# Inverse Dynamics - 2 Rigid Bodies connected with hinge

1. May 15, 2013

### leonj

I am trying to simulate and animate inverse dynamics. So I need to calculate torques. I started out by just taking a rod which rotates about its COM, applied a torque and then just used $\tau=I\alpha$ to make sure I get back the same torque I applied. Next, I attached the rod to the ground with a hinge joint. This time to get the torque I used the parallel axis theorem to get $\tau=(I_{COM}+Mh^2)\alpha$. So my next step is to attach a second rod to the end of the one attached to the ground. What I am stuck on now is I am not sure how the two rods interact.

For example how do I calculate the torque on the rod connected to the ground? When it is just one rod I used the aforementioned $\tau=(I_{COM}+Mh^2)\alpha$, but now with the other rod attached to it, what else do I need to take into account? Is there a negative torque on the opposite side?

Note: For now I have disabled gravity.

I would appreciate any insight. Thanks

2. May 15, 2013

### SteamKing

Staff Emeritus
If you are not familiar with Lagrangian dynamics, this would be a good excuse to do so.