A uniform disc of radius r and mass m is charged uniformly with charge q  This disc is placed flat on a rough horizontal surface having coefficient of friction on μ.  A uniform magnetic field is present in a circular region  (a>r) but varying at  kt3 as shown in fig. Then the time (in second) after which the disc begin to rotate is  2x. Find  x=? Given  r=1m,m=18kg,q=(1/3)c,μ=0.1,k=4(S.I.units),g=10m/s2