• # question_answer A spring 40 mm long is stretched by the application of a force. If 10 N force required to stretch the spring through 1 mm, then work done in stretching the spring through 40 mm is                   [AIIMS 1998; AFMC 2000; JIPMER 2000] A)             84 J     B)             68 J C)             23 J     D)             8 J

Here $k=\frac{F}{x}=\frac{10}{1\times {{10}^{-3}}}={{10}^{4}}\,N/m$ W $=\frac{1}{2}k{{x}^{2}}=\frac{1}{2}\times {{10}^{4}}\times {{(40\times {{10}^{-3}})}^{2}}=8\,J$