In the spring$-$loaded pendulum system shown below,

let's assume that the spring force acting on the pendulum is zero when the pendulum is in the vertical position $($that is$,$ when $=0).$ Also, assume that friction is negligible and that the swing angle $$ is small. Obtain the mathematical model of the system $($Note: The air friction is $B=0,$ and also the elasticity of the suspension and the rope is $K=0).$