I would like to study a two mass-spring system, considering also the gravity forces. I have this system: Figure 1
So, I took the equations from this site: http://scipy.github.io/old-wiki/pages/Cookbook/CoupledSpringMassSystem
And I put the masses in a vertical position and I considered these equations: $$ m_1 x_1'' = m_1 g + k_2\cdot (x_2 - x_1 - L_2) - k_1\cdot(x_1-L_1) - c_1x_1' $$ $$ m_2 x_2'' = m_2 g + F - k_2\cdot (x_2 - x_1 - L_2) - c_2 x_2' $$
With L1 and L2 the lengths of the springs when subjected to no external forces, and F the applied force.
Then I plot the positions: Figure 2
And I think it's weird because, in reality, the two masses must continuously going down, but as you can see in the graph, they are oscillating around their initial position.
What did I forgot? Thank you!