It seems that we have to plug the numeric values into context and get the corresponding equation of motion. Is it possible to plug in symbolic values and get EoM with e.g., q1, q2?
context = plant.CreateDefaultContext()
plant.SetPositions(context, q)
if v is not None:
plant.SetVelocities(context, v)
M = plant.CalcMassMatrixViaInverseDynamics(context)
Cv = plant.CalcBiasTerm(context)
tauG = plant.CalcGravityGeneralizedForces(context)
B = plant.MakeActuationMatrix()
return (M, Cv, tauG, B)