Let the length of the pendulum be l, then we can represent the co-ordinates of the motion of the pendulum as per the given question
x(θ)=lsinθ
y(θ)=−lcosθ
now, differentiating with respect to t,
dtdθ=θ˙
dtd(x(θ))=lcosθdtdθ
⇒dtd(x(θ))=lθ˙cosθ
dtd(y(θ))=lsinθdtdθ
⇒dtd(y(θ))=lθ˙sinθ
Now, applying the conservation of energy
L=T−V=KE−PE
=2mlθ2˙−mgl(1−cosθ)
We know that the Lagrange equation for the generalized co-ordinate
dtd(dθ˙(∂L))−∂θ∂L=0
⇒θ¨+lgsin(θ)=0
Hence, natural frequency of the simple pendulum
ω=lg
T=2πgl
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