A ring of mass m, radius r and made of an insulating material carries uniform charge. Initially it rests on a frictionless horizontal table top with its plane vertical. In the region right side of vertical plane AA’ is established a uniform horizontal magnetic field of induction B pointing everywhere parallel to the axis of the ring. The center of mass of the ring is given an initial velocity of $v_0$ without any angular velocity. The ring is found to start pure rolling on the tabletop when it had just entered completely into the region of magnetic field.