mujoco

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 Copyright 2019 DeepMind Technologies Limited

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<mujoco>
  <option gravity="0 -9.81 0" />
  <worldbody>
    <geom name="ground" type="plane" size="5 5 1" euler="-90 0 0"/>
    <body name="parent" pos="0 10 0">
      <geom name="parent_shape" type="sphere" size="0.5"/>
      <joint type="free" />
      <body name="child" pos="2 1 0">
        <geom name="child_shape" type="sphere" size="0.5"/>
        <joint type="hinge" axis="1 0 0"/>
      </body>
    </body>
  </worldbody>
</mujoco>