in particle dynamics, a formula equating work applied to a system to its change in kinetic energy is obtained as a first integral of newton's second law of motion. it is useful to notice that the resultant force used in newton's laws can be separated into forces that are applied to the particle and forces imposed by constraints on the movement of the particle. remarkably, the work of a constraint force is zero, therefore only the work of the applied forces need be considered in the work–energy principle.