I have a urdf file for NAO robot . I converted it to XML using compile with options collision= "predefined". inertiafromgeom="true" and integrator="RK4. When I simulate the converted XML , I cannot see effect of gravity on the entire body. Only the limbs are moving due to gravity. But the entire bosy remains a fixed position. What options should I to get gravity act on the entire body? Here is the link to the drive containing the mesh folder and the urdf file. https://drive.google.com/drive/folders/1BySb9Zd00jrjQEShUs81AnuH-FSej6Hz?usp=sharing  This is solved by adding joint of type free to the torso. But there are random vibration in the limbs when the robot lands the floor like in this gif.