title: Pydrake - How to output reaction forces from plant
body: <p>How do I evaluate the output spatial force reaction forces?</p>
  <p>Currently I have the following implementation:</p>
  <pre><code>simulator_context = simulator.get_mutable_context()
  reaction_forces_output_port = plant.get_reaction_forces_output_port()
  plant_context = diagram.GetMutableSubsystemContext(plant, simulator_context)
  reaction_force_vec = reaction_forces_output_port.Eval(plant_context)
  spatial_force = reaction_force_vec[int(man_suc_joint.index())]
  </code></pre>
  <p>How do I evaluate the components of this spatial force?</p>