=+Problem 1.9 A mass m slides without friction on a rod positioned at a fixed angle ???? with respect to the horizontal direction (Figure 1.15.d). The mass is fixed to a nonlinear spring developing a force kx3 where x is its extension. The spring is aligned with the rod. To the sliding mass a single pendulum is attached. The pendulum consists of a massless rod and two masses attached to it: a mass m at a distance ∕2 and a mass m∕2 at a distance . Gravity acts in the vertical direction.