A block attached to a spring, pulled by a constant horizontal force, is kept on a smooth surface as shown in fig. Initially, the spring is in the natural length state. Then the maximum positive work that the applied force F can do is (given that string does not break)
Applying work-energy theorem on the block, we get
Fl - 12kl2 = 0
l = 2Fk or work done = Fl = 2F2k