A ring of radius ‘r’ made of an insulating material has mass ‘m’ and carries uniform charge. Initially it rests on a frictionless horizontal tabletop with its plane vertical. A uniform horizontal magnetic field of induction ‘B’ pointing everywhere parallel to the axis of the ring is established in the region to the right of a vertical plane AA as shown in the fig. The ring is pushed forward to acquire a velocity $v_0$ without any rotation.

When the ring starts entering into the magnetic field it experiences torque, hence ring also gets rotation in addition to the retarded translation. When the ring completely enters into the magnetic field the total angular impulse about centre of mass can be written as $\lambda R\left|\Delta \varphi \right|.$  Where  $\lambda$ is linear charge density, R is the radius,  $\Delta \varphi$ is the change in the magnetic flux. Magnetic flux is defined as  $\varphi =\overrightarrow{B}.\overrightarrow{A},$ where B is magnetic induction field and A is the area of the loop.
Take  $v_0=4\sqrt{2}m/sand\frac{Br}{m}=4$ S.I units