The internal forces present in a continuous body represent interaction between molecules and ensure the existence of a solid body. These forces act when no external forces are applied to the body but are not in themselves the object of study here. However, under the action of external forces a body will deform and the mutual position of molecules changes and also the distances between them. The action of external forces that produce deformation gives rise to additional internal forces. Because the study of elastic and plastic deformation is primarily concerned with the external forces involved it is necessary to convert the internal forces into external forces. This is achieved by employing the so-called method of sections.
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