Unable to convert URDF to sdf to use in Gazebo, but works fine in Rviz

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Hi all,

I am new to ROS and was trying to follow along the urdf tutorials on URDF to create my own robot (a cart that should be pushed) and using it in Gazebo. I managed to create cart_v0.urdf.xacro that is visualized by

roslaunch urdf_tutorial display.launch model:=`rospack find reemc_cart`/urdf/cart_v0.urdf.xacro

like this (not enough karma to upload files). Then I try to

roslaunch urdf_sim_tutorial gazebo.launch model:=`rospack find reemc_cart`/urdf/cart_v0.urdf.xacro

according to the next tutorial in order to spawn the model in Gazebo, producing this console output (this indicates that the spawn has been successful right?). However, the produced world is empty besides the ground_plane, supposedly created by Gazebo?

I already tried converting cart_v0.urdf.xacro to cart_v0.urdf by

xacro --inorder cart_v0.urdf.xacro > cart_v0.urdf`

to test it for sdf generating by Gazebo according to

gz sdf -p cart_v0.urdf

However, the generated sdf file just consists of a sdf version and model name:

<sdf version='1.6'>
  <model name='reemc_cart'/>
</sdf>

I suppose this is also the reason why the world is empty without Gazebo throwing warnings/errors, but have not been able to find anyone suffering from the same problem. Is the fact that Rviz is able to show my urdf file without warnings enough indication that the URDF file itself is not faulty? And if the URDF is not faulty, where should I start looking in order to use this URDF in Gazebo?

I'm running Ubuntu 16.04 with ROS Kinetic and an install according to the relevant REEM-C tutorial. I tried to adhere to the post guidelines, but if I forgot to attach relevant information, please indicate it and I'll try to add it.

Thanks in advance!