cmd vel - control robot

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Hello. I have a problem using ROS. I'm a beginner. I want to move this robot in the Gazebo simulation.

What I'm doing is the following: I open a terminal and type:

roscore

Then I open a new tab and I type:

cd catkin_ws

roslaunch p3dx_gazebo p3dx_empty_world.launch

So Gazebo opens with the robot inside.

Now I want to move the robot (send command via cmd_vel topic). What I'm doing here is this:

rostopic pub /cmd_vel geometry_msgs/Twist -r 10 -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 0.0]'

But the robot doesn't move.

What should I do?

Thanks in advance.