50.1.2 is the cross-sectional area of the rod; the longitudinal stress is then the force applied to either end of the rod divided by the rod's cross-sectional area. Here Fn is the force applied...50.1.2 is the cross-sectional area of the rod; the longitudinal stress is then the force applied to either end of the rod divided by the rod's cross-sectional area. Here Fn is the force applied normal to the area A,L0 is the original (unstressed) length of the rod, L is the stressed length of the rod, and ΔL=L−L0.