You have to forgive me, I don't speak english very well. "I" is the moment of inertia.
If really the plane would not have a friction the sphere would not rotate. I supposed that the sphere spins without crawling, in this way friction's work is equal to zero, but is the friction that allows the ball to rotate.
We have to think at "a little friction" that doesn't change significantly our hypotheses
You have to forgive me, I don't speak english very well. "I" is the moment of inertia.
If really the plane would not have a friction the sphere would not rotate. I supposed that the sphere spins without crawling, in this way friction's work is equal to zero, but is the friction that allows the ball to rotate.
We have to think at "a little friction" that doesn't change significantly our hypotheses
Indeed, this difference in meaning is subtle but important