Hi, I don't understand the difference between rolling with and without slipping. To put my question into some kind of context, consider a ball of radius r which rolls to the right along the x (horizontal) axis. The ball is travelling at a constant velocity v and continues rolling until it reaches a curved hill, eventually stopping at some distance H above the x-axis. If there is no friction between the x-axis and the ball then H can be found by solving (1/2)mv^2 + (1/2)Iw^2 = mgH...(1). That is assuming that there is no 'slipping.' But what does slipping actually mean in this context? I was told that if there was 'slipping' then the value of H would be less than that obtained by solving equation (1). So if there is no slipping then does that mean that there cannot be any friction? It's all quite confusing to me. In particular, I don't understand the following means. Let G be the COG of the ball which is rolling. Let point A be the point of contact between the ball and the x-axis (as a visual aid, the line segment AG is perpendicular to the x-axis). I vaguely remember certain consequences of slipping and no slipping in this situation but I'm not really sure. If there is slipping (or no slipping), then can something be said about the relative velocity of the points A and G? Any help would be great thanks.