One U shaped conducting frame is placed in a plane perpendicular to uniform magnetic field B. One conducting rod of mass m and length l is kept perpendicular to parallel sides of this frame. At time t = 0, this rod is pushed perpendicular to its length with initial velocity `v_g`. Prove that its velocity at the end of time t is `v_t=v_0 "exp" ((-B^2l^2)/(mR)t)` where R=resistance of external circuit and l=perpendicular distance between two parallel slides .