ODE friction parameters and max_vel parameter in SDF format.

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Hello! I'm trying to understand how the parameters that I set in <ode><friction> and <ode><contacts><max_vel> influence the simulation.

I build a basic gripper and I wanted to set the maximum friction on the gripper surface, so I put the following parameters in my SDF:

` <surface>

   <surface>
      <friction>
        <ode>
          <mu>1</mu>
          <mu2>1</mu2>
        </ode>
      </friction>
    <contact>
        <ode>
          <max_vel>0.0</max_vel>
        </ode>
      </contact>
    </surface>

` The questions are the following:

1. What exactly represent mu an mu2? I have read in the ODE manual that they are Coulomb friction parameters, so I would expected that mu is the static friction parameter and mu2 the dynamic one, but in the manual and in the SDF documentation there are some references to different directions...
2. Why there still subsist a relative motion between my gripper and the object?