Some complex system can be studied as SDOF systems,either exactly or under some simplifying assumption 1.SDOF rigid body assemblages. Principle of Virtual Displacements the D'Alembert Principle 2.Simple structural systems.(in an approximate manner) Assume a fixed pattern of displacement,whose amplitude(the single degree of freedom)varies with time. 目 土本2程季院 Some complex system can be studied as SDOF systems, either exactly or under some simplifying assumption  1. SDOF rigid body assemblages. Principle of Virtual Displacements the D’Alembert Principle  2. Simple structural systems. (in an approximate manner) Assume a fixed pattern of displacement, whose amplitude (the single degree of freedom) varies with time