how to make Nao standing on the ground?

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Hi, here is a really simple question?? How to make Nao standing on the ground in Gazebo? I'm using Ubuntu 16, ros-kinetic on the VMware player 14. I type following code in my terminal:

roslaunch nao_gazebo_plugin nao_gazebo_plugin_H25.launch

everything looks good, and the joint position trajectory controller is successfully loaded as well. except the Nao seems slippery on the ground, and later it will fall on the ground. In thenaoFoot.world file, I've checked the friction coefficient, it seems sufficiently large (mu=50), so I believe it's some other reason to make the robot not able to stand on the ground. Here is the screenshot of the Nao falling down Any ideas?

Best Yuan