Imagine a ball rolling down a hill or inclined plane without slipping from a particular height. The conservation of energy law says that the final energy remains the same as the initial energy. But, when a ball is rolling without slipping, there must be a friction. So, what I think is E = E' mgh + Ffriction s = (1/2) mv^2 + (1/2) Iω^w But, in every book I read says that E = E' mgh = (1/2) mv^2 + (1/2) Iω^w Why is there no work done by friction ? I questioned this since if the ball is not rolling, it must be mgh + Ffriction s = (1/2)mv^2 But, when it's rolling, there is no work done by friction.. Why?