set gravity on each object Gazebo7

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I am tryng to set the gravity on each individual objects in gazebo. I am spawning a robot model that needs to have 0 gravity. But The objects it needs to interact with need to have gravity,so they don't float away. I am not sure how to accomplish this I can turn on/off gravity for the entire world, but not individual models. Here is my world file,

    <?xml version="1.0" ?>
  <sdf version="1.4">
    <world name="default">

      <include>
        <uri>model://sun</uri>
      </include>


      <include>


        <uri>model://ground_plane</uri>
      </include>

         <include>
        <pose>0 10 0 0 0 0</pose>
         <gravity>0 0 -9.81</gravity>
        <uri>model://button_panel</uri>

      </include>

      <gravity>0 0 0</gravity>

    </world>
  </sdf>

I would greatly appreciate any guidance.