The Modifications to the robot URDF section in Chapter 2, Creating Your First Two-Wheeled Robot (in Simulation) describes changes that need to be made to the URDF model so that Gazebo recognizes it as an SDF. The next section identifies the changes needed for our robot arm,
Specific elements unique to the Gazebo simulation environment are grouped in the following areas:
<material>tags are used to specify the Gazebo color or texture for each link
<mu2>tags are used to define friction coefficients for the contact surfaces for four of the robot's links
rrbot(included here but described in the Adding a Gazebo ...