CHAPTER XXI 

 STRENGTH OF MATERIALS 



280. Need of Knowledge of Strength of Materials. 

 Mechanics are often called upon to determine the size of 

 rod or beam required to support a certain weight or force. 

 Not all pieces of material have the same strength. The 

 strength of any piece of material depends upon the nature 

 of the material (cohesion of the particles composing it) and 

 upon the position, shape, and bulk of the piece. Therefore, 

 it is absolutely necessary to know something about the 

 properties and laws governing the strength of materials 

 used in industry. When a force acts on beams, structures, 

 or bodies of any kind, it may be considered as weight, and 

 may be measured in pounds. 



281. The Effects of a Load of Force on a Body. When 



a body is supporting a load, a force is acting on it. This 

 force will produce a change, perhaps not very noticeable, 

 in the form of the body. Unless this load is so great as to 

 cause a break or fracture, the elasticity, which has previ- 

 ously been defined as the tendency of the particles of a body 

 to unite, or return to their original positions, will support 

 the load. The forces of the body resisting the pull or pressure 

 of the load are called stresses. The change of shape of the body 

 producing these stresses is called a strain. To illustrate: The 

 molecules of a piece of iron are held together by the force 

 of cohesion, which is stronger in iron than in some other 



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