The applet simulates the motion of two pendula, coupled by a spring.The pendula are identical in shape (a bar of length $ l$, width $ w$ and depth $ d$) and have the same mass $ m$. They are coupled by a spring of force constant $ k$ placed at a variable distance $ c$ from the suspension point. The equilibrium length of the spring is taken equal to the distance between the pendula. The distance from suspension point to the center of mass of the pendulum is called $ \ell$. The angle $ \alpha\,(\beta)$ gives the deviation of the left (right) pendulum from the equilibrium position.
The forces acting on the pendula are the gravitational force (with a gravitational constant $ g=9.81$ N/m) and the harmonic force due to the spring. Friction is not taken into account.