In the picture above you can see a SCARA robot, with its end effector (the point x,y). The torques c_a and c_b are the ones of the motors in the joints, while f is an external force. My teacher has written the virtual work principle equation (for the static case) without considering the opposite c_b torque acting on the first link (the red one, which I drew). My question is why? They are internal torques but are on two different links that also have different virtual angular displacement, so I think we have to consider it.