The restoring torque is J = - 2 × kx (2L​) cos θ = I (dt2d2θ​)
Now, x = 2L​ sin θ Therefore, J = - k (2L​) cos θ = I (dt2d2θ​) ⇒ (4−kL2​) sin 2θ = I (dt2d2θ​) For small θ, sin 2θ = 2θ. Therefore, 2−kL2θ​ = I (dt2d2θ​) where I = 12ML2​. Therefore, dt2d2θ​ = (M−6k​) θ = −ω2θ (SHM) ⇒ ω = M6k​​ Hence, the frequency of oscillation is 2πω​ = 2π1​M6k​​
