If friction is present, the motion is not rolling without slipping, but rolling with slipping. If there were a static friction force, the centre of mass would slow down and the motion would be uniformly decelerated instead of uniform. But if the CoM decelerated, since it is a motion of pure rolling, we would derive that its angular acceleration should be non-zero (negative) and the angular velocity of the sphere
should therefore decrease accordingly. Therefore the mechanical energy of the sphere (translational kinetic energy + rotational kinetic energy) would decrease in time. But in pure …