mujoco

<mujoco model="energy conserving pendulum">
  <option integrator="RK4">
    <flag constraint="disable" energy="enable"/>
  </option>

  <worldbody>
    <light pos="0 0 2"/>
    <geom pos="0 0 -.5" type="plane" size="1 1 .01"/>
    <body pos="0 0 0">
      <joint type="hinge" axis="0 1 0"/>
      <geom type="cylinder" size="0.02" fromto="0 -.02 0 0 .02 0"/>
      <geom type="capsule" size="0.02" fromto="0 0 0 .1 0 0"/>
      <body pos="0.1 0 0">
        <joint type="slide" axis="1 0 0" stiffness="200"/>
        <geom type="capsule" size="0.015" fromto="-.1 0 0 .1 0 0"/>
        <body pos=".1 0 0">
          <joint type="ball"/>
          <geom type="box" size=".02" fromto="0 0 0 0 .1 0"/>
          <body pos="0 .1 0">
            <joint axis="1 0 0"/>
            <geom type="capsule" size="0.02" fromto="0 0 0 0 .1 0"/>
          </body>
        </body>
      </body>
    </body>
  </worldbody>
</mujoco>