Hi folks...can someone tell me where I'm going wrong with this question, or back me up if I'm right? A string of natural length 2a and modulus of elasticity b has its ends attached to fixed points A and B where AB = 3a. Find the work done when the midpoint C of the string is pulled away from the line AB to a position where triangle ABC is equilateral. My attempt to solve is as follows: I ignore B and focus just on A and C if the natural length of the string = 2a then the midpoint represents half this length and therefore = a AB = 3a, and so AC = 3a/2... the extention x = a/2 AC when streched to the equilateral triangle = 3a and so...x = 2a The question asks me how much work is done from pulling the midpoint from its initial position on the line AB to its final position Surely I want to subtract the work taken to pull the string to from its natural length to AC1 from the work required to pull the string to AC2..using Work = (b(x)^2)/2a... (b((2a)^2-(a/2)^2)/2a = 15ab/8. The books answer is given as 4ab however....I've tried to find a problem with my working and cannot.