In the shown figure, two parallel rails in a vertical plane of negligible resistance are connected by a resistance  $R=1\Omega$ at one end. A uniform horizontal magnetic field is present  $(B=0.5T)$. A thin ring of mass  $m_1=1kg$, radius  $r=25cm$ is fitted over a wooden disc of mass  $m_1=1kg$. Resistance of ring  $R\text{'}=4\Omega$ and there is no friction between the ring and the disc, ring and upper rail. Friction between ring and lower rail is sufficient to support pure rolling of  the ring. Horizontal force  $F=2N$  is applied at centre of the disc. When the terminal  velocity is reached