What is ?
 can be defined as that science which describes and predicts the conditions of  or  of bodies under the action of forces. It is divided into three parts:  of  bodies,  of  bodies, and  of .
The  of  bodies is subdivided into  and , the former dealing with bodies at , the latter with bodies in . In this part of the study of , bodies are assumed to be perfectly . Actual structures and machines, however, are never absolutely  and [5-動詞] under the loads to which they are subjected. But these [5-名詞複数] are usually small and do not appreciably affect the conditions of equilibrium or  of the structure under consideration. They are important, though, as far as the resistance of the structure to failure is concerned and are studied in  of materials, which is part of the  of  bodies. The third division of , the  of , is subdivided into the study of   and  . An important subdivision of the study of   is , which deals with problems involving water.