After losing, the [D'Alembert system] requires bettors to increase their next bet by one unit. So after losing aR$10 bet on heads, the next bet should beR$20 on heads. In the event that a wager is a winner, the system requires the bettor to then decrease the next bet by one unit.
D'Alembert's principle generalizes the principle of virtual work from static to dynamical systems by introducing forces of inertia which, when added to the applied forces in a system, result in dynamic equilibrium. D'Alembert's principle can be applied in cases of kinematic constraints that depend on velocities.