Q 12-05-008NEETJEE MainEAMCET 2008 (Medical)Medium
Two bar magnets are placed in a vibration magnetometer and allowed to vibrate. They make $20$ oscillations per minute when their similar poles are on the same side, while they make $15$ oscillations per minute when their opposite poles lie on the same side. The ratio of their magnetic moments is
Answer: (B) $25 : 7$
$f^2 \propto M$ (same moment of inertia). Similar poles together: $M_1 + M_2$; opposite: $M_1 - M_2$.
$$\frac{M_1 + M_2}{M_1 - M_2} = \frac{20^2}{15^2} = \frac{16}{9} \;\Rightarrow\; 9M_1 + 9M_2 = 16M_1 - 16M_2 \;\Rightarrow\; \frac{M_1}{M_2} = \frac{25}{7}$$
Solution by Sreeraj P, M.Sc Physics