Plastics wave 8 11-1 Movement differential equations of elastic objects This chapter we still adopt the assumptions (1) Elastic objects are ideal elastic objects (2) The displacement and distortion are tinny The two assumptions are equal to the basic assumptions when we discuss. static force questions. So the physic and geometry equations and elastic equations where stress component is expressed by displacement component, still are the same with movement equations at any instantaneous time. The only difference is that the equilibrium differential equations of static questions must be substituted by movement differential 7 equations7 §11-1 Movement differential equations of elastic objects The two assumptions are equal to the basic assumptions when we discuss static force questions. So the physic and geometry equations and elastic equations where stress component is expressed by displacement component , still are the same with movement equations at any instantaneous time. The only difference is that the equilibrium differential equations of static questions must be substituted by movement differential equations . This chapter we still adopt the assumptions: （1） Elastic objects are ideal elastic objects. （2） The displacement and distortion are tinny