I have a tough time understanding this part. My attempt at the question: I think that the static friction at the top and bottom of the roller will provide the external torque. This external torque will provide the rotating motion. V_roller is the velocity due to the friction between the ground and the roller? rω is due to the friction between the roller and the plank? And one more question I know what is pure rolling but i am wondering how does non pure rolling looks like, is it rolling or not ?