1. The problem statement, all variables and given/known data This is a general situation that I'm trying to figure out. Say there are two massless rods connected with a revolute joint, and one of the rods' other free end is connected to the wall with a free pin joint. There are two point masses in the system (see attachment), so the system is in a double pendulum configuration. Now a torque τ appears between the rods only. What are the equations describing the motion of the two masses? There's no gravity (i.e. this is sideways in a plane, for example) Intuitively both the bottom and top masses will rotate and displace around some pseudo-center-of-mass point (the green dot P?), and with different displacements. I drew this guess onto the figure as well. 2. Relevant equations These are not correct: τ = I(m2 about P)*α(m2) where I(m2 about P) = m2 * r(b to P)^2 and τ = I(m1 about P)*α(m1) where I(m1 about P) = m1 * r(a to P)^2 3. The attempt at a solution The problem is that the wall is causing some strange behavior of the system, and I don't know how to approach this in closed form due to the constraint of the wall... Hints?