.py

    import pybullet as p
    
    def apply_external_force(self, force, link_index, position=[0,0,0]):
    """
    Apply the specified external force on the specified position on the body / link.
    Args:
        link_index (int): unique link id. If -1, it will be the base.
        force (np.array[float[3]]): external force to be applied.
        position (np.array[float[3]], None): position on the link where the force is applied. See `flags` for
        coordinate systems. If None, it is the center of mass of the body (or the link if specified).
        frame (int): Specify the coordinate system of force/position: either `pybullet.WORL   D_FRAME` (=2) for
        Cartesian world coordinates or `pybullet.LINK_FRAME` (=1) for local link coordinates.
    """
      p.applyExternalForce(
        objectUniqueId=self.model_unique_id, linkIndex=link_index,
        forceObj=force, posObj=position, flags=p.LINK_FRAME)
      
    
    #This is essential, as this disables the default motor imposed by bullet. These default motors prevent overspeeding, and functions as
    #friction drives.
    p.setJointMotorControl2(self.model_unique_id, self.secondary_joint_id,
                              controlMode=p.VELOCITY_CONTROL, force=0)
    spring_angle = np.pi/2.0
    TORQUE_K = 10.0
    torque = spring_angle * TORQUE_K

    #apply external force
    self.apply_external_force(force= torque * self.scaled_force_axis_c, link_index=self.child_link_id, position=self.force_pos_c)
    self.apply_external_force(force= torque * self.scaled_force_axis_p, link_index=self.parent_link_id, position=self.force_pos_p)
    # print("torque: ", torque)