A conducting light string is wound on the rim of a metal ring of radius r and mass m. The free end of the string is fixed to the ceiling. A vertical infinite conductingsmooth plane is always tangent to the ring. The arrangement is shown in the figure. The ring is released and simultaneously a uniform magnetic field of strength B perpendicular to the plane of the ring is switched on. The string and the plane are connected by a resistance R. Find the terminal velocity of the spring as a function of time .
Ans : mgR/4B^4r^4
