How do I use force sensor & bumper sensor in Gazebo?

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I am designing a biped robot model in gazebo. I am trying to add force sensors on the bottom of the foot, but I don't know what parameters I need to do that.. so far this is what I have written in my urdf.xacro file. If I correct if the sensor got implemented then it should publish the sensor data, right?? .Please let me know if I am missing something or have written it wrong.

<link name="R_leg">
  <inertial>
    <origin xyz="0 0.0 -0.124" rpy="0 0 0 "/>
    <mass value="0.15" />
    <inertia  ixx="1.0" ixy="0.0"  ixz="0.0"  iyy="1.0"  iyz="0.0" izz="1.0" />
  </inertial>
  <visual>
    <origin xyz="0 0.0 -0.124" rpy="0 0 0 "/>
    <geometry>
      <box size="0.1 0.12 0.25" />
    </geometry>
  </visual>
  <collision>
    <origin xyz="0 0.0 -0.124" rpy="0 0 0 "/>
    <geometry>
      <box size="0.1 0.12 0.20" />
    </geometry>
  </collision>
</link>
<gazebo reference="R_leg">
  <material>Gazebo/White</material>
  <turnGravityOff>false</turnGravityOff>
</gazebo>

<link name="R_foot">
  <inertial>
    <origin xyz="0 0.0 -0.010"  rpy="0 0 0"/>
    <mass value="0.15" />
    <inertia  ixx="1.0" ixy="0.0"  ixz="0.0"  iyy="1.0"  iyz="0.0" izz="1.0" />
  </inertial>
  <visual>
    <origin xyz="0 0.0 -0.010"  rpy="0 0 0"/>
    <geometry>
      <box size="0.2 0.12 0.02" />
    </geometry>
  </visual>
  <collision>
    <origin xyz="0 0.0 -0.010" rpy="0 0 0"/>
    <geometry>
      <box size="0.2 0.12 0.02" />
    </geometry>
  </collision>
</link>
<gazebo reference="R_foot">
  <material>Gazebo/Red</material>
  <turnGravityOff>false</turnGravityOff>
</gazebo>

<joint name="R_leg3_joint" type="revolute" >
  <origin xyz="0 0.0 -0.250" rpy="0 0 0" />
  <parent link="R_leg" />
  <child link="R_foot" />
  <axis xyz="0 1 0"/>
  <limit effort="30" velocity="1" lower="-1.0" upper="1.0"/>
  <joint_properties damping="0.0" friction="0.0"/>
</joint>

<body:empty name="R_foot_force_model_name">
  <sensor:contact name="R_foot_contact_sensor">
    <updateRate>15.0</updateRate>
    <geom>R_foot</geom>
    <controller:gazebo_ros_bumper name="R_foot_contact_controller" plugin="libgazebo_ros_bumper.so">
      <alwaysOn>true</alwaysOn>
      <updateRate>15.0</updateRate>
      <bumperTopicName>R_foot_bumper</bumperTopicName>
      <interface:bumper name="dummy_bumper_iface" />
    </controller:gazebo_ros_bumper>
  </sensor:contact>
</body:empty>

Second Question : I dont understand how gazebo_ros_f3d works.. cause i get force and torque zero all the time... should it not show gravity as one of the forces... this is what i have written:

<controller:gazebo_ros_f3d name="gazebo_ros_Lf_force"plugin="libgazebo_ros_f3d.so">
  <alwaysOn>true</alwaysOn>
  <stepTime>0.001</stepTime>
  <updateRate>1000.0</updateRate>
  <bodyName>L_foot</bodyName>
  <topicName>L_foot_force</topicName>
  <frameName>map</frameName>
  <interface:position name="p3d_L_foot_force"/>
</controller:gazebo_ros_f3d>