ABSTRACT
Whenever a body is subjected to external forces, internal forces are generated. Internal forces are the forces that one part of the body applies on the other part. Figure 3.1 shows a rod fixed at one end. It is pulled by a net force of F along the longitudinal direction applied at the other end. For convenience of understanding, it can be assumed that the force is uniformly distributed over the cross section of the rod. To obtain the internal force at a distance of x from the fixed end, the rod is hypothetically cut at this point. Consider the free body diagram of the right-hand side of the rod. This portion is subjected to the applied force F and an internal force that is applied by the left-side portion of the rod. As the entire body is in equilibrium, by Newton’s first law of motion, the internal force on the right-side portion of the body must be equal to F directed toward the fixed end. The force is uniformly distributed over the cross section, but the figure shows a net force F passing through the centroid of the cross section. By Newton’s third law, the right-hand-side portion exerts a force F on the left-hand-side portion of the rod. This force is directed toward the free end. Thus, the magnitude of the internal force at a distance of x from the fixed end is F. The magnitude of the internal force is independent of x in this case. Hence, the magnitude is the same at all the sections of the body. The direction of the force depends upon which portion of the rod is being considered.
