Hello Im new here, I hope I'm not disturbing anyone. Following this guide below, im trying to find two 2. order differential equations, one for q1'' and one for q2'', describing the movement of the double pendulum. I have no problems with the mathematics, but when the guide tells me to use Mathematica, I have some problems in getting the same result as in the guide and elsewhere. Could you help me? I'm attatching my recent try in mathematica. Note: I'm not trying to numerically solve the differential equations, as done later in the mentioned guide, I'm just trying to "create" those differential equations. The guide mentioned is: http://myphysicslab.com/dbl_pendulum.html Sorry for my bad english.