Robot slips on ground while moving arm

edit

Make sure you're setting the virtual joint with the correct robot base link. Assuming you're using the WidowX arm from RobotnikAutomation, you should be using base_footprint as the child frame name. You can check the root link for a robot URDF/Xacro using e.g.

check_urdf <(xacro --inorder $(rospack find widowx_arm_description)/robots/widowx_arm.urdf.xacro)

Source

Often, Gazebo-specific code is placed in a separate .xacro file that incorporates the original URDF. Such a file might look like:

<?xml version="1.0" ?>
<robot name="widowx_arm" xmlns:xacro="http://ros.org/wiki/xacro">
  <xacro:include filename="$(find widowx_arm_description)/robots/widowx_arm.urdf.xacro"/>

  <!-- Used for fixing robot to Gazebo 'base_link' -->
  <link name="world"/>
  <joint name="fixed" type="fixed">
    <parent link="world"/>
    <child link="base_footprint"/>
  </joint>
</robot>

You can put static tags to the base link. It might cause problems with the simulation as it introduces more constraints. You have to try and see

<link name="base_link">
    <static>true</static>
    ...
</link>