Q.
As shown in the above figure, two strings and a bob system of length are maintained at an angle of with the horizontal and the system is in equilibrium in the vertical plane. Now, if the bob is slightly displaced normal to the plane of the strings, find the time-period of oscillation
Question

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Solution:

The pendulum will oscillate in plane perpendicular to XY axis.
therefore, effective legth of oscillation is
= and