Abstract: This article discusses the problem of three force equilibrium, mainly involving the three force intersection equilibrium theorem and its corresponding Lamys theorem, the three force parallel equilibrium theorem, and its corresponding lever law. Lamys theorem essentially belongs to the triangle sine theorem. This article expands the conditions for the use of the Lamys theorem, thereby simplifying the difficulty of solving the three force convergence equilibrium problem. This article provides examples to illustrate the application of Lamys theorem and levers law in solving problems such as statics and friction. For the first time, the scope of application of Lamys theorem has been extended to kinematics and plane equilibrium force system composed of a couple and three forces. It can be used to calculate velocity in problems such as rigid body plane motion and composite motion of points. The research results help to improve the quality of teaching and cultivate students interest in mechanics. 

Key words:  lever law, law of force triangle, statics, kinematics, theoretical mechanics