 Solution

The original diagram has three horizontal spans $L$, $2L$ and $L$. In the small-slope approximation the massless string segments are straight and their transverse force components are tension times slope. The horizontal tension components cancel to first order.

For the first bead, the left segment contributes $-T{y}_1/L$ and the middle segment contributes $T(y_2-y_1)/(2L)$. For the second bead, the middle contribution is $T(y_1-y_2)/(2L)$ and the driven right segment contributes $T(h-y_2)/L$. Newton's second law therefore givesWith ${\omega }_0^2=T/(2ML)$ and $h=h_0\cos \omega t$, the coupled equations of motion areThe factor two in the driving term follows from the right span having length $L$, while the middle span has length $2L$. Treating all three spans as equal would change both the coupling and the normal mode frequencies.