stretch term rs in the above equation 1 in order to derive the parameters of an optimum rod string one begins with the basic stress strain equation in a steel and fiberglass rod string the average stress on the fiberglass string u03c3 1 becomes ignoring the effect of buoyancy on the weight of the rod equ1 similarly the average stress on the steel portion of the rod sub u03c3 s becomes equ2 thus the total strain on the rod string can be calculated by summing the individual strains of the steel and fiberglass portions of the rod string so that the total strain becomes equ3 where e f and e s represent the modulus of elasticity of fiberglass and steel respectively l f represents the total length of the top fiberglass rods in feet l s represents the total length of the bottom steel rods in feet e f represents the modulus of elasticity of the top fiberglass rods in pounds per square inch e s represents the modulus of elasticity of the bottom steel rods in pounds per square inch and 12 converts feet of rod into inches the above equation represents the strain on a stationary rod string resulting from gravity without taking into account strains produced by acceleration however the effects.