body acceleration in contact between two bodies

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Hey,

I am trying to simulate a wheel(cylinder) on a surface (ground_plane). I launch the gazebo with sdf file which contains both models and give it a 3 seconds to get rest with parameters Kp =380 and Kd=10. I can see the spring rest when I look at the output of position and velocities commands of z axis (GetWorldCoGPose() and GetWorldCoGLinearVel() gazebo API commands), but when I use GetWorldLinearAccel() command I get acceleration z value as zero although the wheel is moving. Sombody knows why? maybe the GetWorldLinearAccel() is not the right command?

Thanks in advance