[Two ball of mass 60g are attached with mass less rubber thread and held in vertical position as show in figure. In this position length of rubber thread is 40cm and it is not stretched. The upper sphere is slowly raised vertically upward until the lower ball just becomes unsupported by ground. At this time length of thread is 1m. The rubber thread exerts a force which is proportional to its extension. Q. What is force constant of rubber thread ? Ans 1N/m Attempt 1 On Applying work energy theorem x = 60 cm Mgx = 1/2kx^2 K= 2N/m Attempt 2 x = 60 cm On Applying Newtons Equatns Mg = kx k = 1N/m Doubt: Why is first Attempt wrong.