The equilibrium analysis of a rigid body is concerned primarily with the calculation of external reactions (forces that act external to a body) and internal reactions (forces that act at internal connections). In mechanics of materials, we must extend this analysis to determine internal forces—that is, forces that act on cross sections that are internal to the body itself. In addition, we must investigate the manner in which these internal forces are distributed within the body. Only after these computations have been made can the design engineer select the proper dimensions for a member and select the material from which the member should be fabricated