Before simulating a robot in a robot simulator, such as Gazebo, V-REP, and so on, we need to define the robot link's physical properties such as geometry, color, mass, and inertia, and the collision properties of the link.

We will only get good simulation results if we define all these properties inside the robot model. URDF provides tags to include all these parameters and code snippets of base_link contained in theses properties as given here:

<link> 
......     
<collision> 
      <geometry> 
      <cylinder length="0.03" radius="0.2"/> 
      </geometry> 
      <origin rpy="0 0 0" xyz="0 0 0"/> 
    </collision> 
 
    <inertial> 
    <mass value="1"/> 
    <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/> 
    </inertial> 
........... 
</link> 

Here, we define the collision geometry as cylinder and the mass as 1 Kg, and we also set the inertial matrix of the link.

The collision and inertia parameters are required in each link; otherwise, Gazebo will not load the robot model properly.