The period of oscillation of a thin magnet at a place is T. When it is stretched to double its length and its pole strength is reduced to  14 of its initial value, then its period of oscillation is

We know time period of oscillation T=2πIMBGiven : Pole Strength : m1=m,  m2=m4;Length    l1=l,  l2=2l;Moment of Inertia I1=I=ml212, I2=m(2l)212=4I;Dipole Moment     M1 = ml = M,  M2=m4(2l) = ml2=M2;Now, T1T2=I1I2×M2M1⇒TT2=I4IM2M⇒T2=22T