I used the sdf2usd command from gz-usd to convert the
mimic_fast_slow_pendulums_world.sdf
example world from SDFormat to USD.
mimic_fast_slow_pendulums_world.usdahas angular drives enabled for each joint that hold the joint positions to zero, preventing the pendulums from swinging.mimic_fast_slow_pendulums_world_without_drives.usdahas no angular drives (they were manually removed), so the pendulums swing in IsaacSim but energy is not conserved.