) of prismatic bars subjected to tensile or compressive centroidal loads is proportional to both the load (P) and the length (L) of the bar, and inversely proportional to the axial rigidity (EA) of the bar; this relationship is called a force-displacement relation

a)      The summation of the displacements of the individual segments of a non-prismatic bar equals the elongation or shortening of the entire bar ()

i)        

b)      If axial forces and/or cross-sectional areas vary continuously, an integral expression is required