本文证明常力是有势力并应用常力势能求解理论力学问题.在平衡问题中，常力势能的引入可以用势能驻值建立受常力作用的有势系统的平衡条件并判断稳定性.借助常力势能可以将保守系统的拉格朗日方程应用于受常力作用的有势系统，在拉格朗日函数不显含时间或广义坐标时，得到相应的广义能量守恒和广义动量守恒.

A unchanged force is demonstrated as a potential force with its potential energy to solve problems in theoretical mechanics. In equilibrium problems, the potential energy can be used to establish the equilibrium conditions for a potential system subjected to unchanged forces and the stability is determined via the stationary value of the potential energy. The potential energy generalizes the conservative Lagrangian equation to a potential system with unchanged forces. For Lagrangian functions independt of the time or the generalized coordinates, the generalized energy conservation and generalized momentum conservation are obtained.