Suppose we have a rod with Young's modulus (Y) and length L and a constant area (A). Now we apply a force (on both ends so that rod is in equilibrium) which increases it's length by suppose ∆l. By equation stress = Y strain. I can write F= YA∆l/l. Now suppose I increase the force by ∆F which causes a increament in length by ∆x. Then what will be the new equation, ∆F = YA∆x/l or ∆F=YA∆x/l+∆l. What length do we consider in the equation, just before the force ∆F is applied or the length when no force was applied to it?
