where in the last step we substituted o=P/A. If the strain (or stress) in the bar is not uniform, then Eq. (2.6) is invalid. In the case where the axial strain varies with the x-coordinate, the elongation of the bar can be obtained by integration, as stated in Eq. (2.3): __ e dx. Using g=6 /E= P/(EA) where P is the internal axial force, we get