In 1743 Jean le Rond d'Alembert generalized the concept we now call virtual work to dynamical systems with rigid constraints, like rods or string under tension, a form that became known as the d'Alembert principle. In the case of static (in equilibrium) rigid bodies without friction, the principle of virtual work says the net work of all applied forces ( F i ( a ) ) under variation of positions ( r i ) is zero: