Dynamic In the first part, statics, we started from the axioms of statics, then obtained the equilibrium conditions of a rigid body by simplification of the system of forces. They are used to investigate equilibrium problems of rigid bodies and of system of rigid bodies. In this chapter, we shall introduce a theorem which is suitable to study equilibrium problems of any system of particle in general. Using the concepts of displacements and work, the theorem defines the equilibrium conditions for any system of particles. It is called the theorem of virtual displacements. It is the most general theorem to study equilibrium problems. In addition, combining it with D'Alembert's principle, we can obtain a general equation of dynamics which can be used to solve the problems of dynamics3 In the first part, statics, we started from the axioms of statics, then obtained the equilibrium conditions of a rigid body by simplification of the system of forces. They are used to investigate equilibrium problems of rigid bodies and of system of rigid bodies. In this chapter, we shall introduce a theorem which is suitable to study equilibrium problems of any system of particles, in general. Using the concepts of displacements and work, the theorem defines the equilibrium conditions for any system of particles. It is called the theorem of virtual displacements. It is the most general theorem to study equilibrium problems. In addition, combining it with D‘Alembert’s principle, we can obtain a general equation of dynamics which can be used to solve the problems of dynamics